The tulip belong to a group of bulbous plants that need a cold period to ensure flowering and development of sufficient stem length. In regions where temperatures are sufficiently low in winter, tulip bulbs planted in the field undergo this cold period naturally.
Research has shown that tulip bulbs which receive their cold treatment before the winter months can be brought into flower before their natural flowering period. This method is called forcing. Depending on the regional climate in which the tulip bulbs are produced, flowering as early as December is possible. The climate influences the development of the new tulip flower shoot until the bulbs are lifted and, provided the bulbs are not treated further in a cold room, even afterwards.
As well as advancing development, the flowering time of tulips may also be delayed. Tulip flowers have been marketed as early as September, October and November for some years now. These so-called ice tulips are produced from bulbs that are stored at low temperatures for an extended period of time.
In order to advance the cold period, the bulbs are subjected to a number of temperature treatments. It is vital that the bulbs are not cooled until the flower has developed fully in the bulb, mostly even later.
Cooling temperatures for tulips are 9�C and below. Different temperatures will yield different results. For example, �high� as opposed to �low� temperatures will produce:
- shorter flower stems
- larger flowers touxiang
- longer greenhouse period
- less risk of flower blast (in early forcing)
- faster growth of stem and roots.
There are three methods to bring tulips into
flower: forcing in trays, cultivation in the border soil of the greenhouse and delaying in trays (ice tulips). Each of these methods relies on one or more cold treatments. A diagram of these treatments is shown in the figure.
1.2 Forcing in trays: 9�C (pre-cooled) and uncooled tu
Bulbs are planted in trays which are then stored in a rooting room or a plunging bed. This method, whereby the bulbs receive the entire cold period in the rooting room or naturally out-side, produces so-called �uncooled tulips�. Part of the cold treatment can also be given to the dry bulb (9�C tulips) before it is planted in a box to continue and complete its cold treatment.
1.3 Forcing in the border soil of the greenhouse:
Dry bulbs receive the entire cold period while stored in a cold room at 5�C or 2�C. Immediately following the cold treatment, the bulbs are planted in the greenhouse, ready for flower production to start.
1.4 Forcing in the border soil of the greenhouse:
Uncooled tulips receive most or all of the cold period naturally in the greenhouse. For 9�C tulips the dry bulbs receive part of the cold treatment at 9�C in the cold room.
1.5 Delaying flowering in trays: Ice tulips
Ice tulips are planted in trays in November, where they remain to root for 2 to 4 weeks at 9�C. After the rooting phase, the trays are frozen and stored at -1.5 to -2�C. In order to prevent trays from drying out, they are usually wrapped in plastic. From september, trays are generally placed in the greenhouse or in a cool place outside where the bulbs are to flower.
A limited range of tulips is forced into flower in autumn in this manner. Unfortunately, both the flower and the keeping quality of these tulips leave a lot to be desired. We will therefore not go into detail of this technique in this book.
2.1 Stage G
The new shoot develops quite early in the tulip bulb. The leaf and later the flower is formed shortly before and after the bulbs are harvested. The time at which the flower is fully developed in the bulb is called �Stage G� (�G� is short for the Greek word Gynoecium, meaning pistil).
As the development of the leaf takes place largely before harvesting, the weather influences the rate with which this takes place and indirectly the following phase and the time at which stage G is reached.
Of crucial importance for the right treatment of tulip bulbs is a timely start to the 5�C or 9�C treat-ment for early-flowering bulbs. This must take place after the bulbs have received the necessary minimum period at the intermediate temperature. Damage may occur when the cold period is started too soon. However, if it starts too late, valuable time is lost. If stage G is late, flower growers must expect that (pre-cooled) 9�C tulips for forcing in trays and early 5�C tulips destined for planting in the border soil in the greenhouse cultivation are supplied later and flower later.
2.2 Intermediate temperature
The intermediate temperature is the temperature the (dry or planted) tulip bulbs receive from the time the flower is fully developed, i.e. Stage G, until the start of the cold period. The optimum intermediate temperature is 20�C. It promotes root growth and will prevent flower blast in most cultivars. From 15 October the intermediate temperature is reduced to 17�C. The minimum duration of the intermediate temperature treatment depends on the method of cultivation and the cultivar characteristics. It is advisable to comply with the recommended number of weeks.
The minimum number of weeks stated here applies to bulb sizes 12/+. If bulb size 11/12 or 10/11 is used for forcing, the intermediate temperature must be extended by 1-1.5 or 2-3 weeks. However, since the bulbs receive the necessary intermediate temperature treatment during storage in the Netherlands, this is of no concern to bulb flower forcers.
2.3 Stage properties
Leaf formation phases:
Stage I apical meristem flat, leaf formation
Stage II apical meristem round, leaf formation completed
Flower formation phase:
- Stage P1 the outer 3 perianth segments are formed
- Stage P2 the inner 3 perianth segments are formed
- Stage A1 the outer 3 anthers are formed
- Stage A2 the inner 3 anthers are formed
- Stage A2+ the gynoecium is visible, but still flat
- Stage G triangular swollen gynoecium
- Stage G+ all flower parts, including stamens, are clearly visible
2.4 Stage determination
The cold period may be started once stage G is reached and the treatment at the intermediate temperature has been completed.
Unfortunately, no fixed date can be given for this period, as it depends on the stage development in that particular year and batch. A wide range of factors play a role in the development, such as weather conditions in the previous growing season, the time the bulbs were lifted, the temperature treatment after lifting, cultivar characteristics and bulb size. Use stage examination to determine whether or not the flower has developed fully. To do this, carefully cut open a bulb. Take out the apical meristem without damaging it and examine it with a binocular microscope (magnification of 25 to 30 times), or, if the flower has developed, a 10-x magnifying glass.
The following botanical terms are used internationally for the flower�s organs:
- P = perianth (petals)
- A = anther (stamen)
- G = gynoecium (pistil)
- The numbers indicate the various stages. Plus and minus signs after the stage indicate the
- transition between the stages.
3.1 Advantages of forcing in trays
Forcing in trays is the most commonly used forcing method for tulips. There are a number of reasons for this, including:
- forcing in trays allows for better planning, with-out being too dependent upon outside
- forcing in trays allows better labour planning.
- relatively few diseases due to the low temperatures in the rooting room. This is an important advantage in view of likely future restrictions or even ban on the use of pesticides.
- reduced energy consumption due to shorter periods in the greenhouse.
- forcing in trays achieves a higher production of tulips per m2 per forcing season.
The cold period ensures that the tulips develop a sufficiently long stem. It also promotes faster growth of the crop. When forcing in trays, the bulbs receive part of the cold period in the cold room as dry bulbs (9�C-(pre-cooled) tulips). After the bulbs are planted, they receive the remainder of the cold period in the rooting room or in a plunge bed. Alternatively, the bulbs receive the entire cold period as planted bulbs. These bulbs are called �uncooled tulips�. The duration of the cold period depends on the cultivar, the bulb size and the scheduled date for housing.
The minimum number of weeks at the intermediate temperature is based on bulb size 12/+. If bulb sizes 11/12 or 10/11 are used, the intermediate temperature must be extended by 1-1.5 or 2-3 weeks.
