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The cold weather of winter is actually necessary for most woody plants of temperate regions so that their buds will burst (flush) normally and commence growth in spring. This necessity for exposure to cold is referred to as "chilling requirement". After the chilling requirement is satisfied warm weather results in rapid bud burst, but if unchilled dormant plants are exposed to warm weather, bud-burst is delayed. This delay usually results in loss of growth.
Chilling required to break bud dormancy is akin to the stratification (low temperature treatment of imbibed seed) required to break dormancy, or hasten germination, of seed. Physiological changes occur during chilling which allow the bud to respond rapidly to favourable conditions when spring comes.
Chilling requirements of 2-0 Douglas-fir plants, was measured in terms of exposure to temperatures below 4.4°C (40°F) This means that, when 4.4°C is taken as the threshold temperature, one hour at 4°C is one hour of chilling, and equally one hour at -25 C is one hour of chilling. Douglas fir plants required 2,000 hours of chilling measured in this way.
Flushing occurs rapidly at an air temperature of 13°C when plants have received 2,000 hours chilling, but with less chilling the air temperature must be increased to obtain rapid flushing. For example, with only 1,250 hours chilling the air temperature must be about 24°C for rapid flushing. Weather records show that nursery stock would normally be exposed to at least 2,000 hours chilling at all nurseries, except during some winters at Surrey and Green Timbers. This may cause delayed flushing and loss of first season growth in stock from these two nurseries. It is also likely that chilling requirements will not be satisfied if container stock is kept in greenhouses over winter. The extent to which cold storage in darkness satisfies the chilling requirement is unknown as yet, but is to be investigated.
Detailed information on this subject may be obtained by writing to the Information Division, B.C. Forest Service, Legislative Buildings, Victoria, B.C. V8V 1X5 for Research Note No. 71 by R. van den Driessche titled "Flushing response of Douglas fir buds to chilling and to different air temperatures after chilling."
[Abstract contains full text of memo.]
BC Forest Service - Research Division. 1975. Chilling requirement of Douglas-fir. British Columbia Ministry of Forests, Lands, Natural Resource Operations and Rural Development. Research Memo (FLNRORD). RM2
Topic: FLNRORD Research Program
Keywords: FLNRORD, Research Memo, British Columbia
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