http://www.bulb.com/ibc/binaries/pdf-bestanden/hyacinth_forcing-guide.pdf "Temperature treatment: Introduction
The growth cycle of the hyacinth bulb encompasses distinctive periods - leaf formation,
flower formation and stem elongation. Forcing is achieved by advancing the flower-formation
period and then providing the most effective type of cold period. This is why hyacinth bulbs
grown in Mediterranean countries are used to produce very early flowers. The leaf-formation
period is accelerated by higher temperature during the bulb’s cultivation stage, hereby
starting the flower formation period earlier.
In contrast to other bulbous plants, early flower- formation in the hyacinth is achieved by
providing specific temperatures. One way of achieving this is to increase the temperature of
the soil in which the bulbs are grown (“Herald hyacinths”). Because of its expense, this
method is very rarely used. More commonly, the bulbs are lifted early and then given a
temperature treatment in an air-conditioned chamber to advance flower formation. The
lifting date and temperature treatment are the factors which separate the two methods of
production pot and cut flowers.
Temperature treatment: Prepared hyacinths
Bulbs are lifted around 20 June after which a specific sequence of temperatures is applied to
advance flower formation. Depending on where the bulbs are grown, the cultivar and the
lifting date, the flower is completely formed during the second half of August. In technical
terms, this is described as having reached “Stage G”. Since the hyacinth has a flower
cluster, this stage is not reached until the last floret is differentiated. The stem elongation
period can then proceed. Prepared hyacinth bulbs are used for producing pot plants intended to bloom until late December, and cut blooms until mid-January."