The number of cold weeks also applies to bulb size 12/+. If bulb sizes 11/12 or 10/11 are used for early-mid or late forcing of a cultivar also suitable for early flowering, the cold period may be reduced by 0-1 or 1-2 weeks. In that case these smaller bulb sizes will produce somewhat shorter stems.
As soon as the bulbs have had the necessary cold period, the trays can be placed in the green-
Extending the cold period has both advantages and disadvantages. By extending the period by one week, the greenhouse period is shortened by three days, which reduces energy consumption. Associated disadvantages include:
- the development of long necks, especially in susceptible cultivars and smaller bulb sizes
- smaller blooms
- higher risk of topple
- limp leaves
3.3 Cooling temperature
The temperature during the cold phase depends on the time the cold treatment is given, regardless of whether it involves dry or planted bulbs. From mid-October it is advisable to cool the bulbs after planting. Table 1 sets out the optimum cold temperatures per period.
You may decide to provide the cold period to dry instead of planted bulbs for the following rea-sons:
- cooling dry bulbs takes up less room and is therefore cheaper;
- the time you can or wish to start the cold period (indirectly the date you wish to bring the trays into the greenhouse).
With regard to the last point, if this is be-fore mid-September, the cold period must be given to dry bulbs. This because before mid-September the bulbs won�t root or will root insufficient.
The duration of the cold period for dry bulbs may range from 2 to 8 weeks. A cold period of less than 2 weeks is not advisable; instead, plant earlier. A cold period of more than 8 weeks may lead to problems due to an early and steady development of the roots and shoot. Moreover, it will leave insufficient time for the rooting period after planting. Therefore tulips are generally not planted after 15 December.
After the tulip bulbs are planted in trays, whether or not following a cold period as dry bulbs, they must receive the necessary cooling in the rooting room or in a plung bed outside. From 1 December the temperature in the rooting room must be reduced in time, depending on the growth of the shoot. Leave a gap of at least 2 cm between the tips of the shoots and the bottom of the tray above.
3.4 Determination of flowering time
When the first flowers are harvested depends on, amongst others, the time the cultivar reaches Stage G. As soon as this occurs, bulbs are
stored at the intermediate temperature for at least one week. The intermediate temperature is immediately followed by the cold treatment. Before 15 September all bulbs must be cooled dry, after that date you may choose to provide the cold period to dry bulbs or to bulbs planted in trays. The bulbs must remain in the trays for at least 6 weeks. By adding up the total time spent in the greenhouse, the harvesting date can be calculated. The first bloom can be calculated from the time Stage G is reached, alternatively the beginning of the cold period may also be determined based on the required flowering time. For example, the desired flowering period is around 6 March, minus 21 greenhouse days gives a housing date of 14 February; deduct 16 cold weeks, which means that the cold phase must start from 25 October. Here, too, part of the required cold may be given to the dry bulbs, if the bulbs are to be stored in a planted condition for around 6 weeks and the dry cold period is completed at the latest around 15 December.
4.1 Greenhouse and greenhouse equipment: Introduction
The forcing of tulips in trays does not place any special demands on the greenhouse. Green-houses built from glass or plastic are suitable for this purpose. Tulips do not have any special light requirements either, as they are not very light-sensitive. It is however preferable to choose a greenhouse which is approx. 3 metres high as this makes it possible to control the environment more effectively and an energy screen can also be fitted. Another very important point is good ventilation and there are many suitable systems available.
4.2 Greenhouse and greenhouse equipment: Space utiliza
In the greenhouse the trays are usually placed on benches that may be made from a variety of materials. Based on the average height of people working in the greenhouse, use the following sizes:
- width side benches: 60 cm
- width other benches: 120 cm
- height of bench: 65 cm
- path width: 45-50 cm
- main path, if used: 200 cm
In a standard-size greenhouse this will yield an efficient capacity of appr. 70%. An efficient capacity of 85% may be achieved if mobile staging is used.
The trays may also be placed directly on the floor instead of on benches. Do not remove the trays after harvesting but simply stack the next layer of trays on top. This also creates a form of �bench�.
As a rule of thumb to calculate the number of trays per gross greenhouse surface area of 500 m2 (75% capacity use): trays (dimensions 60 x 40 cm) divided by 3 times the gross greenhouse surface area = 1500 trays.
4.3 Greenhouse and greenhouse equipment: Heating
A greenhouse heating system is absolutely vital to keep temperatures at a constant 18-20�C. The system must have the following standard capacity in the coldest months of the year: 290 Wh per m2 greenhouse surface area. During other seasons 235 Wh per m2 suffices. For production in the border soil of the greenhouse appr. 350 Wh per m2 is standard.
For the forcing of tulips pipe heating is preferred for its heat distribution capabilities. Although hot-air heating is also suitable, it is not ideal, particularly because of its inadequate air distribution capacity in the greenhouse. The resulting temperature fluctuations will yield an uneven crop and a greater number of rejects. Temperature fluctuations may be prevented by providing horizontal air ducting. Make sure that the hot-air heaters in the greenhouse (including hot-air blowers and CO2 burners) are adjusted properly. Failure to do so or leakage in flue pipes may mean ethylene is released during combustion. Besides uneven growth, slow growing crops and leaves with a thin surface wax layer, even the smallest concentration of this gas (0.1 ppm) in the greenhouse may cause great damage through flower blast. Therefore, have the heater checked before each forcing season.
4.4 Greenhouse and greenhouse equipment: Shading syste
Shading is essential in climate control. Moreover, it reduces energy consumption. Although tulips do not place high demands on light, it is still a quality factor that benefits leaf colour and keeping quality. The light-sensitivity of the tulip cultivar in question is decisive. Cultivars that produce a lot of foliage or tend towards a pale leaf colour are for example more susceptible to light deficiency than others. The degree of light screening will depend on the range grown and the speed of forcing. The use of mobile shading systems towards the end of spring is preferred.
Although shading systems may also be used to reduce energy consumption, quality should always come first. Therefore, the use of mobile systems is preferred. If necessary, permanent shading may be used. Only use material with an open woven structure. The use of non-permeable materials such as plastic sheeting increases the risk of excessive RH levels in the greenhouse. We therefore strongly discourage the use of plastic sheets if adequate ventilation is not provided. High relative humidity may lead to flower blast, infection with botrytis or leaf, stem or flower topple. To prevent this, inspect carefully and ventilate on time or, when necessary, remove the sheeting or screening in spring.
4.5 Greenhouse and greenhouse equipment: Watering syst
In order to meet the tulips� water requirements, the plants must be watered regularly throughout cultivation. Watering by hand with a hose is very labour-intensive. Flower growers are increasingly changing to either fully or semi-automated systems. There are two modern watering systems: sprinkler installations and drip irrigation systems. Since not all batches will require the same amount of water, it is essential to regulate the water given.
- Greenhouse sprinkler: This inexpensive system is highly suitable for -the initial phase of cultivation. However, the more the crop develops, the greater the risk of disease. The crop gets wet, which in turn leads to a large increase in the relative humidity in the greenhouse and reduced evaporation of the crop.
