Lee Valley Tools Gardening Newsletter
Vol. 2, Issue 2
April 2007
How Plants Know When to Rise and Shine

There are two kinds of winter dormancy, but a number of names for each, so I'll make up my own. In one kind, which I'll call snoozing, the plant will grow only if the conditions are right, which usually means higher temperatures. In the other kind, which I'll call sleeping, even if the air is warm and conditions are excellent, the plant will not grow. This is because it requires a certain amount of time at a low temperature – called chilling requirements or chilling hours – before it will break dormancy.

This need for a period of cold explains why a warm spell in early winter is generally non-threatening – most plants have not chalked up enough chill time. It's why cutting forsythia early in the winter for forcing inside will
produce no results. It's also the reason many plants will not grow in warmer climates – it simply does not get cold enough for long enough.

Chilling requirements vary, not only for different plants, but also for different cultivars of the same plant. For example, certain apple cultivars need only about 50 hours, while others need more than 1,500.

First blossoms
The abundant blossoms of this hardy crab apple tree will be followed by colorful fruit if conditions are right.

Chilling requirements are, therefore, strongly related to hardiness. If an apple cultivar that needs 50 hours of cold is growing in a northern zone (unlikely in reality, as it would only be recommended for warmer zones), it may be damaged by a mid-winter thaw because it has sufficiently chilled out and is ready to wake up. However, a cultivar requiring 1,500 hours is unlikely to be damaged because it hasn't clocked enough cold hours to break dormancy.

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