Dormant Trees

Part 25 of a series by Eugene Howell

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Dormant treesYou’ve heard that the best time to do major branch pruning and root work on deciduous trees is during the winter when the trees are dormant. But what does it mean that the bonsai is dormant?

If you look in a horticulture dictionary you will see that dormancy is “a resting phase; a state of temporary cessation of growth and the slowing down of other activities in whole plants”. In this phase transpiration almost stops, production of food stops, and leaves turn color and eventually drop during fall. The only part of the tree that stays fairly active is the root system. During this period the roots take the opportunity to continue their development.

Interestingly, the only way to research dormancy in trees is by observing a particular species as it goes in and out of its winter dormancy. Each plant species has its own set unique environmental conditions that cause dormancy and within a given species each individual tree will have a slightly different set of conditions that cause its dormancy. Therefore, it’s possible to have two bonsai of the same species, where one goes dormant as much as two or three weeks earlier than the other one and comes out of dormancy with equally dissimilar timing. For example, I have two Bald Cypress’ that do this. At the time of writing this article we were within 4 days of Christmas. One of these trees had been fully dormant for three weeks. The other one, however, was still covered with bright green leaves, some of which were not more than a month old. These two bonsai sit side by side and receive the same light and temperatures, yet every year this happens. The one that goes dormant first is also the last one to come out of dormancy in spring.

The subject of dormant trees is one that has been of keen interest to researchers since before the beginning of the 1900’s. There is an annual International Symposium on Plant Dormancy that takes place so researchers from around the world can exchange findings on the subject. The reason dormant trees are of such great interest to scientists is that if various food crops can have their dormancy characteristics changed by even small amounts, the ability to feed the world’s masses is greatly increased. An excellent book on the subject is Plant Dormancy: Physiology, Biochemistry, and Molecular Biology.

So what causes dormancy and when can it typically be expected to occur? We will cover that in the next post.

Photo credit: Trident maple, Acer burgerianum by Walter Pall

Part 24 of this series : Part 26 of this series