Hard water will leave white marks on leaves and flowers. The disadvantages of this system may be countered by placing the sprinkler pipes between the crop, if necessary with additional covers. The applied water can be mea-sured relatively easily by means of cups placed throughout the greenhouse.
- Risers: Risers with spray or irrigation nozzles are placed amongst the crop. Plants stay drier than with the sprinkler and spraying is also possible during harvesting. A disadvantage of the system is that the installation of the risers before and in between the various crops and replacing the risers pulled out during harvesting is very labour-intensive.
This system ensures that water is released very slowly and that the crop remains dry. This not only reduces the occurrence of diseases, but also ensures the stems remain clean and less energy and water is used. The water volume can be adjusted to meet the needs of each individual tray.
When the system is first installed, the soil on the bud in the tray may be too dry for the use of the drip-irrigation system alone. In that case it is recommended to increase moisture levels in the soil immediately after installing the unit by providing water by hand or via a sprinkler installation. There are two drip systems that are both suitable for use.
- Drip hose system: This system comprises a hose with openings which will release not more than 1 to 4 litres of water per hour, despite a high circulation rate. By accelerating the circulation rate of the water, blockages in the system are prevented. The hose has drip openings at regular intervals. The number of openings depends on the soil structure. Peat usually requires four drip openings per tray, releasing 1 litre of water per hour. Thanks to the pressure-compensating characteristics of the hose openings, the water release in the first and last opening is identical.
- In-line system: This system basically resembles the drip hose system. The difference is that the distance between the drip openings can be adjusted. Since the system does not feature pressure compensation the maximum hose length is 60 metres.
Depending on the size of the batches, the greenhouse is divided into a number of watering segments. A sprinkler unit ensures that automatic watering takes place when needed.
4.6 Forcing trays: Requirements
For tulip forcing, using the right trays is vital. In practice, use is normally made of so-called tulip export trays. These 60 x 40 cm plastic trays with a height of 18 cm and an inside depth of at least 8.5 cm allow for the use of sufficient tulip substrate. The layer underneath the bulbs must be at least 5 cm deep. This is important, not only because it supports the plants, but because it acts as a water and oxygen buffer by preventing the bulbs from suffocating due to an excess of water and attendant with that an oxygen deficit. When not enough water is supplied, the tips of the petals will fade, the buds will desiccate and the leaves turn yellow.
Another important factor is the bottom of the tray which must be provided with sufficient openings or slits. More openings means less roots at the bottom of the tray, which reduces the risk of Trichoderma. An insufficient number of openings carries a risk, especially in the rooting room, that the bulbs drown and suffocate due to higher humidity levels. Openings that are too wide (wider than 2 mm) will dry out the soil and damage the shoots growing in the trays below. Too many openings in the sides may dry out or flush out the soil. Always stack trays in the roo-ting room at least 7 cm apart, preferably 10-11 cm. Above all, the trays must be sturdy and easy to handle and stack.
4.7 Forcing trays: Cleaning
Rhizoctonia, Pythium and Olpidium (see Augusta disease), the cause of grey bulb rot, root rot and Augusta disease, may survive in forcing trays. Therefore, disinfect used wooden forcing trays and clean used plastic trays. To prevent damage to a later crop, disinfect the wooden trays well before the forcing season begins and leave to dry for a few weeks. Plastic trays are cleaned with a strong water jet
- remember to clean in the corners!
4.8 Rooting medium
The substrate most commonly used for tray forcing is a peat mixture with sand, which must meet the following standards:
- The rooting medium must have the right composition. A good mixture is 40-80% 1-year-frozen black peat (garden peat) + 60-20% peat moss. Preference is given to a mixture of 60% garden peat and 40% peat moss. To make sure that the plants do not fall over and to improve the water-air ratio in the substrate, but especially to control the fungi Pythium Trichoderma and Botrytis cinerea, it is advisable to add disease-free potting soil or course sand (15%) to the mixture.
- The structure should be neither too course, nor too fine. A rooting medium with a course structure is difficult to work and will lose water too quickly. A structure that is too fine reduces the water-draining capacity and the air content of the soil. Flooding must be avoided. During transportation moist soil will settle even more, resulting in even further deterioration in the soil structure.
- Good water and air balance. Provide a maximum of 80% moisture content and minimum air content of 10% (at a suction tension of -10 cm). Rooting mediums processed with peats that have shrinking qualities are unsuitable. A maximum settling of soil of 30% is allowed.
- The rooting medium should not be too acid. Low pH levels will lead to root rot. A pH level of 6-7 is ideal. Even if an analysis is supplied with the rooting medium, have it tested. If the pH level is too low, the medium requires additional treatment. 1 kg of calcium carbonate per cubic metre will raise the pH value by a factor 0.3.
- The medium should not be too salty. High salt levels in the soil will affect the rooting of the tulips. A maximum of EC 0.5-1.0 at 25�C applies. This includes the addition of any fertilizer, which we do not recommend.
- The medium should not be too light. 1 cubic metre of slightly moist rooting medium must weigh at least 300 kg. If this is not the case, the plants will not sit tightly in the trays.
- The medium must be disease-free.
5.1 Planting period
The planting period depends on the following:
- plant only between 15 September and 15 December
- the date scheduled for bringing the trays into the greenhouse
- a minimum rooting period of at least six weeks after boxing
- temperature in the rooting room (without climate control) or plunging bed
- available space in the rooting room
- allocation of labour.
Tulips intended for late flowering (March and April) have always been planted too early. Because the bulbs received too much cold, the shoots would snap and the crops would grow long and limp in the greenhouse. Early planting also produced fast-growing crops with small flowers, which would suffer from bud desiccation and topple.
Therefore, do not start the cold period too early (see Determination of the flowering period).
Lower the temperature in the rooting room or standing ground to appr. 9�C in autumn before planting. Failure to do so will not only increase the risk of an attack by Augusta disease, Fusarium and/or Pythium, but also require an modification of the duration of the cold period.
5.2 Bulb size
The following applies to bulb sizes:
- the largest bulbs produce the highest percentage of flowers, which is particularly obvious in early forcing.
- smaller bulb sizes generally produce smaller flowers, with longer necks that are less likely to flower satisfactorily.
- later during the season the stem length and flower size may decrease due to faster growth rates; larger bulb sizes then yield the best results.
5.3 Planting method: Introduction
Place the bulbs on a layer of rooting medium of at least 5 cm depth on the bottom of the tray. This allows bulbs to sit firmly while providing room to develop a sturdy rooting system. Moreover, the layer provides a good water and oxygen buffer, which drastically reduces the risk of suffocation or desiccation. The tulips are placed upright by hand in the trays. This will achieve a uniform crop with level flowers, which greatly facilitates harvesting.
5.4 Planting method: Planting density
The planting density depends on the foliage produced by the crop, which in turn depends on the cultivar and planting time. The following table sets out the number of bulbs per tray.
5.5 Planting method: Covering layer
After planting, cover the tulip bulbs with a 1 to 2 cm thick layer of sand to prevent the bulbs from pushing up through the rooting medium during their development, causing the plants to fall over upon harvesting. The sand will also help to keep the tulips clean.
