University of Vermont
Department of Plant and Soil Science

Fall News Article


Dr. Leonard Perry, Horticulture 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 of 20 to 30 degrees (F) below zero.  Some could tolerate even colder temperatures.  Knowing how plants prepare for winter, you can manage plant choices and their culture for better overwintering success.

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 and become dormant.  This is termed “acclimation”-- the development of cold hardiness or "hardening off." A sedum plant that may be killed by freezing soil temperatures in September, may withstand soils below zero degrees (F) in January.

But first, plants must stop growth.  Growth slows for most plants as the days shorten (actually it is the lengthening nights) and grow colder.  Scientists have found that short days and cold temperatures trigger the development of cold hardiness.  Some believe it is the former daylength, which is stable from year to year, that triggers the first stage of dormancy and that cold triggers the true or subsequent deeper dormancy stage, sometimes called “mid-winter dormancy”.
When a plant becomes dormant, cellular components change and processes slow, allowing them to withstand lower temperatures.  You may see this deep dormancy termed “endo dormancy”, referring to these internal (“endo”) changes.  Since freezing water can burst cells, some water leaves cells during acclimation.  Sugars and other protective chemicals, similar to antifreeze, with lower freezing points are increased.  So it makes sense that you don’t want plants to enter winter waterlogged.  On the other hand, you want them to not be stressed from prolonged drought periods.  Ideal is to make sure that plants have sufficient water in the fall, but keep them on the dry side if possible.
Plants have a maximum level of cold they’ll tolerate, this varying genetically by species and even cultivar (cultivated variety). Even within a species, the amount of cold tolerated may be determined by location or “provenance”.  A rhododendron from the south may be much less hardy than the same species from the north. 
Plants parts can vary as well in the amount of cold they’ll tolerate.  This is the reason some years you may have leaves but not flowers on forsythia, flowers buds generally being less hardy than leaf buds.  Or peach trees may live in an area and may leaf out, but have no flowers.  Since roots are in the ground, which stays much warmer than the air, they usually tolerate less cold than the tops of plants.  So the popular Japanese pachysandra groundcover, in winter, has evergreen tops hardy to -30 degrees (F) while the roots only survive in soils down to 15 degrees.

Once a plant is dormant, it tracks the winter progress through “chilling units”, the number of hours not below freezing but that are at a certain level above freezing—often 40 to 50 degrees (F), ranging between 500 and 2000 hours depending on the plant.  (Sugar maple requires about 2000 hours.) After this chilling, they enter a lighter stage of dormancy or “eco dormancy” (“eco” meaning external) just as they experience during fall acclimation. In this “standby mode” they can respond to specific periods and degrees of warming (varying by plant), as with the warming in spring or a prolonged “winter thaw” period.  In this spring period of “deacclimation” buds are   much less hardy, so may be damaged by subsequent severe cold (as happened in March 2015 in Vermont).

To help plants prepare for best winter survival, 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.  Herbaceous perennials are the opposite—good fertility right into fall, stimulating vigor and storage of more food for winter, often helps them survive better and regrow more vigorous next spring.  We’ve seen this time and again in our cold climate hardiness research with perennials.
Fertilizer and lime applied (according to soil test results) in October or November, 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.  While it is best to lime in the fall—most lime takes some months to change the soil acidity—it is best to wait until growth resumes in spring to fertilize if not done by mid fall.

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), 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.  It is illegal in Vermont and in some other areas to apply phosphorus to lawn areas, unless recommended by a soil test, as this can end up in and pollute waterways.  For the same reason, you should use a fertilizer with water insoluble nitrogen.  A rule of thumb is to apply three pounds of nitrogen per 1,000 square feet, as from 30 pounds of 10-0-10 fertilizer. 

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.  For established plants, most professionals recommend to mulch late, just before snow fall.  This allows the soil to cool, and plants to harden off sooner.  The exceptions to late mulching are bulb beds, including those for garlic, and newly planted perennials.  Mulching these sooner will keep the soil warmer longer, allowing for more root growth in fall.  You want them well-rooted so they don’t “heave” out of the ground in spring, a phenomenon known as “frost heaving”.  Roots will continue to grow until the soil temperature drops below about 40 degrees (F)—usually early to mid November in Vermont.

You can learn more about winter hardiness, particularly in perennial plants, in Understanding Perennials, A New Look at an Old Favorite, by William Cullina.  This readable reference covers many other aspects of perennial anatomy, growth and factors affecting them such as soil—a mini course in these topics written for the lay reader. 

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