The Effects of Cold Weather on Landscape Plants
By Ashley Appling, Virginia Cooperative Extension – Culpeper County, Horticulture Extension Agent
The frosty winter weather we will experience in the coming months will impact our homes and landscapes in many ways. We will complain about the cold mornings and gray afternoons, along with the slick roads and sidewalks. Our plant friends also experience the cold and may respond with brown leaves, dead branches and injured flower buds that will not be evident until spring. So, what is it about winter that bothers plants, and what protection measures can we put in place? In addition to heavy snowfalls breaking branches and uprooting Leyland cypress, rapid temperature changes may injury plant tissues. Warm 60˚F days followed by cold fronts lowering the temperatures below freezing can wreak havoc on plant tissues, from dead flower buds to branch and root death. Newly expanded leaves and buds exposed to freezing temperatures may appear wilted and turn black within hours or days after the event. The damage is caused by the formation of ice crystals, which rupture the walls of plant cells. Frozen soil in turn, can lead to desiccation of needled and broad - leafed evergreens. Windy, sunny days can lead to rapid tissue damage as leaves warmed by the sun begin to transpire. The water lost through transpiration cannot be replenished under these circumstances since the roots are frozen solid. This often leads to brown leaf margins or the loss of whole leaves as is commonly the case with camellias, hollies, and junipers. The damage is most evident on the windward side of the plant. Frost heaving (Figure 1) can damage plants as the soil alternates between freezing and thawing. This process can push shallow - rooted plants out of the ground, exposing the roots to wind damage. Trees and shrubs with smooth bark may also experience bark splitting (another name is southwest injury since the damage is usually seen on this side of the tree) because of sudden temperature changes. Sunlight can warm the bark on clear days, which may cause splitting as the temperature rapidly declines after sunset or during cloudy weather. This decline in temperature freezes the water within the tree trunk, causing it to split open. Trees such as cherries, maples, and young fruit trees are most susceptible. Help prevent the split by wrapping the trunk with burlap strips, painting white, or shading the trunk. Some preparations can be undertaken before the cold weather hits. The first step is to grow plants that are cold hardy in our USDA Plant Hardiness zones 6a, 6b, or 7a. Don ’ t fertilize in fall, or prune in late summer (which may lead to new growth that is damaged by frost). Be sure to water evergreen plants during dry fall weather and add a two to four - inch layer of mulch to slow moisture loss and prevent upheaval of newly planted plants. It is best to wait to spring to determine whether or not dead limbs will need to be pruned from trees and shrubs. The spring flush of growth should hide the damaged leaves of evergreens. Patience is the best practice because some damaged plants such as crape myrtles may not leaf out until June after severe winters. If patience is not your virtue, use a pocket knife to scrape back the outer layer of bark to check for green cambium tissue. Seeing green tissue is a good sign that the plant is not totally dead, even though it is not a guarantee. Also, keep an eye on winter - damaged plants if drought conditions are experienced the following summer. Winter damage plus summer drought could lead to the demise of your perennial, shrub, or tree.
For more information contact the Orange Extension Office by phone (540) 672 - 1361 or by email ashappling@vt.edu.
Page 15 | November 2020
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