The quality of the covering sand is very important. If it is too fine or contains too many fine particles or silt, the medium may pan after watering, causing the bulbs to suffocate due to a lack of oxygen. Therefore, always use course-grain sand (e.g. fluvial sand).
5.6 Planting method: Watering
Next, thoroughly water the rooting medium in the trays. Also remember to water the corners of the trays. As a rule of thumb to determine whether the medium is moist enough it should be squeezed. If hardly any water can be squeezed out, the mois-ture content in the medium is just right. Remember that this is only a guideline. The right medium composition and structure encourages rapid and abundant root development. If you are planning on using dry wooden trays, test the moisture levels of the soil shortly after planting and water again if necessary.
6.1 Rooting room: Temperature
After the trays are placed in the rooting room, the following temperature regimes must be used:
Cold temperature per period
9�C until October 25
7�C October 25
- November 5
5�C from November 5 onward
5�-2�C from December 1 onward
2�-0�C (max. -2�C) depending on height of shoot
From 1 December the room temperature may be reduced to 4-2�C, depending on the development stage of the shoot. This may later be reduced even further to 2-1�C with -1�C as an absolute minimum. So reduce from 1 December the temperature in time before shoot elongation accele-rates and remember to leave a gap of at least 2 cm between the shoots and the tray stacked over it. Never let the temperature rise, as this may cause shoot development. The tempe-rature must be reduced gradually degree by degree. If the temperature has already fallen to around 0 to -2�C and subsequently rises again, the shoots experience dramatic growth which, once started, cannot be stopped.
In the first few weeks after planting a tempera-ture of 11�C is permitted. 13�C is too high and must be compensated by a longer cold phase. The following rule of thumb applies: for each week and each degree above the tolerated temperature, the cold period must be extended by one day.
In practice, this will be necessary when the bulbs cannot be stored in a temperature-controlled rooting room during mild autumns.
6.2 Rooting room: Humidity and relative humidity
With regard to the sound development of roots and shoots in the rooting room, the planting soil must contain sufficient water and oxygen. Not enough or too much water will soon lead to problems. For example, the bulbs may suffocate, or suffer from poor rooting, Pythium, topple, hollow stem and flower blast. Regularly check the moisture content of the soil in the trays in various places in the rooting room (especially in the vicinity of the ventilators). As a rule of thumb: squeeze the soil. If hardly any water is released, it is sufficiently moist.
To prevent the soil from drying out and to stop bulb roots from emerging from the trays, provide plenty of water directly after planting and maintain a relative humidity of 90
- 95% in the rooting room. Dried-out bulb roots provide a perfect target for Botrytis cinerea and even Trichoderma if the bottom of the tray is covered with roots instead of medium.
By regularly watering the floor, relative humidity levels may be maintained without problem at 90-95%. Ceiling-mounted sprinklers are not recommended as too much water may cause the bulbs to suffocate.
6.3 Rooting room: Ventilation and air circulation
Sufficient ventilation will be achieved by simply opening the doors upon inspecting the crop. Ventilation in rooms without a climate system to reduce the room temperature may dry out the medium and roots. The risk of desiccation also poses a real threat in the vicinity of evaporation units. Check regularly and water if necessary.
For even temperature distribution low air circulation will suffice. Greater air circulation may dry out the roots.
6.4 Rooting room: Crop protection
There is not much point in trying to control certain diseases in the rooting room as tempera-tures are too low and the forcing trays are not easily accessible. Many diseases, such as Botrytis cinerea and Trichoderma, may be prevented by providing the right storage conditions for the bulbs. By disinfecting the bulbs before planting (see Bulb treatment), diseases may also be prevented. For a detailed description of the rooting room, please refer to Rooting Rooms.
6.5 Standing ground: Introduction
In regions where the average temperatures in winter do not exceed the temperatures set out in the table, the planted trays may also be stored on a standing ground outdoors.
6.6 Standing ground: Description
A level, well drained site, near to the greenhouse, free from pests and diseases and protected from vermin. The traditional method of cooling bulbs, the trays being placed in a single layer and covered with 20-30 cm of straw.
6.7 Standing ground: Soil requirements
The soil in the standing ground must be disease-free. It is therefore recommended to make a new standing ground every year, as is common in crop rotation. After all, this has also proved its worth. Infected soil may also be treated with an additional soil treatment.
The most suitable substrate for the bulbs is fresh rooting medium (see also).
This all serves to prevent diseases. The soil from the standing ground may be used as substrate only if it is disease-free. The standing ground must also drain freely and be level.
The trays are placed on the ground or dug in. Make sure that the bottoms of the trays are positioned flat onto the surface. If this is not the case, the space created between the surface and the tray will have an insulating effect, frustrating heat transfer from the soil. This increases the risk of frost damage. The bulbs in the trays are then covered with a layer of appr. 5 cm coarse-gain sand, followed by cultivated soil. Lastly, the standing ground is carefully covered with peat or straw. Application of this last layer is weather dependent. In a mild autumn, for example, it may be wise to apply the layer early to prevent excessive temperatures.
When the trays are dug in, it is best to apply a thin layer of sand between the bulbs and the soil of the standing ground. When the trays are brought into the greenhouse, the soil will come off by simply shaking the tray, without damaging the shoot. The standing ground should also be covered with soil or peat during frost. Take extra care in covering the edges of the standing ground.
In severe frost, cover the standing ground with bubble plastic. We recommend you always have this sheeting in stock!!
Only plant at low daytime temperatures. In planting, the soil is lightly pressed. Soil that has a loose structure contains more water. This reduces the insulation capacity of the soil, allowing frost to enter the soil. Thoroughly water the standing ground in the evening, preferably with cold water.
6.8 Standing ground: Temperature
The temperature in the standing ground should be the same as in the rooting room (see table). To avoid temperatures that are too low or even frost damage, cover the standing ground with straw (2 kg/m2), plus (bubble) plastic sheeting in the event of frost.
If the temperature in the standing ground is too high, sprinkle the bed with cold water to lower it. If the temperature in the standing ground is high at the beginning due to high outside temperatures, the same rules apply as for the rooting room.
6.9 Standing ground: Housing
If the trays are brought into the greenhouse du-ring temperatures below zero, there is a danger that the shoot dries out or freezes during transportation. If this is the case, postpone bringing the trays in, or, if this is not possible, take the following points into consideration:
- transport the trays quickly, preferable in the afternoon
- remove the covering layer after arrival of the trays in the greenhouse
- cover the trays with plastic sheeting during transportation
- to prevent damage to the bulbs, do not provide any water in the greenhouse until they are fully thawed.
Frequently inspect the moisture of the medium in the trays, the temperature and relative humidity in the rooting room. Also check the temperature and moisture in the standing ground. In dry conditions the temperature will drop rapidly and is difficult to raise to the right temperature during periods of continuing frost. In order to avoid any unplea-sant surprises due to sudden frost, provide adequate cover on the standing ground, preferably bubble plastic.
All thermometers and hygrometers used for ta-king readings should be calibrated once every year. Here frugality may prove expensive.
As soon as the required cultivars have received the necessary cold period, the forcing trays may be moved into the greenhouse.
