A.F.G. Slootweg
Until recently the only procedure to force tulips to flowerproduction in winter was to box the bulbs in the autumn and to store them either outside or in a rooting room. Roughly 14–17 weeks after planting the boxes can be taken into the glasshouse and forced into bloom. If flowering before Christmas or in the beginning of January is desired, the bulbs have to be subjected to a storage at 9°C for 6–8 weeks before planting (pre-cooling at 9°).

Gill, Stuart and Beijer developed a method to tacle the problem of too short stem-length of garden tulips in regions with a mild winter climate. This can be done by storing the bulbs prior to planting for 7–9 weeks at 5°C. This was the starting point of the experiments of Hoogeterp for the purpose of direct planting into the soil of a heated glasshouse of tulips precooled at 5°C (five degree tulips).

The use of five degree tulips is very attractive as it is saving labour, avoiding difficulties with standing ground temperatures and avoiding the tricky decision whether the bulbs are sufficiently developed to bring them into the glasshouse. More valuable varieties can be used as five degree tulips for Christmas flowering (Apeldoorn). At that time the quality of these flowers is often superior to boxed tulips of the same variety. Five degree tulips can be grown as an intermediate crop in the rotation scheme of holdings with glasshouse facilities which are not specialized in flower production. As disadvantages have to be mentioned the longer glasshouse period of five degree tulips and as a consequence higher costs of heating. Five degree tulips require very accurate preplanting and forcing procedures, similar to those required for bulbs pre-cooled at 9°C. Varieties and stocks have to be chosen with greatest consideration. The results may differ from one year to another. Environmental circumstances as soil temperature, texture of the soil, watering conditions and plant diseases (Fusarium oxysporum, Rhizoctonia solani) can affect five degree tulips more than tulips treated and forced in the traditional way.

Some of the most important parts of the pre-treatment of the bulbs, influencing the final result with five degree tulips will be discussed.

Slootweg, A.F.G. (1969). SOME CONCEPTS OF TULIPS PRE-COOLED AT 5°C. Acta Hortic. 14, 177-186
DOI: 10.17660/ActaHortic.1969.14.19

Acta Horticulturae