Winter is Tough on Landscape Plants
by Don Janssen, Extension Educator
Cold temperatures, drying sun and wind, road salt, and snow and ice can take a toll on landscape plants. Homeowners can choose plants adapted to the growing conditions in their yards, maintain them properly and even take steps to protect them, but injury may still occur.
Winter injury to tender tissues can occur any time the temperature drops below freezing. Late-season fertilization or mild weather and fall rains that keep woody plants actively growing can dispose them to freezing injury when the temperature drops.
Likewise, unusually warm weather in early spring can cause plants to break dormancy and begin to grow, leaving them susceptible to damage by a spring cold snap.
Plants that become fully dormant in the fall have a built-in ability to withstand temperatures down to a certain level. This is the basis of the U.S. Department of Agriculture's hardiness zones, those maps in the plant catalogs that divide the country into zones where various plants can be expected to survive a normal winter -- or not.
A good strategy when selecting plants is to choose those that are hardy in your zone or the next one north. You may get away with plants from warmer zones, especially if you're blessed with a series of mild winters or you plant in a protected area, but the odds are that you'll lose them to cold injury sooner or later. In evergreen plants, winter injury to foliage is common. Bright sun and drying winter wind remove moisture from foliage that the plant's roots cannot replace from frozen soil. The result is brown foliage in the spring.
Brown needle tips in pines are a common sign of this drying injury. As long as new growth comes in healthy and green, the plant is probably okay, even though the brown needles will stay brown.
Brown evergreens that are producing green growth may not look too good for a while, but they will probably recover. But if needles -- especially new ones -- are falling off in the spring, the plant may have a serious problem. Unlike deciduous trees, which can replace new leaves, evergreens are unlikely to survive being defoliated.
Fall needle drop is normal. Though the plants stay green overall, individual needles don't hang around forever.
Sometimes evergreens appear to have survived being flattened by a heavy load of snow or ice only to die later in the summer. The cause is damage to the plant's vascular system, which carries nutrients produced in the foliage and water taken up by the roots. Severe damage kills the plants by starving the roots.
A common problem of smooth-barked deciduous trees is called frost crack or southwest disease. Both are misnomers -- frost crack doesn't have anything to do with frost, though the bark does split and crack open; and southwest disease isn't a disease. Frost crack occurs when the sun warms the tree's bark, which expands. As night approaches, the outer bark cools and shrinks faster than the inner tissues. Finally the outer bark splits.
The crack poses no particular problem to the tree, but it can open the trunk to infection by disease organisms or attack by insects.
Planting hardy plants, maintaining them with a regular program of watering and fertilizing, pruning to remove poorly placed branches and dead or diseased wood, shading smooth-barked trees on the southwest side, and protecting evergreens against sun, wind and salt spray with burlap or canvas screens, can go a long way toward reducing problems with winter injury.
When winter injury does occur on deciduous plants, don't be too quick to remove them. A plant that doesn't leaf out in the spring isn't necessarily dead -- it may have suffered frost injury to leaf and flower buds. Secondary buds will probably open later.
On the other hand, a thoroughly brown evergreen that looks dead probably is dead, and no amount of waiting and hoping is likely to bring it back to life.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office