Tulips may be placed in the greenhouse at a temperature of 18-20�C. From a quality point of view it is, however, recommended to keep temperatures just a few degrees lower which will extend the cultivation period by a few days. From 1 February the temperature should be 16-18�C. Fluctuations in temperature should be avoided. Temperature fluctuations will halt the development and increase the risk of flower blast and �long necks� in the flowers. High greenhouse temperatures also pose a danger as they stimulate excessive growth, which leads to flower blast. If the cold period is exceeded by 2 to 3 weeks, lower the temperature in the greenhouse by 1 to 2�C to ensure a quality crop.
The supply of bottom heat is not recommended as it promotes root rot, Trichoderma and particularly flower blast.
7.2 Relative humidity
The relative humidity can make or break a crop. Insufficient humidity slows the development of the crop down, thereby increasing energy consumption. Excessive humidity increases the risk of topple, infections by Botrytis tulipae, lighter and limper crops and flower blast. Relative humidity should be between 60-80% and must be frequently checked, preferable just above the crop. Most suitable for this are hygrometers, which must, of course, give an accurate reading.
For computer-controlled greenhouses, test the recorded values with a hygrometer. Especially in mild, wet springs, the relative humidity can rapidly rise to above 80%. Therefore, check carefully. The relative humidity can also be reduced by regular ventilation. In still, dull weather the greenhouse is heated a little and ventilated at the same time. Horizontal air blowers encourage the evaporation from crops and lower the relative humidity. To raise the relative humidity when necessary, reduce the ventilation and sprinkle the path ways with water.
Production under low light conditions, i.e. in the event of closed insulation, in combination with high relative humidity will in a number of cultivars lead to pale leaves, long and limp plants, late emergence of the leaves (so-called �cigars�), topple in leaf and stem and short keeping quality of the flowers. These cultivars require more light. Towards the end of the spring it may be advisable to introduce a light shading when necessary to boost the flower quality. For further information, see �Shading systems�.
8.1 Crop advancement
The tulips may be advanced by placing them under the greenhouse benches for a while to produce one or two additional crops per season. This method will reduce the forcing period by up to one week at the beginning of the forcing season, and by up to a few days later in the season.
The same effect may be achieved by placing the trays, either stacked or not, in a so-called �pre-forcing room� before bringing them into the greenhouse.
How long the trays must remain under the benches or in the pre-forcing room depends on the cultivar, the shoot length, actual temperature and the stage of the forcing season. If the trays are stacked, the distance between the trays determines the shoot development. Pre-forcing takes place at temperatures of not more than 16�C. When stacking the trays, leave sufficient room for the emerging shoots. Besides the obvious advantages, pre-forcing also has disadvantages. It is a labour-intensive method that later in the forcing season may lead to tall crops and stems that are crooked at the base. There is also a risk that the roots growing out of the stacked trays which are infected with Botrytis cinerea cause marks on the plants. Cultivars susceptible to Trichoderma may not be stacked. Instead, place them on a greenhouse bench covered with a thin layer of sand to protect them from infestation.
Covering the tulip trays with plastic sheeting, for example to produce longer stems, is not effec-tive. On the contrary: the risk of spotting (Botrytis tulipae) and topple increases. However, if you choose to use this method, make sure you remove all plants that are affected or that fail to emerge, usually caused by Fusarium or Botrytis, immediately after housing.
8.3 Watering measures
The tulips, which are very sensitive to desiccation, must be watered during production as the water supply in the trays is relatively limited. Watering depends on three factors: method, time and volume. Correct watering helps to prevent problems such as flower blast, topple, hollow stems, �oedema� and Trichoderma. As a guide-line, water three to four times per week. No guidelines can be given for the amount of water required. After all, this depends on the growth stage, weather, ventilation, cultivar and soil type. The soil must be kept moist at all times, including even the weekends. Pay particular attention to the bottom of the trays. Too much or not enough water may lead to damage during the greenhouse period. If more than half of all tulips have been harvested, the risk of rejects due to desiccation becomes even greater. Therefore, provide water at soil level, preferably after each harvesting, until the last flowers have been harvested. Throughout the production period it is best to provide water in the morning to allow the crop to dry thoroughly. Pay particular attention to the edges and corners of the trays that are located in the vicinity of heating pipes.
Regular watering is highly labour-intensive when it is done manually with a hose pipe. This accounts for the popularity of automated watering systems. There are two watering systems that are suitable for the cultivation of tulips. (see Watering systems).
8.4 Period in the greenhouse
The duration of the greenhouse period for each tulip cultivar depends on the storage temperature and the cooling of the bulbs, plus the relative humidity in the greenhouse during production.
8.5 Brilliant Star and Joffre
These short cultivars are only forced into flower around Christmas when they are marketed as short cultivars. Every year every effort must be made to ensure the cultivation goes as planned. Risks are flower blast and late flowering. Read the following to avoid these problems. Your supplier will be able to tell you when the bulbs may be planted. This may be possible straight after delivery or after storing the bulbs at 17�C for a while. After a 10-week cold period during which temperatures are gradually lowered from 9 to 7 to 5�C the tulips are ready for housing. Never allow the temperature to drop below 5�C during this phase. Refer to Temperature, for an overview of the temperature treatment during the cold period. At the end of November/beginning of December the tulips are brought into the greenhouse.
One or two days after housing, the temperature is raised to 23-25�C. During the entire 3-week period in the greenhouse this temperature should be maintained to ensure the flowers will bloom at Christmas. Since the bulbs have received relatively little cold they not only produce short flowers, but also grow slowly, which may lead to late flowering. Dropping the greenhouse temperature is risky as this may stunt the development and lead to flower blast. As soon as the flowers are ready for marketing, gradually reduce the temperature to 18-19�C.
8.6 Disease control
Plants that develop insufficiently may be infected by fungi such as Fusarium (�sour�), Pythium or Botrytis tulipae (�tulip fire�), or may have received insufficient cold. Plants with hollow stems will also be stunted.
After bringing the plants in, remove all those plants infected with Fusarium or Botrytis. This will at the same time combat ethylene damage to the neighbouring plants and remove a potential source of Botrytis infection (the so called �primaries�) for the remainder.
8.7 Cultivation in pots: Methods
There are three different ways to achieve a suitable stem length for the cultivation of tulips in pots.
The first option is to choose genetically short-growing cultivars.
Secondly, use relatively short cut-flower cultivars, whereby the bulbs undergo a temperature treatment. Contrary to cut flowers, the stem growth is retarded by extending the intermediate temperature by 1 week and shortening the cold period by 1 to 2 weeks. Suitable species are Abra, Colour Cardinal, Irene, Page Polka, Rococo and Yellow Present.
Thirdly, use chemical growth inhibitors. However, since the effect of these agents is not very reliable, most are no longer marketed and thus we will not discuss this method any further.
8.8 Cultivation in pots: Cultivation
Plant each pot with one, three, five or six/seven bulbs. The pot sizes must be 10, 12 and 15 cm respectively. When planting more than one bulb per pot, plant the bulbs with the flat sides facing towards the side of the pot. The bottom leaf of the plant will then fold across the rim of the pot, leaving the flower to emerge in the cen-tre.
