By Tim Johnson
Special to Tribune Newspapers
December 18, 2012
Do I need to be worried about winter burn on my evergreens? If so, is there something that I should do to prevent it?
— Leo Jackson, Lincolnshire
Winter burn is discoloration caused when the leaves or needles of evergreens dry out.
During the cold months, evergreens continue to lose water vapor through their leaves (or needles, which are modified leaves). The leaves must replace the water by pulling it up from the roots. But when the ground is frozen, the plants' roots cannot absorb water to supply it to the leaves. If the weather turns warm and sunny while the ground still is frozen, evaporation from the leaves increases and the water cannot be replaced. Discolored or "burned" foliage may start to appear.
This type of winter damage may be misdiagnosed as a disease or as damage from excessively cold temperatures. In fact, winter burn symptoms typically develop during warm weather in late winter and early spring.
The brown or yellowed foliage generally is on the side of the plant facing the sun and/or the side exposed to the wind, where the rate of evaporation from the needles or leaves is greatest.
Winter burn can be more prevalent in years in which the ground freezes early before plants are acclimated to cold weather or when there is little snow. Without snow cover or mulch to insulate the soil, the ground can freeze more deeply.
If you see winter-burned foliage in spring, you may simply trim it out.
You do not specify the type of evergreens that are in your garden or whether they were recently planted. Recently planted evergreens are more vulnerable because they have not had time to grow a large root system to store water. It is important to water evergreens planted over the last three years as needed so that they do not go into winter dry. Keep watering until the ground freezes.
Evergreens that have been growing in your garden for more than three years are well established and usually are fine without any special maintenance. However, older evergreens also may benefit from supplemental water this year because the 2012 growing season was unusually hot and dry.
Water evergreens slowly and deeply by letting the hose trickle for half an hour or so near the base of the plant.
Anything that dries out evergreen leaves can cause winter burn. Since salt draws water from plant tissue, salt spray from traffic can cause winter burn in evergreens near driveways or streets. Hot air from dryer vents or furnace vents also can dry out nearby plants.
Planting evergreens too late in the fall, leaving them little time to grow roots, can increase the chance of winter burn. Gardeners will provide different advice on how late to plant evergreens, but generally, installing them after mid- to late October is risky.
I planted several 6-foot-tall upright yews in the middle of November one year, although a cold spell had frozen the root balls before planting. These plants had a good amount of winter burn the next spring. They did recover, though they were thin the first summer.
The type and size of plants and their site in the garden will affect their vulnerability to winter burn. Broad-leaved evergreens such as holly, Japanese Andromeda and rhododendron are touchy in this climate because their large leaves allow more evaporation than needles. They should be sited so that they have protection from winter sun and wind. When they are newly installed, they can be protected with a burlap screen for shade and a windbreak.
It may or may not help to apply anti-dessicant sprays to limit water loss through the leaves. The Chicago Botanic Garden does not use these products, focusing instead on proper plant selection, placement, planting and maintenance.
Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe (chicagobotanic.org). Send questions to: Gardening Q-and-A, Sunday, Chicago Tribune, 435 N. Michigan Ave., Chicago, IL 60611-4041; email to firstname.lastname@example.org.