Why is this important?

Many deciduous plants have a dormancy period that allows the plant to survive the winter. Dormant buds can withstand cold, freezing temperatures of winter whereas actively growing buds would be destroyed. Once a bud has become dormant, these plants needs x amount of hours between 0 and 7 degree (chilling hours) before the bud can break dormancy and start growing again.

Again why do plants need this chilling requirement?

Can you imagine what would happen if the buds came out of dormancy every time there was a warm spell in the middle of winter? All the buds – potential flowers and fruits– would be killed when the temperatures dropped again.

The “chilling requirement” — total number of hours needed annually, below 7 ºC and above 0 ºC — are cumulative and need not be continuous. The chilling hours vary with each plant species and even different varieties within each species.

Without a sufficient chilling period the plant may grow well but will either not set fruit very well or not all. Lack of chill makes buds slow to break in spring or not at all. The flowering period may be 4-6 weeks rather than 1-2 weeks, with very irregular bud break all over the tree. This may mean that some of the cross pollinating variety’s flowers are over while others are not yet in bloom. Some varieties will set fruit but the fruit quality and maturity will be non-uniform on the tree.

Most of the “chilling requirement” research has been done on fruit and nut trees. Go figure…follow the money. Anyways, this research revealed that the major fruit and nut varieties grown in the world need 800 to 1500 hours of chill. These amounts of “chilling hours” are usually found within USDA hardiness zones 4 to 8.

In the last 30 years extensive research has gone into producing “low chill” fruiting varieties for the warmer countries. So there are low chill varieties available for almost every variety of fruit tree with a chilling requirement. In Canada, research on other side of the chill factor, has led to fruiting varieties with a higher chill requirements (especially peaches).

Chilling requirements have been loosely classified into three categories, high chill varieties (800-1500 hours), medium chill varieties (450-800 hours), and low chill (50-450). For example, apples have high chill varieties (Gala, Delicious and Fuji), Medium chill varieties (Gravenstein, Granny Smith and Pink Lady) and low chill varieties (Ann, Tropic Mac, and Tropic Sweet). There are also low chill varieties available for peach, plum, nectarine, apricot, cherries and pears. Choose the variety appropriate to your area. Do not pick a low chill variety if you live in an area with 800 plus chill hours. The tree will break bud too soon and the buds will be killed by frost!

Avoid setting yourself up for disappointment by finding out the average chilling hours your area receives and only plant trees, shrubs and perennials that will have their chilling needs met. Take a look at the chilling hours US map (produced by University of Maryland) to give you approximate chilling hours in your area.

Chilling requirements for most commercial fruiting trees and shrubs have been examined. There is little or no information on chilling requirement for ornamental deciduous flowering trees and shrubs. The Stanford website and ehow website contains the main fruit, nut and berry plants and their chilling requirements.