The remainder of production in pots is identical to forcing in trays. For an attractive article, use nice-looking pots, which are cleaned before delivery and supplied with a photo label and sleeve, possibly with text and an illustration.
14.1 General soil treatment: Steaming
In order to revitalize the soil in standing ground and greenhouses, and rooting medium that has been used before, and to reduce the risk of diseases such as Botrytis cinerea and Pythium, it is advisable to apply steam treat-ment.
Steaming means heating the soil down to a depth of 25-30 cm for at least an hour at 70-80�C.
This will destroy most soil diseases, with the exception of Pythium which must have additional chemical treatment.
Steaming is best done with underpressure. In muddy soil with a low pH steaming may lead to damaging levels of magnesium.
14.2 General soil treatment: Flooding
This method entails flooding the (greenhouse border) soil for 6 weeks. It has proven to be an adequate solution against a variety of soil fungi, eelworms and weeds. Rhizoctonia tuliparum and Botrytis tulipae, two dangerous tulip diseases, are particularly easy to combat using this method. Soil infected with these fungi may be used for tulip production after flooding.
Unfortunately, the fungus Rhizoctonia solani and Pythium do not respond. Not only that, but due to the absence of any competitors after flooding, they may even spread rapidly under the right circumstances.
14.3 Additional soil treatment
Since Pythium fungi grow (back) quickly, it is not controlled adequately by the annual general soil treatment alone. Therefore, also treat the soil before planting. If the soil has not been treated with a general soil treatment before cultivation, treat the soil before planting not only to control Pythium, but also Rhizoctonia solani. Mix the fungicide evenly through the upper 20 cm of the soil.
Although fresh planting soil should in principle be disease-free, Pythium will always be present. It therefore pays to treat the peat mixture with fungicides effective against Pythium.
Information about suitable fungicides and applications required is available from your local information service, your supplier or in the Bulb Flower Information Bulletin.
14.4 Treatment of planting soil
In the treatment of planting soil a distinction is made between fresh and used planting soil. Since Pythium is present even in fresh planting soil, it is advisable to add a fungicide against Pythium.
Used planting soil is given a general soil treatment by means of steaming. In addition, add a Pythium fungicide. If the used soil has not been given a general treatment, use a fungicide to combat Pythium as well as Rhizoctonia solani. All fungicides must be worked through the soil evenly.
14.5 Bulb treatment
There are different methods to treat tulip bulbs. The following applies with regard to the concentration of the dip.
The following tables show which of the diseases that may occur during production in trays and in the greenhouse can be controlled by means of a bulb treatment.
Information about suitable fungicides and applications required is available from your local information service, your supplier or in the Bulb Flower Information Bulletin.
14.6 Fungal diseases: Augusta disease
Diseased plants grow crookedly and remain short while brown, longitudinal patches and stripes appear on the leaves. The flowers of red-flowering cultivars develop thin, dark veins.
The disease has a few typical characteristics. It often leads to great damage, whereby the infection may be limited to a few patches in a parti-cular cultivar, while other cultivars remain healthy under identical circumstances. There are great differences between the susceptibility of the various cultivars. It may for example affect only two cultivars on the standing ground, but not any of the others. Many growers will assume that the entire batch was diseased upon delivery, which is usually not the case. Susceptible cultivars include Angelique, Apricot Beauty and Prominence.
The Augusta disease is caused by the tobacco necrosis virus, which is transferred to the roots by the swarm cells of the fungus Olpidium brassicae. This is particularly encouraged by soil temperatures above 9�C. Infected plants do not necessarily show symptoms. The disease may suddenly appear when the plants are planted out during frost. Augusta disease is most common in tulips that are planted early at high temperatures after storage on a standing ground.
- Use fresh planting soil, although this is not always sufficient since the virus can survive on other plants and weeds.
- Disinfect the soil of the standing ground or cold storage facility as well as any infected trays by means of steaming (thirty minutes at 100�C).
- When using soil suspected of being diseased, do not plant the bulbs until the ambient temperature has dropped to below 9�C.
14.7 Fungal diseases: Botrytis cinerea
This fungus occurs mainly in damaged or affected parts of the plants and beneath the skin between the roots. Both the bulbs and the roots beneath the skin may be infected.
The symptoms of the infected bulbs are:
Various bulbs are completely or partly soft and dark brown. There are large, mat black sclerotia on the diseased tissue. In the parts above the soil the infected plants are brittle and will break abruptly. The flowers have a different colour to normal ones. Heavily infected plants remain short or don�t emerge at all.
The symptoms of the infected roots are:
Only the roots or part of the roots of tulips planted in the rooting room are infected. The fungus thrives in thick layers of roots at the bottom of the tray or on parts of the root that lie beneath the skin of the bulb. When the infected roots touch the tulip shoots emerging below, they
may cause marks there, leading to so-called �spotted tulips�. It does not lead to �fire� spotting (Botrytis tulipae).
The disease is caused by the fungus Botrytis cinerea, also known as grey mould. It is distributed by means of spores. Moist conditions promote the infection. The disease is most commonly found in long-stored bulbs that are planted late. It is encouraged by the use of steamed soil or fresh peat-based substrate as this does not contain any of its natural competitors. In the border soil cultivation of the greenhouse the use of loose or lumpy soil and the addition of organic material will further add to the problem.
- Do not plant the tulips in a purely peat-based substrate.
- Always mix some coarse-grain sand or disease-free soil in the peat product (20%).
- Give the bulbs a fungicide treatment before planting.
- After planting, cover the bulbs with a layer of course sand.
- Prevent the roots growing through the tray openings from drying out by using high humidity levels (90-95%).
- Always force the tulips on open benches.
14.8 Fungal diseases: Botrytis tulipae
Heavily infected plants do not open or are retarded in growth (�primaries�). The lowermost, often twisted, leaves at the base of the stem develop greyish brown fungal hyphae with spores. The parts of the plant beneath the soil develop 1-2 mm large, mat black sclerotia.
Germinating spores cause small, watery marks on the leaves and flowers. These marks are green at first but later turn to large white and brown spots. Susceptibility to �fire� spotting depends on the cultivar.
The disease is caused by the fungus Botrytis tulipae. The sclerotia and spores only germinate in moist conditions (surface water), at temperatures starting from appr. 1�C. The spores are spread quickly through e.g. watering or even air circulation, and can cause the �fire� spots within 24 hours on the leaf and within as little as 10 hours on the flowers. From 15 February the spots will intensify. The fungus can survive up to two years in the soil by means of its sclerotia, even if no tulips are planted in the meantime.
The spores of the fungus Botrytis cinerea may also cause spots. However, these are much smaller and only occur in the flower buds.
- Infected soil must receive a general soil treatment (see General Soil Treatment).
- The bulbs must be dipped before planting.
- Remove all bulbs that do not emerge immediately after the trays have been taken into the greenhouse.
- Ensure that the crop remains dry during production, especially at night. It is advisable to water and ventilate early in the day to allow the crop to dry.
- Just prior to flowering, the greenhouse is smoked with suitable fungicide.
- Avoid surface water collecting on the plant. The relative humidity must be 85-90% and the air must be circulating continuously.
