Have you been noticing brown leaves and needles on some evergreens in area landscapes? Many of these are located where they could not possibly be victims of the injury caused by deicing salts that we discussed several weeks ago. The likely cause of the brown leaves and needles on these evergreens, particularly on boxwood, holly, cherry laurel, rhododendron and evergreen euonymus, is winter burn.
Winter burn, also referred to as winter dessication, occurs in frigid weather when the soil is frozen, making the water in the soil unavailable to plants. This is bad news for evergreens because they continue to lose moisture through their leaves and needles during freezing weather.
Because they can not replace that moisture, evergreen leaf tissues become desiccated, die, and turn brown. Typically this damage shows up as the browning of entire leaves and needles or, when less severe, browning of leaf edges and needle tips. Severe winter burn can also cause twig and branch dieback.
Environmental factors contribute to the severity of winter burn. Damage will generally be more severe where water loss is the greatest, such as on the side of a plant more exposed to wind or direct sun. Damage can also be more severe if the plant was already drought stressed at winter’s onset.
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Cultural factors predispose a plant to winter burn. Evergreens with restricted or girdling roots that limit the uptake of water; recently planted plants without adequate root systems; or plants planted too deep are more likely to suffer winter burn damage. Plants weakened by drought, insect or disease are also more vulnerable to damage.
What can you do to help plants suffering from winter burn? Do not rush to prune off branches and twigs with brown leaves or needles. New growth may develop if the buds and stems are not dead. To tell if a branch or twig is alive, use your fingernail or the edge of a pocket knife to gently scrap under the bark to look for signs of green tissue. Prune off the portions that are obviously dead. If you see green tissue, wait to continue pruning so that latent buds have a chance to grow.
We cannot easily predict when our area will experience another winter when the weather stays severely cold for a long period of time. However, there are some steps you should take to help minimize potential winter burn and promote healthy landscape plants.
1. Select plants that are not prone to winter burn. Recalling past severely cold winters, I typically advise against planting boxwood, holly, cherry laurel and evergreen euonymus because of their vulnerability to winter burn.
2. Water evergreens, especially those planted within the last two years, properly through the entire growing season and also during mild fall and winter weather when the soil is dry. Check the soil periodically to determine moisture levels. Keep in mind that fog and light rain typically do not provide enough water to keep the soil moist.
3. Mulch plants with a 3- to 4-inch layer of shredded bark or wood chip mulch to conserve soil moisture.
4. When placing new plants, avoid planting broadleaf evergreens where they will be subject to winter winds and sun.
5. Refrain from planting evergreens in late fall when they will not have time to develop adequate roots before cold weather arrives.
6. Protect existing plants from drying winds by placing a screen of lath fencing or burlap two feet away from the plant on the windward side.
Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.