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 data.
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 zone.
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.