Why Trees Go Dormant Over Winter

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dormant trees in winter

We use the phrase “winter dormancy” a lot, especially when talking about why it’s good to prune trees during the winter (dormant pruning).

But what does it really mean? And why is dormancy important?

Dormancy in plants is like winter hibernation in animals; the tree is conserving energy and living off stored nutrients to get through the long, cold Northeast Ohio winter.

How Dormancy Works

During summer, when trees are actively growing, they generate “food” in the form of carbohydrates and sugars through the process of photosynthesis. Sunlight triggers the process in the tree’s leaves to turn carbon dioxide and water in carbohydrates that the tree uses for energy throughout the growing season and/or stores for use over the winter.

As temperatures cool down and days shorten in autumn, photosynthesis slows down. The tree stops actively growing. Leaves of deciduous trees change color and eventually fall off the tree, leaving it without its “food factory”. Remaining sugars and carbohydrates, as well as moisture inside the tree, move down into the roots where they are used to sustain the tree during winter dormancy.

When temperatures warm up in spring, the tree comes out of dormancy – sap flows quickly, nutrients make their way into the tree canopy, buds swell and leaves emerge to provide food for the tree through photosynthesis.

What Would Happen Without Winter Dormancy?

Several things would happen during freezing winter weather if trees didn’t enter dormancy:

  • Water inside the tree trunk, branches and leaves would freeze, expand, and tear apart the tree from the inside out.
  • The frozen ground would prevent moisture uptake, leaving the tree without the necessary moisture to sustain itself.
  • The tree would have to expend tremendous energy to maintain itself (especially the leaves) during a time when short days limit the amount of photosynthesis that can occur. As a result, the tree would decline, even if it survived internal ice damage and moisture loss.
  • During snow or ice events, snow- and ice-covered leaves would add tremendous weight, often leading to significant damage to the tree.

Is That Tree Dead or Dormant?

Without leaves, it can be difficult to know whether a tree is dead or just dormant.

Some things are obvious. For example, if a tree doesn’t start putting out leaves by early summer, it’s probably dead. But if buds start to swell in spring, that’s a clear sign that the tree or shrub is breaking dormancy.

But during the winter or very early spring when there are no leaves on the tree, how can you tell?

Here are some easy ways to differentiate dormant wood from deadwood:

  • Bend a twig. Deadwood is brittle so if the twig snaps then it’s likely dead, but if it’s flexible then the plant is probably just dormant.
  • Look at the color and texture of the bark. If it looks different from nearby trees of the same kind (or if a branch looks different from the rest of the tree), then it may be dead. Trees or branches that have been dead for a while often lose their bark and turn a silvery color, but if the change has only happened recently then the difference will be more subtle.
  • Check for bent or broken branches. Anything beyond that point is probably dead.

You may have heard about the “scratch test” as a way to identify whether woody stems are still alive. It involves scratching the bark off a small branch to see if the wood below is grey (dead) or green (living). While it does tell you whether the branch is dead or alive, it also creates a wound that leaves an entrance for disease and decay. It’s best to reserve this method for situations where you’re fairly certain you’re dealing with deadwood.

If you’re not sure whether or not your tree is dead or dormant, give us a call. Professional arborists are experienced in differentiating between the two, even when it may not be obvious to the untrained eye.

Take Advantage of Winter Dormancy

Because of the prevalence of Dutch elm disease and oak wilt here in Northeast Ohio, elm and oak trees should only be pruned during dormancy. Both diseases are carried by beetles that are attracted to fresh cuts on host trees. Pruning during winter avoids this problem.

Winter, while trees are dormant, is an excellent time for major pruning work. For details, see this article describing six reasons to choose dormant pruning.

In Summary

Dormancy is a natural process that all cold-hardy plants go through in Northeast Ohio to help them survive our cold winters. While we may not appreciate the look of dormant trees as much as their colorful appearance throughout the rest of the year, it’s a necessary process that gives them the energy needed to put on a show of spring blooms and brilliant fall colors.

If you’re not sure whether a tree is dead or dormant, give us a call at 440-564-1374 to schedule an inspection.


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