Gardening Topic for November 2002
Winterizing the Landscape

The storms, ice and rapid temperature fluctuations above and below zero characteristic of winter weather take their toll on trees and shrubs across the North. Even for species native to colder regions this is a stressful time. And this is particularly true for the exposed and isolated trees of the residential landscape. Some of this stress is unavoidable. However, there are things that you can do to minimize the damage caused by winter conditions.

Sun Scald: Sun scald is a form of injury to the trunk of many landscape trees. Often called southwest injury, it usually occurs on the southwest side of young trees. On mild winter days, as the sun shines on the tree trunk, cells within the tree break dormancy and become active. When the sun sets and temperatures get colder, the active cells are killed. This injury may appear as sunken and discolored bark. By spring, the bark may crack and fall off in areas along the trunk. Dead tissue is revealed under the cracked bark. The injury is most severe on thin-barked deciduous trees, such as honey locusts, fruit trees, ashes, oaks, maples, lindens, and willows. Young, newly planted trees are also susceptible to sun scald. Trees that have been pruned of their lower branches to allow more sunlight, or transplanted from a shady to a sunny location, are sensitive as well, because the lower trunk is no longer shaded. Older trees are less subject to sun scald because the thicker bark can insulate dormant tissue from the sun's heat, ensuring the tissue will remain dormant and cold hardy. In the case of young trees, the tree owner might consider wrapping the bark as part of the fall maintenance procedure. Remove the wrap in the spring to prevent insect and disease problems.
The impact of sudden early frosts on late growth:  Late season tree growth is vulnerable because it does not have the same time as established growth to prepare for the cold weather. Ice crystals can rupture the cell walls on the new tips of branches leading to die off the following season. I know the temptation is great to get out there while the weather is still mild, but pruning too soon might encourage new growth and increase the risk of frost damage. To avoid this, you should avoid pruning until after the tree has gone into dormancy in the fall. Trees can certainly benefit from proper fall fertilization, but it's important to avoid using fertilizers with high amounts of quick-release nitrogen, which would stimulate growth.

Winter Drought:  At times during the winter, particularly for evergreens, drying out can be a real problem. Winter drought occurs when a tree loses more water than it can absorb from frozen ground, and is especially acute during the early spring when the ground remains frozen while the spring sun begins warming the rest of the tree. Windy conditions can worsen the problem. It's important that trees and shrubs go into the winter well watered. Laying down a thick layer of organic mulch around the base of the tree in late fall can help to slow moisture loss and runoff, while acting as a temperature buffer for the roots. We have been the victims of one drought after another, and this stress alone makes trees and shrubs more vulnerable to winter stress. If I were only allowed one suggestion in this article, making sure your plants are well watered before the ground freezes would be the one I would consider the most important.
Winter Desiccation:  Yew, arborvitae, and hemlock are most susceptible, but winter browning can affect all evergreens. Nearly all broadleaf evergreens are susceptible to winter desiccation, and especially those with the larger leaves, such as rhododendron. New transplants or plants with late season growth are particularly sensitive. There are several ways to minimize winter injury to evergreens. One is proper placement of evergreens in the landscape. Yew, hemlock, and arborvitae should not be planted on south or southwest sides of buildings or in highly exposed (windy, sunny) places. Winter injury can often be prevented by constructing a barrier of burlap or similar material (never use plastic) on the south, southwest, or windward sides of evergreens.  If a plant has previously suffered injury on all sides, surround it with a barrier, but leave the top open to allow for some air and light penetration. Anti-desiccant and anti-transpiration sprays are often recommended. Read the label.

Branch Breakage:  Branches are more vulnerable to breakage during the winter. Heavy snow and ice storms cause damage by bending and breaking branches. Upright evergreens, such as arborvitae and juniper, and multiple leader or clump trees, such as birch, are most subject to snow and ice damage. Evergreens under eaves may benefit from protection with a teepee-shaped wooden frame. This would protect against damage from snow sliding from the roof and water drips that cause ice buildup. The key to minimizing branch breakage lies in good fall tree maintenance, particularly pruning. Proper pruning, to eliminate multiple leaders and weak branch attachments, will reduce snow and ice damage. For trees with large wide-spreading leaders or large multi-stemmed trees, the main branches should be cabled together by a professional arborist. Pruning weak and vulnerable branches and removing one limb of a pair sharing a deep "V" crotch can make the entire tree less susceptible.

Rodents and Deer:  Winter food shortages force rodents and deer to feed on bark, twigs, flower buds and foliage, injuring (and sometimes killing) trees and shrubs. Apart from deer in the more rural areas, the major culprits are mice, voles, and rabbits, all of which chew bark and can girdle trees. Squirrels can also become a problem. To prevent this,  leave a space between the mulch and the trunk of the tree and check frequently. Trees can be protected from rodent damage by placing a cylinder of ¼-inch mesh hardware cloth around the trunk. The cylinder should extend 2 to 3 inches below the ground line for mice and 18 to 24 inches above the anticipated snow line for rabbit protection. The best way to deal with deer is an 8-foot fence. With light feeding, repellents may be useful. The most effective method is to dip heavy rags in the repellent concentrate and place near the feeding areas. But if deer are starving, little can be done to prevent feeding.

Salt damage:  Salt used for deicing walks and roads in winter can cause or aggravate winter injury and dieback. Salt runoff can injure roots and be absorbed by the plant, ultimately damaging the foliage. Salt spray from passing autos can also cause severe foliar or stem injury. To prevent salt damage, do not plant trees and shrubs in highly salted areas. Avoid areas where salty runoff collects or where salt spray is prevalent, or use salt-tolerant species in these areas. Burlap barriers may provide protection to some plants from salt spray. When sidewalks become icy, consider using sand or kitty litter to improve walking conditions.

All things considered, I think plants possess a remarkable ability to withstand severe winter conditions. By taking a few hours to water, mulch, prune and protect where necessary, you'll help ensure that your trees and shrubs come through the winter in the best possible condition.