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How Plants Prepare for Winter

by Dr. Leonard Perry, Extension Professor,University of Vermont    

Outdoors, a major change is taking place. Your trees and shrubs are preparing for winter. By January most of them will be able to withstand temperatures 30 degrees below zero. Some could tolerate even colder temperatures.

However, these same plants would be killed now if the temperature suddenly dropped that low. That's because something happens inside plants that allows them to prepare for winter. This is the development of cold hardiness or "hardening off."

But first, plants must stop growth. Growth slows for most plants as the days shorten and grow colder. Scientists have found that short days and cold temperatures trigger the development of cold hardiness. But it isn't known whether the plant mechanism that slows growth also starts cold hardiness development or whether they just happen to occur simultaneously.

You should avoid practices that stimulate late summer growth of trees and shrubs. For example, nitrogen fertilization in August or early September may encourage a late flush of stems that can't turn off their growth before frosts.

Fertilizer and lime applied (according to soil test results) in October when temperatures are cooler and days are shorter will not stimulate top growth until spring. Roots take up the nutrients and store them in the roots and stems, stimulating vigorous growth next spring.

For shade trees and shrubs, a surface broadcast application of readily-soluble or slow release, high nitrogen fertilizer over the whole root zone is probably the most effective and easiest means of applying fertilizer. Other methods are liquid injection feeding, the poke and pour method (divvying up fertilizer into holes drilled into the soil in concentric circles around the base of the tree), foliar feeding, or placing fertilizer pills, packets, or spikes in the root zone.

If the soil already contains adequate phosphorus and potassium (as in a well-fertilized lawn), nitrogen is often the only element needed to enhance growth. Follow recommendations from your soil test. Or, as a general rule of thumb, apply 3.5 pounds of nitrogen per 1,000 square feet. This computes to roughly 17 pounds of 20-5-5 or 35 pounds of 10-5-5 fertilizer.

A word of caution: Do not use lawn fertilizers containing herbicides within the spread of trees. Otherwise, the herbicide will be taken up by the tree and kill it just as it kills broad-leaved weeds.