The Difference Between Plant Hardiness and Chilling-

Chillin’ isn’t just for teenagers…plants need it too. Spelled a bit differently, though.

For temperate plants, this is serious business. Without chilling, the plant won’t be able to grow in the spring. Without hardiness, the plant may be damaged by cold weather. Let’s break this down a bit.

There are two things at work here. Hardiness. And Chilling. 

• Hardiness: The ability of the plant to survive cold. Light frosts improve the ability of a plant to withstand deeper cold. Once plants begin growing in spring they gradually lose their cold hardiness.
• Chilling: Time, in hours, needed in the 33-50 F range before the plant will grow in spring. Freezing has no effect on chilling, and time spent below freezing is not ‘logged’ in the plant’s countdown. Temperatures above 60 are detrimental to chilling. Unmet chilling requirements are what prevent a plant from growing too soon in the spring, and if chilling requirements are met and the plant is still not growing it’s because the weather is too cold. Once temperatures are in the 40’s trees generally begin growing.

So. What does this mean to those who grow temperate species as bonsai?

• Some plants have a lot of chilling requirement or need a higher temperature in spring before growth is initiated, and burst out so late in spring you might wonder if they’d died, such as beech. Others have very little chilling requirement, like some southern blueberries. You may notice differences between coastal trees and high elevation or northern trees. Knowing these differences help us to anticipate and protect those that will begin growing earlier in spring.
• You might notice some kinds of growth, like flowering, that may happen while the snow is still flying on camellia or ume (flowering apricot). These plants will still take a while to begin shoot growth, however, which is more tender to cold.
• Understanding chilling also helps us to program trees that are in need of chilling for health, such as temperate trees we’re trying to keep in warmer climates. In those warm areas, we may need to place temperate bonsai in the shade, keep them cooler, and keep that up for as long as we can in the winter to help them meet their chilling requirements.
• Although understanding whether our trees can or cannot survive freezing is very important, hardiness maps for plants are nearly useless to bonsai growers, who have root hardiness to consider. Roots are considerably more tender to cold.
• Be very careful in the spring, when bonsai are growing and are beginning to lose their hardiness. Even very hardy kinds of plants like spruce, once growing, can be damaged by cold. And cold damage is irreversible, one of those things where one night of prevention is worth years of bonsai training. Isn’t there a saying like that?

Also take a look at the first in this set of articles on cold and bonsai: http://crataegus.com/2015/11/19/seasonal-care-for-cold-weather/