14.9 Fungal diseases: Fusarium
Infected bulbs develop greyish brown spots during storage, occasionally with concentrated rings and a clear yellow rim. They give off a distinctive, acrid smell and release ethylene. The bulbs shrivel up and sit loosely in their skins.
Even healthy bulbs may contain spores of this fungus. Depending on the circumstances (especially the soil temperature) the bulbs are infected from the base of the root crown in the first weeks after planting. Diseased bulbs will not emerge. In less serious cases growth will be retarded, flower tips will turn yellow and flower buds will desiccate. A longitudinal dissection of a bulb clearly shows that the stem turns brown from the base upwards. Bulbs infected with Fusarium release ethylene into the soil which may lead to retarded growth and even bud desiccation in neighbouring plants.
The disease is caused by fungus Fusarium oxysporym f-spec. tulipae. In tray cultivation it thrives in high planting temperatures and during long periods in the greenhouse. In the cultivation of 5�C tulips there is a high infection risk especially in early crops, as the bulbs undergo temperature treatments that encourage the development of the fungus.
Infected bulbs release ethylene gas, which during storage may lead to �gumming�, open shoots, multi shooted crops, bud necrosis and a somewhat greater risk of blind flower buds in the other bulbs.
- Ventilate infected batches thoroughly when in storage.
- Remove infected bulbs.
- Disinfect bulbs (see bulb infection) or plant at soil temperatures of 9�C or less (5�C tulips 12�C or less). Never plant in temperatures above 13�C. If necessary, plant later.
- Remove all bulbs that do not emerge.
14.10 Fungal diseases: Pythium
- root rot
The symptoms at low levels of infection are restricted to a partly rotted root system, without the plant actually being affected. Serious cases may lead to flower blast and shorter crops. The infected roots in the soil are glassy or watery, often marked with a narrow brown stripe, and break off easily. When the disease develops the entire root system turns brown.
The disease is caused by fungi of the Pythium genus, especially P. ultimum. The fungi are active in soil temperatures above freezing. The risk of disease increases as the temperature and moisture content of the soil increases. An attack from Pythium occurs both during tray cultivation and in the greenhouse. Susceptibility depends on the cultivar.
Symptoms caused by salt in the tulip roots.
- Use fresh soil for the standing ground, greenhouse and in the trays.
- Soil infected with Pythium generally requires an additional soil treatment.
- The (border soil of the) greenhouse must have good structure and drain water freely.
14.11 Fungal diseases: Pythium
- soft rot
Bulbs that are infected at an early stage often develop short shoots. The bulb tissue is weak and frequently coloured pink. The bulbs give off a typical, unpleasant odour similar to Fusarium bulbs. The shoot and root may seem healthy for a while, but will rot eventually.
If the attack occurs later during cultivation, the bulbs are retarded in growth, the tips of the leaves turn yellow, the plants fall over and under certain circumstances the blooms may desiccate at a very late stage.
The disease is caused by certain strains of the fungus Pythium ultimum. The fungus attacks the bulbs mainly in the first few weeks after planting, in soil temperatures of 12�C or higher. Infected plants are found throughout the greenhouse. Seriously diseased tulips may even be surrounded by completely healthy plants.
The fungus strains can also cause root rot.
- See under Pythium root rot.
- Carry out a bulb treatment (see bulb treatment)
- Ensure the temperature in the greenhouse is below 12�C, preferably even below 10�C, in the first two weeks after planting the tulips in the border soil of the greenhouse.
14.12 Fungal diseases: Rhizoctonia solani
The symptoms of this disease vary according to the cultivation method used. Symptoms during cultivation in the greenhouse include:
Orange-brown spots and stripes which develop on the outside of the shoot. Later the tissue splits as if it has been gnawed at. The plants flower normally, but the tips of the bottom leaves are curved outward. Remove these leaves when bunching. In a more serious attack, the leaf is damaged and the bottom part of the stem develops oval, deeply-set patches. These plants may be retarded in growth and will break easily when handled. The symptoms gradually worsen, resulting in a poor growth of the shoot and later rot. This is because the fungus attacks the bulb directly.
Symptoms in tray cultivation:
When the trays are brought into the greenhouse the shoots have small, brownish black patches and stripes. The plants will flower normally, although there is some damage, especially to the tips of the lowermost leaves.
The disease is caused by the fungus Rhizoctonia solani, which attacks the tulip shoot from the soil. After the plants emerge, further growth is halted. Susceptibility to the disease will differ per cultivars. The fungus is found on a range of agricultural/horticultural crops including potatoes, salads, tomatoes, chrysanthemums and cereals as well as many other bulbous plants. It also thrives on organic material (straw, leaves, roots, etc). This means it can occur even if the previous crop grown in the soil was not tulips. Unlike Rhizoctonia tuliparum, this fungus thrives at higher temperatures (15-18�C).
- Give infected soil a general soil treatment (see General Soil Treatment) or treat the border soil of the greenhouse with an effective fungicide. Mix the fungicides carefully through the top layer (10 cm) of the soil. Pay particular attention to the soil over the bulbs.
- Clean the forcing trays.
- Carry out a bulb treatment (see bulb treatment).
- Preferably plant the bulbs for 5�C forcing and for forcing in trays such that the noses remain uncovered. Strip the bulbs!
- Cover the trays with coarse-grain sand or gravel, leaving the noses bare.
14.13 Fungal diseases: Rhizoctonia tuliparum
Bulbs throughout the crop fail to emerge. Upon inspection the roots of the diseased plants appear intact. The shoot, which develops normally, is diseased in the soil and starts to rot. The bulbs and shoot are occasionally covered with mycelium-clotted soil often containing whitish grey to blackish brown sclerotia. The shape and size of these sclerotia vary greatly (from 1 mm to 1 cm), but they are easy to remove from the plants. The bulb scales develop large, greyish-brown marks with grey fungal tissue. Characteristic, brown rings are visible upon cutting the bulb in half. The entire bulb usually dies through rot. The focus of infection is usually delineated by plants that are retarded in growth and die prematurely. In a heated greenhouse disease development is arrested by applying higher temperatures.
The disease is caused by the fungus Rhizoctonia tuliparum. This fungus, which does not produce any spores, usually attacks bulbous plants in winter, especially at soil temperatures lower than 13�C. Besides bulbous and tuberous plants, it will also attack perennial plants such as Sedum spectabile, Lychinis, Helleborus and Valeriana officinalis.
The fungus may survive unchecked by means of sclerotia. Infected soil may cause serious damage to the crop. The fungal disease will spread by means of hyphae and sclerotia sticking to the bulbs, tools, machines, shoes, rooted cuttings, corms, tubers and rhizomes that are cultivated in infected soil. Major damage only occurs when the bulbs are planted in the same spot regularly. The disease is more common in uncooled tulips than in cooled tulips as they stay on the standing ground longer.
- Use fresh soil on the standing ground and fresh planting soil.
- Steam infected soil or treat it with a suitable fungicide.
- Clean infected forcing trays.
- Destroy infected plants.
- Do not store infected soil in the vicinity of the standing ground.
- Treat the bulbs with a suitable fungicide before planting.
- Planting the bulbs late (appr. from mid November) will dramatically reduce the risk of disease occurring.
