Warm winter puts North Dakota trees at risk

2012-02-04T22:00:00Z Warm winter puts North Dakota trees at riskBy KAREN HERZOG | Bismarck Tribune Bismarck Tribune

While this winter's unprecedented mild weather has been delightful for people, those balmy conditions are putting some trees and plants at risk.

Ron Smith, extension horticulturalist at North Dakota State University in Fargo, said that the more recently-planted the tree, the greater the risk of winter dessication and winter kill.

Trees recently planted, or set in this past summer, have root systems that have had no chance to get established, he said. For trees, the deeper the root system, the safer they are, Smith said.

Even mature trees are not immune to the vicissitudes of our weather, he said, and the summer's flooding along the Missouri River has already put trees under stress from weeks of being waterlogged.

What happens when warm weather lingers in the winter is premature dehardening, Smith said. Plants and trees respond to the warmth by putting out buds.

"If we go into a cold snap, we could have a minus 25 temperature, after 30- to 40-degree days and only modest freezing at night," he said. "That would take out and injure quite a few plants."

Flower buds particularly have a lower tolerance to survive a sudden freeze than foliage buds, Smith said. Flowering trees - starting with cherries, then plums and finally apples - may start to swell and open, he said.

A cold snap would burn the buds, meaning no fruit for the coming season, he said.

Shade trees would be less affected, Smith said.

"Shade trees established around here have gone through the school of hard knocks," he said. "They might get foliage burned off by a cold snap (but) in Mother Nature's wisdom, they have built a reserve."

About 75 percent of their leafing might be burned off initially, but the tree keeps the remaining 25 percent in reserve to leaf out later. This keeps the tree alive and able to manufacture food for the next year, Smith said.

The added stress to trees immersed for weeks last summer by floodwaters means that those trees were going to struggle, anyway, he said. Adding in the warm winter means that disease and insects, which normally get wiped out by severe winters, will be present in greater numbers to prey on the weakened trees.

In landscaping, the trees and shrubs most at risk are the marginal plants, Smith said. "As horticulturalists and homeowners, we like to push the envelope," he said, planting trees and shrubs at the edge of our hardiness zone to see if we can get them to grow.

Marginal trees that have been alive for three or four years - maybe because of a good microclimate or protection from a house or fence - could be taken out by this anomalous winter, he said.

However, because of a general, gradual warming trend, areas of North Dakota have been reclassified for the USDA Hardiness Zone Map; that will allow people to go ahead and try once-marginal plants, he said.

There's not much homeowners can do in this situation, Smith said: "Really just let sleeping dogs lie and see what happens, and watch if trees leaf out."

"Don't think North Dakota has become Camelot," he advised. "We can even go through February not seeing bitter cold, but then a Manitoba Clipper can come along and that's going to hurt."

Reach reporter Karen Herzog at 250-8267 or karen.herzog@bismarcktribune.com.

Copyright 2016 Bismarck Tribune. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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