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It’s hard to trust March if you’re a tree

"Growing Together" columnist Don Kinzler says the false spring starts and rapid temperature swings of this time of year can cause issues for non-native trees.

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Tree types that stay dormant longer remain safe through the temperature swings in March.
David Samson / The Forum
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FARGO — Have you heard the simplest explanation of an acorn? In a nutshell, it’s an oak tree.

All trees will soon be waking from their winter nap. What happens to a tree when beautiful springlike weather spurs sap to start flowing and buds to start swelling, only to be followed by a winterlike cold snap?

According to North Dakota State University Extension Forester Joe Zeleznik, “One of the challenges for trees in the northern Great Plains is the extreme variability in our weather. In March 2021, the temperature swings were huge. Daily highs were in the 50s and 60s. Sometimes the nights were below freezing while other nights were relatively warm.”

Zeleznik continues, “As an example, on March 19, 2021, in Bowman, N.D., the temperature was 16 degrees at 7 a.m., and it rose to 65 by 4 p.m. Less than 12 hours later, it was below freezing again.”

Massive temperature swings are common in February and March. Zeleznik calls it “the midwinter fakeout,” when a warm spell makes it seem like spring has arrived, only to be pulled back into winter soon after.

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Are trees injured by dipping in and out of spring? Zeleznik says it depends. “Our native trees, such as ash, bur oak, box elder and others, generally do just fine. They’re not fooled by these false signals and they stay dormant.”

Our native tree species are specially protected from late winter’s mood swings with a special type of dormancy called endodormancy. These trees are dormant not only because of cold weather, but because of an internal plant inhibitor system that prevents them from growing, even if weather conditions turn warm in mid- to late winter.

Once a tree enters endodormancy in autumn, it won’t grow again until it receives enough cold to overcome this internal security system. Tree species differ in how many days of chilling are required to break this internal dormancy. Our native tree species generally require enough days of chilling to keep them safely dormant past the “midwinter fakeout.”

Zelenski notes that sometimes non-native ornamentals are misled and will break dormancy and start growing, which can be their downfall. When temperatures fall back to normal, these trees can’t reharden quickly enough and they get damaged.

Last spring, I received many calls and emails about birch trees that were not leafing out. Zelenski observed the same. “We saw this in 2021, especially with birch trees. Throughout the state, ornamental birch trees had a lot of dieback, and there was nothing we could do about it.”

Addressing whether there’s anything else we can do for our trees this time of year, Zelenski says March is a great time to prune trees for several reasons. The trees are dormant, which means the insects and fungi that might cause problems are also dormant and won’t infest the fresh pruning wounds.

Zelenski also says it’s easier to choose which branches to remove and which to keep because there are no leaves on the trees, making branches and their structure more visible.

If you’re questioning whether certain tree branches are dead, perform the scratch test. With your thumbnail or a knife, gently scratch away the outer grayish brown bark from twigs. The presence of a fresh green layer immediately beneath the outer bark indicates life. If that layer has turned brown, and the twigs are dry and snap when bent, that portion is dead and can be pruned away.

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Live twigs have a green layer under the outer bark.
David Samson / The Forum

Upcoming webinar

Don Kinzler will present a live Zoom webinar at 7 p.m. Thursday, March 24, describing successful methods for growing vegetables. It's tailored especially to those who have never had a vegetable garden, although gardeners of all experience levels are invited.

The live and interactive webinar will include tips for growing vegetables in traditional gardens, raised beds and gardening in containers on patios, decks and balconies. The hourlong webinar is free, but online registration is required at How to Grow a Vegetable Garden ( https://docs.google.com/forms/d/e/1FAIpQLSeJq4WuZxxXWrBnr7_KqM21suKAypN5urD67fDEwZvVo9QhbQ/viewform ).

For more information, contact Don Kinzler at donald.kinzler@ndsu.edu. The webinar is live only, and recordings won’t be available.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.

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