14.14 Fungal diseases: Trichoderma
Under unfavourable circumstances the roots that lie at the bottom of the tray may be infected by this fungus. The roots, which will eventually rot, develop a glassy look and are covered in fungal hyphae. The tips of the leaves of the diseased plants turn bright grey. At a later stage the tissue turns white and rapidly dries out. The disease is most common during the late forcing of tulips planted in trays in a pure peat substrate. Susceptible cultivars are Ad Rem, Angelique, Coriolan, Kees Nelis, Pax, Prominence, Rosario and many others.
The disease is caused by the fungus Trichoderma. It releases a toxin that is transported through plants and causes the symptoms described in the tip of the leaves. The fungus is
naturally present in all soil types and in peat-based substrate. However, it will only affect roots that are not surrounded by sufficient soil and are therefore weakened.
- Never plant tulips in purely peat-based substrate.
- When using peat products, always mix these with an equal quantity of course-grain sand or disease-free garden soil. In forcing Tricho-derma susceptible cultivars it is advisable to apply a layer of 1 cm fine-grain sand in the bottom of the tray.
- Keep the bottom of the tray sufficiently moist.
- Maintain a high relative humidity (90-95%) in the rooting room to ensure that the roots growing through the openings in the trays do not dry out and do not grow too long.
- Place the forcing trays on open benches with sufficient space below to allow the roots that grow out from underneath the trays to dry as quickly as possible.
14.15 Disorders: Flower blast
This is the disorder which prevents flowering. Common symptoms are bud desiccation, green petals, white leaf-tips, desiccation of pistil and stamen and incomplete opening of the blooms when the flowers are stored in water during transit to the consumer.
Flower blast is caused by a variety of factors, such as cultivar-dependent characteristics, small bulb size, brief intermediate temperature, short cold period, long transport or storage, ethylene damage during storage or in the greenhouse released by Fusarium bulbs or heaters, high relative humidity in the greenhouse, water shortage, root suffocation and diseases.
Early detection is possible by checking the root systems. When roots are not damaged and are white, there is no salt or Pythium problem. Damage from salt is usually localized and turns the tips of the roots brown. In the event of a Pythium infection, the roots are covered in brown, watery patches. Damage may be limited by keeping the soil moist at all times.
- Avoid the above situations.
- Avoid ethylene concentrations of more than 0.1 ppm by removing Fusarium bulbs from the batches, sufficient ventilation, keeping bulbs apart from cut flowers, vegetables and fruit, and by avoiding exhaust fumes (e.g. from forklift trucks).
- Avoid ethylene gas in the greenhouse by having the burner of the heating unit checked every year and by using heating equipment that uses outside air for combustion.
14.16 Disorders: Topple
A calcium deficiency may produce glassy stems during the growth phase which later fall over.
The top part of the stem becomes dark green and watery. The tissue shrinks, and the above-lying stem with the flower falls over (topple).
Topple in leaves is expressed as watery, dark marks in the middle of the second or third leaf. They often release water drops, and in serious cases the epidermis rips at right angles with the longitudinal direction of the leaves. Another symptom are leaves displaying grey parts (especially in the middle). It is difficult to avoid leaf topple by low relative humidity. The disorder is most common in very large bulbs, insufficient rooted bulbs or in tunic-less, part mouldy bulbs.
Topple is caused by a high relative humidity in the greenhouse and/or poor rooting of the bulbs. This means little water is transported through the plants, leading to a calcium deficiency in the fast-growing parts of the plant. This in turn affects nitrogen absorption.
The susceptibility depends on the cultivar. Highly susceptible cultivars include Gander and Kees Nelis.
- Avoid high RH levels (over 80%) in the greenhouse at all temperatures.
- Avoid excessive cold treatment.
- Ensure healthy roots.
- Avoid excessive growth of the plants.
- Fans that circulate the air horizontally ensure that the relative humidity between the crops will not drastically exceed the RH elsewhere in the greenhouse.
- After harvest place the batches with topple symptoms in a solution containing 1% calcium nitrate.
14.17 Disorders: Veinal streak
The so-called veinal streak shows as twisted leaves, retarded growth and discoloured leaf veins. At an advanced stage the leaf develops glassy, withered patches. The flower buds of the diseased plants later develop watery, sunken stripes and often green marks.
Especially susceptible cultivars include Merry Widow and Monte Carlo and their sports. A disorder related to the veinal streak is evident in cultivars including Snowstar and Gander and their sports. Although these tulips develop sufficient length and the discolouration of the veins is hardly evident, leaves will easily split lengthways.
The vein disease is a physiological disorder which may occur in susceptible cultivars under wet and cold conditions in the rooting room or greenhouse.
- Do not place susceptible cultivars in the standing ground, but rather in the rooting room.
- Use planting soil with a good water/air ratio.
- Before housing, do not water as long as the soil below the bulbs is still moist. Only water when really necessary!
- Do not place the trays on a moist surface in the greenhouse, but instead on open benches (see Trichoderma).
- Keep the greenhouse temperature constant and never allow it to drop below 16�C.
14.18 Disorders: Oedema and hollow stems
Immediately after bringing the plants into the greenhouse, they develop dark-green, watery patches that resemble frostbite. Thick drops of moisture are released in these damaged areas. In forcing, this phenomenon is known as �oedema�. As soon as a larger leaf surface permits greater evaporation, the plants will stop sweating, without any further problems.
In the cultivars Monte Carlo and sports, Cassini, Snowstar and others, the symptoms may even occur up to two weeks after housing. During the rapid development of the plants a tear occurs lengthways in the stem which then develops into a hollow stem. The outside of the hollow stem contains small tears. Although the plant will flower, the bottom part of the stem is unusually thick and remains short. Although the hollow stems may occur in all tulip cultivars following frost damage, the two diseases only occur together in Monte Carlo and sports.
This typically physiological disorder occurs mainly in batches planted early (September/October). It seems there is slightly less risk during longer, warmer storage. Individual batches will vary greatly in their susceptibility to the disease. Plants that grow from the largest bulbs are particularly susceptible.
The cause of the disorder is excessive water absorbtion combined with limited evaporation. Climatic conditions that hinder evaporation of the plants or bulbs stimulate the development of oedema and hollow stems. This is for example the case when humidity levels of the air and soil in the rooting room are too high. Batches that are buried in standing grounds and covered with straw are less susceptible and rarely suffer from these disorders. Planting soil is not normally used as this entails a higher risk of disease. After all, plants in this soil grow relatively more roots and the soil usually contains a great deal of moisture.
Other factors that promote these disorders are the use of plastic trays and a high relative humidity. Laboratory research has shown that there is a connection between the material of the forcing trays (wood or plastic), the soil type and humidity.
- Never use peat products as rooting substrate without mixing with 20% coarse-grain sand or potting soil.
- If any glassy plants emerge, add 0.5 to 1.0 kg gr calcium nitrate per cubic metre substrate mixture or spread at least 50 gr calcium nitrate per square metre of tray surface area and water lightly.
- Cover the tulips with a plastic sheet after housing them.
- Stimulate the evaporation of the plants and maintain a relative humidity of no more than 75%.