Articles
THE ‘FORCING DISK’, AN EXPEDIENT IN PROGRAMMING TULIP FLOWERING
Since then, many investigations have been carried out in several countries to formulate principles for practical application.
In Holland in recent years the studies of Hoogeterp have greatly improved the methods of forcing.
Especially the introduction of the concept of cold requirement must be mentioned in this respect.
Recent surveys of studies on bulb forcing have been given by Rees (1972) and De Hertogh (1974).
Tulips, like most other bulbous plants, require a warm-cool-warm temperature sequence for normal development.
By optimalizing the temperature treatments, the normal annual periodicity can be reduced to about 7 months.
The three essential and successive steps after harvest of Dutch-grown bulbs are briefly:
- Heat treatment: for earliest forcing: (1 week at 34°C) + 20°C (17°C after Oct. 15).
- Cold treatment: y weeks at low temperature (9°C or lower), either as dry bulbs (= precooling) or after planting.
- Heat treatment: z days in the greenhouse.
For earliest forcing (Dec., Jan.), in many cultivars, if lifted early, the treatment at 34°C accelerates flower initiation.
In all forcing periods(x), y, and z are to some extent interrelated, e.g. by increasing y by a week z can be shortened by a few days.
In recent years the essential parameters x (for earliest forcing), y, and z have been established for different forcing periods in a large number of cultivars (De Hertogh 1973, Anonymous 1975). The ultimate choice is always a compromise, taking into account quality (stem length, flower size). temperature during forcing, the periode in the greenhouse, and’a minimum risk of such deviations as flower blast.
Consequently, the figures given by different authors for the same cultivar may vary slightly.
To facilitate the programming of successive crops of tulips in the available greenhouse space, a ‘forcing disk’ has been developed.
The