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Tree Dormancy

Q from Kerstin Jones, Master Gardner, Salt Lake County, UT (received 2/27/20)

How soon do dormant trees need to be watered?


A from Dr. Mike Kuhns, 
USU Extension Forester and Professor (sent 3/09/20)

Kerstin,

I define dormancy as growth and metabolic or biochemical functions stopping in a plant or a part of a plant even though environmental conditions might allow for it. An example is a branch on a tree as it exists in late fall and through the winter with buds closed tight, no growth occurring (in the branch), sugars moved to where they need to be and turned into starch for storage through the winter, and antifreeze chemicals moved to or made where they are needed. This dormancy is usually caused by the plant sensing that the day length has shortened the required amount for that species to be ready to go dormant. The day length is determined by chemical changes that occur in tissues in the bud scales. These changes are actually caused by a change that happens as day length shortens that results in a change in the ratio of near infrared to far infrared light. When that ratio changes enough the plant goes dormant. Then even though the weather might warm, up a bit in December, the plant will stay dormant and won’t be fooled into coming out of dormancy by the weather changing, but will wait for the near infrared/far infrared ratio to tell it that spring is here. 

Roots, on the other hand don’t go dormant because they live in a more protected environment underground. The soil mass prevents quick temperature changes and sudden drying out. Therefore, roots grow whenever the soil is moist and the soil temperature is above about 40F. If the soil is too cold near the soil surface but is warm enough deeper in the soil then roots that are located deeper in the soil will be able to grow. What matters is the soil moisture and temperature at the root tip because the tips are where root growth happens.  

So, having said all of that, I also need to point out that more root growth is a good thing for trees. And since root growth occurs only in moist soil, then water to keep the soil moist at any time of year when the soil temperature at some depth will exceed 40F.
I hope that this helps.

Mike Kuhns

Mike Kuhns