“Cold Damage” is a catch-all phrase we use to explain a huge variety of plant problems that crop up often winter’s over. To protect our plants from the cold, however, we need to understand exactly how low temperatures damage plants. Sometimes, it isn’t really the cold at all, but the fluctuation between warm and cold temperatures that causes problems. As we approach the end of winter, it is a good time to assess our winter protection practices. There are many kinds of cold damage and just as many solutions.
Frost Injury – Frost occurs when surface temperatures drop to 32 degrees F or below and causes one of the most common types of cold damage. We are all familiar with the killing frost that can ravage trees and shrubs after a long warm fall. Damage also will typically occur with a late fall or late spring frost after flowers have begun to emerge. Late fall frosts damage the buds and blooms of winter flowering shrubs and trees, where late spring frosts damage annuals and buds and blooms of many early ornamental and fruit trees. In this area, we have multiple – very cold and then warmer weather which is the same as fall and winter, cold and warm.
The best prevention against frost damage to flowers is to keep buds from opening up too soon, For example, plant magnolias on the east or north side of a building so that early spring sun won’t fool them into thinking spring has arrived. Another prevention is selected late-flowering cultivars.
Soil Heaving – Herbaceous perennials are often damaged by soil heaving. Heaving can push shallow plant roots right out of the soil leaving them exposed to cold and desiccation. Heaving is caused by alternate freezing and thawing of the soil. Poorly established plants and shallow rooted species, such as strawberries, are most prone to damage. The best protection is a 3 inch layer of winter mulch (after the soil has frozen) to insulate it, reducing temperature fluctuations. Be careful to keep mulch pulled slightly back from the crowns of perennials that are susceptible to rot.
Freezing Injury – As opposed to frost injury; is freezing injury damages plants while they are still in their dormant state, fracturing cell membranes and disrupting cellular organization. Buds are most often affected. For example, in some regions, forsythia, peach, and dogwood may not bloom well after an extremely cold winter.
Trees, shrubs, and perennials in containers also are at risk. Roots usually are not as hardy as shoots because they normally depend on being insulated by the soil when in the ground. Some extremely hardy plants may weather the winter in a container, but most species require extra protection, Use a thick pot with a large soil volume to give better insulation, wrap
it with insulation material, and place the container in a protected spot. Another reliable method is to bury pots in the ground and mulch the area.
Freeze Cracks – Bark on some trees may split due to sudden changes in temperature. Thin-barked trees, such as maples, lindens, tulip poplars, sycamores, and cherries, are most likely to crack. To prevent, shield the tree trunk from direct sun. Treatment is not usually necessary for this type of damage. On young trees, cracks may close on their own.
Winter Burn – Damage to evergreen foliage in the winter usually is due to desiccation rather than cold. Evergreens continue to lose water through their leaves even in cold weather. Windy conditions, sun, and warm days can speed this process. Plants will suffer from drought when soil moisture is low or when the above-ground portions of the plant have thawed, but the soil around the roots is still frozen.
The best prevention is to ensure that evergreens enter the winter season well watered. Soil moisture can be conserved by adding a 3-inch layer of mulch in late fall after the soil has gotten cold. Broadleaf evergreens should be placed where they are protected from winter winds and late afternoon sun. Usually, the east or north side of a building or other barrier is suitable. Remember to water plants when the soil thaws during the winter.
Using plant material that is reliably cold hardy in your area is, of course, the first step in protecting plants form old damage. Understanding microclimates on your property will also help you to place plants properly.
Please come by or contact me at the Cooperative Extension for more information on ornamental trees and shrubs. We have many free researched based publications and leaflets for you to take with you.
Nancy Olsen can be reached at firstname.lastname@example.org.