By Leonard Phillips, ASLA Emeritus
Trees take advantage of the fall and early winter to build reserves for supporting life functions during winter dormancy. The following are some keys to protecting trees against winter damage:
Salt spray can cause desiccation because the high concentration of salt draws water out of the root and into the soil. This causes a drought condition to occur in the tree. In desert regions, the lack of rainfall and evaporation in the soil will result in an accumulation of salt in the soil. Many trees can be disfigured and killed by sodium chloride. The worst damage occurs to sensitive species planted near heavily salted roads, especially when they lie downhill or have poor drainage. Winter salt damage can be recognized by looking for “witch’s brooms” (cluster of twigs growing out of branch ends) on deciduous trees or yellow tips on evergreen needles. In early summer, look for marginal leaf scorch on deciduous trees. Look for yellow, brown or fallen needles on evergreens especially on the side toward the road.
Fortunately, many trees will tolerate salt excesses. Well-drained soil is an important factor in reducing salt damage. Flooding the root zone with water should reverse the problem. Melting snow and spring rains will usually flush the salt lower into the soil profile. Reduce salt application rates by lowering the throwing distance from the salting truck and making salt applications before the roads freeze. Use less harmful products and mix in inert materials like sand. Raise the planting site, or shelter the trees with a barrier so salt is easily leached away from trees. Sometimes incorporating gypsum or a similar commercial product into the soil before winter begins reduces salt damage.
Insects and disease
Because plant disease and insect pests tend to linger from one season to the next, an important first step toward preparation for the coming growing season is the application of a dormant-season spray. This activity should only be done if the situation warrants treatment. Apply dormant sprays or horticultural oil on trees in the early spring when the temperature is above 40 degrees Fahrenheit (4 degrees Celsius). Evergreens should be sprayed before winter to kill as many insects and eggs as possible. Before spraying, make sure that plants are truly dormant to prevent devastating results.
Rodents and deer can all cause severe damage to plants in the winter. These animals feed on the tender twigs, bark and foliage during the winter. They can girdle trees and shrubs and eat shrubs to the ground line. Trees can be protected from rodent damage by placing a cylinder of 1/4-inch mesh hardware cloth around the trunk. The cylinder should extend 2 to 3 inches below the ground line for mice protection and 18 to 24 inches above the anticipated snow line for rabbit protection. Plastic tree guards and chicken wire are also effective. Deer feed on branches of small trees and shrubs. Deer can cause significant injury and breakage by rubbing their antlers on trees during the fall. Deer can also be successfully excluded with fencing. To be effective, fences must be high and constructed properly. If deer are starving, there is little that will prevent feeding. Voles or meadow mice whose natural habitat is grassy meadows incur most of the rodent damage. When trees are planted in a turf situation, a habitat for voles is provided. By properly designing a landscape to avoid planting trees with turf, many of these rodent problems are eliminated.
For many trees, repellents may be the best solution. The most effective repellents for rodents are those containing thiram, a common fungicide sprayed on trees and shrubs. Repeat applications are necessary after heavy precipitation. Commercial baits containing poisoned grain may be hazardous to humans, pets and beneficial wildlife. Shelter or containerize baits so that they stay dry and are accessible only to targeted pests. Trapping and shooting, where legal, will also control wildlife.
Winter discoloration of evergreens
Browning or bleaching of evergreen foliage occurs because:
* Winter sun and wind cause excessive transpiration while the roots are in frozen soil.
* Bright sunny days cause warming of tissue; that in turn initiates cellular activity. Then, when the temperature drops the foliage is injured or killed.
* During bright, cold winter days, chlorophyll in the foliage is destroyed and is not resynthesized when temperatures are below 28 degrees Fahrenheit (–2 degrees Celsius). This results in a bleaching of the foliage.
* Cold temperatures persist before plants have hardened off completely in the fall, or late in the spring, after new growth has occurred, and it can result in injury or death of this tissue.
* Foliar damage normally occurs on the south, southwest, and windward sides of the plant, but, in severe cases, the whole plant may be affected. Yew, arborvitae, and hemlock are most susceptible, but winter browning can affect all evergreens. New transplants or plants with succulent, late-season growth are particularly sensitive.
* Minimize winter injury by proper placement of evergreens in the landscape by not planting on south sides of buildings or in windy, sunny places. Also prop pine boughs against evergreens to protect them from wind and sun.
Winter injury can be prevented by constructing a barrier of burlap on the windward sides or surrounding the plant, but leaving the top open to allow for air and light penetration. Never stress plants by under-watering or over-watering. Decrease watering slightly in September to encourage hardening off, and then water thoroughly in October until freeze-up. Anti-desiccant and anti-transpirant sprays are often recommended, but recent studies have shown them to be ineffective.
If an evergreen has suffered winter injury, wait until mid-spring before pruning. Buds are more cold-hardy than foliage and will often grow and fill in areas where brown foliage was removed. Prune dead branches back to living tissue. Fertilize injured plants in early spring and water them well throughout the season.
Leonard Phillips can be reached via e-mail at firstname.lastname@example.org
* Cornell Fact Sheets, “Tree Root Damage”, http://www.cce.cornell.edu/monroe/cfep/factsheets 2007
* University of Minnesota Extension Service, “Protecting Trees & Shrubs Against Winter Damage”, Advocate, Winter 2004