[52]. Species and cultivars vary greatly in the number of chilling hours required. For example, the chill-
ing requirements of peach cultivars grown in Florida should not exceed 300 hr, whereas those grown in
the northernmost parts of the United States may require 800 hr or more.
Bud dormancy is not confined to woody plants. Many herbaceous perennials must be chilled before
growth can resume in the spring. Ornamental bulbs such as tulips and daffodils are planted in the fall.
Cold soil temperatures provide the chilling required to allow normal stem elongation the following spring.
If such bulbs are planted indoors, the flower stalks are much shorter and the flowers themselves may
abort. Florists meet the demand for these flowers out of season by artificially chilling the bulbs, then forc-
ing them in a warm greenhouse. Note that this period of cold temperature stimulates elongation of preex-
isting inflorescences and therefore differs from vernalization, in which chilling stimulates the initiation
of flowers. In some species, however, including Dutch iris (Irisspp.) and Easter lily (Lilium longiflorum),
vernalization indeed occurs. Although the rate of sprout development in onion bulbs is greater at 15°C
than at higher or lower temperatures [53], Abdalla and Mann [54] established that the time required for
sprouting was independent of storage temperature prior to transfer to 15°C. Thus onion differs from tulip
in not requiring chilling for floral stalk elongation.
Similarly, some tubers (e.g., Jerusalem artichoke) must be chilled before buds can grow normally.
This, of course, is not the case with crops, such as the potato, that originated in the tropics. Although
potato has no chilling requirement, the tubers are dormant at harvest. Dry storage at room temperature for
several weeks permits bud development; this parallels the response of seeds of several grains to “after-
ripening” (see earlier).
V. METHODS FOR BREAKING OR PROLONGING DORMANCY
Dormancy or lack thereof can be troublesome to the plant grower. Waiting 6 to 10 weeks or more for seeds
to be after-ripened or buds to be chilled may not be inconvenient in areas where cold temperatures pre-
vent winter production but can reduce profitability in areas where crops can be grown year-round. In the
latter areas, multiple cropping is practiced, with two or more crops being harvested each year. Thus yields
will be maximized if no dormant periods intervene.
170 DENNISFigure 6 Curve used in estimating chill unit accumulation, based on the Utah model, for the breaking of bud
dormancy in deciduous tree fruits. Effective chilling temperature is the mean of the two temperatures measured.
Positive values are assigned to temperatures between 2 and 13°C, negative values to higher temperatures.
(From Ref. 51.)