By Dr. Vern Grubinger
Extension Vegetable and Berry Specialist
University of Vermont

Besides ice dams on the roof and piles
of snow in the driveway, horticultural injury is a fact
of life during northern winters. Surprisingly, there
are many ways that otherwise long-lived plants may meet
their demise in between growing seasons.

Direct injury from exposure to cold temperature
is the most obvious, and as a result, most gardeners
and homeowners pay attention to maps of plant hardiness
zones. These define the average annual minimum temperature
of geographical areas based on years of meteorological

The southern corners of Vermont are in
zone 5a, where the average minimum temperature is between
15 and 20 degrees F below zero. That’s why some people
call it the “Banana Belt.” Up in the Northeast
Kingdom, it’s zone 3b, where the coldest temperature
each year averages 30 to 35 F below.

Avoiding the problem is one way to prevent
winter injury to your plants. Be sure to buy the hardiest
plants you can find. When selecting species or varieties,
never choose those that aren’t suited for your hardiness

Of course, hardiness zones are based on
averages, and that can lead to problems because some
years are colder than average. That’s why it’s advisable
to err on the side of caution when choosing plant varieties
based on hardiness claims. If possible, purchase plants
that are suited to survive in a hardiness zone colder
than yours. If you must put in”marginally hardy”
plantings, try to place them in protected areas, where
the winds are reduced by the presence of structures
or by the topography.

Winter conditions can cause the death
of plants in many ways. Their tops or roots may freeze
or plants may dry out when whipped by persistent winds.
Sometimes, roots are heaved out of the ground by alternate
freezing and thawing.

Sunny winter days followed by frigid nights
create rapid temperature variations that really hurt
plants. Ice storms can cause breakage of stems and limbs.
Mice and deer browsing for food also may cause damage.

What’s a grower to do?

For many valuable crops, winter protection
is essential. But protecting crops too early can interfere
with the plant’s natural ability to adjust for winter
or “harden off.” The first step to take for
winter protection is to wait, so that plants are given
enough time in the late fall to properly harden off.

Don’t apply mulch too early, and don’t
apply nitrogen (N) late in the season. Too much available
N, whether from fertilizers or manure, keeps plants
succulent in the autumn and increases susceptibility
to winter injury.

Mulching plants is a means of stabilizing
soil against freezing and thawing. A thick layer of
weed-free organic residue-like straw is best. Because
straw has hollow stems, it is a very good insulator.

Leaves also can be used, but if they mat
down, their insulating value is diminished, and they
may smother plants. Mulch should be applied in late
November, after a hard frost or two.

When mulching roses, cut the canes back
to 18 inches, surround with chicken wire, and cover
the whole plant to protect shoots as well as roots.
Low-growing perennial flowers, strawberries, and the
like are easy enough to cover completely, but they may
require re-covering if the winds blow off the mulch.

For grape varieties that are not very
hardy, remove the vines from the trellis and lay them
on the ground before mulching. For upright plants that
cannot be lowered to the ground, erecting a fence of
some kind can protect plants against drying winds. Burlap
or other material stretched between posts works well.
So does snow fence. It breaks the wind somewhat even
when there’s no snow, and works really well to protect
plants when there is snow.

Antidesiccants are materials that add
a protective coating to plant leaves, helping them to
retain their moisture. These can be sprayed on leaves
of some plants to reduce the loss of water in winter.

In northern climates, winter injury to
plants is hard to avoid. With proper precautions, it
can be minimized,but be prepared next spring to prune
away winter-killed canes and limbs and to remove dead
plants. These can be a source of plant disease if left
in place.

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