Extension Educator, Horticulture
We have all heard or uttered complaints about winter: dancing on snow packed sidewalks, scraping icy windshields, Jack frost nipping at our noses and for gardeners having to hang up our shovels. Complaints overheard from plants at a winter plant therapy meeting include: being planted in the wrong place, cars and people throwing salt on them and having shiny things draped over them. Besides people produced problems, what is it about winter that bothers plants and how can we protect them?
First of all, tremendous variability exists within and between plant species in their tolerance to cold. Otherwise we would have palm trees lining main street. Using plants that are reliably cold hardy and keeping plants healthy during the growing season is the first step in protecting plants from cold damage.
Sometimes it isn't really the cold temperatures that cause problems, but the fluctuations especially rapid fluctuations between warmth and cold.
The most common frost injury occurs not in winter but in early spring or late fall. Impatiens get that cooked spinach look. Homegrown apricots and peaches are often a dream.
Emerging flowers and leaves are most prone to frost damage. The best prevention against frost damage to flowers is to keep buds from opening too soon. For example plant magnolias on the east or north side of a building so early spring sun won't fool them into thinking spring has arrived. Select late flowering magnolias such as lily magnolia. Mulch also keeps soil temperatures cold.
Perennial plants are often damaged by soil heaving. Alternate freezing and thawing of the soil pushes shallow plant roots out of the soil leaving them exposed to cold and drying winds. Poorly established or shallowly rooted plants such as strawberries and chrysanthemums are prone to heaving.
Heaving can be reduced by applying a 2 to 4 inch layer of winter mulch of wood chips or shredded leaves. As temperatures warm in the spring, be careful to keep mulch pulled slightly back from the crowns of perennials that are susceptible to rot. A blanket of several inches of snow (pun intended) is also a good insulation against heaving.
Dormant plants are susceptible to freezing injury. Flower buds are most often affected. For example forsythia or dogwood may not bloom well after an extremely cold winter. We'll see what El Nino has to say.
Few plants in containers can survive winter without some protection. Use a thick pot with a large soil volume to give better insulation, wrap with insulation material such as a cylinder of chicken wire filled with straw and place container in a protected spot such as unheated porch or garage. Use plastic pots or others that can survive a freeze. Clay pots will break if subjected to freezing temperatures. Another reliable method is to bury the pots in the ground and mulch the area.
Freeze cracks/sun scald
Bark on some trees may split on the southwest side due to rapid temperature changes. Thin barked trees such as maples, lindens, and cherries are most likely to crack. Wrapping with tree wrap may give protection against frost cracks.
Broadleaf evergreens such as holly, boxwood or rhododendrons are especially prone to winter burn from winter winds. Evergreens continue to lose water through their leaves even in cold weather. Windy conditions, sun and warm days can speed the process. Evergreens should enter winter well-watered. Soil moisture can be conserved by adding a 3 inch layer of mulch after the soil has gotten cold. Plant broadleaf evergreens where they are protected from winter winds and late afternoon sun such as the east side of a building. Also anti-desiccants such as Wilt Pruf can be sprayed on evergreens to reduce the amount of water lost through leaves.