This article from the
2000-2001 Winter Newsletter should help Plateau gardeners prepare for the rest of the winter.
Why the Season Is So Hard on Plants
Large temperature changes.
Winter temperatures fluctuate dramatically at high elevations in northern Arizona. During the winter months, the thermometer often varies daily by 30 or more degrees. Daytime temperatures are often in the 40’s and 50’s, while nighttime typically drops well below freezing.
During the winter, the surface cells in plants warm up in above-freezing daytime temperatures. Then
night-time chills plunging significantly below 32 degrees may freeze young tissues. Young tissue is especially prone to damage from alternating freezing and thawing. Sunscald may occur in late winter or early spring when the temperature of bark in direct sunlight may be higher than the surrounding air. After sunset, plummeting temperatures may quickly refreeze the water in the cells, splitting the bark and causing large wounds. During extended periods of intense cold, frost can penetrate into bark and deeper wood cells.
Repeated freezing and thawing of the soil can cause heaving of soil and plant roots. This exposes roots to desiccation and freezing. Once the soil and roots are frozen, the roots can no longer take up more water for the rest of the plant.
Winds. Wind can cause several problems for plants. It can damage young tissue due to its cooling and drying effects. In spring, as the days grow longer and the air begins to warm, plants are induced into higher rates of transpiration, increasing the rate of moisture loss.
However, cool winds and deep mulch may keep the soil from thawing, and the still-frozen roots are unable to replace the moisture lost from the aboveground parts of the plant. The top or all of the shrub or tree may die back, and it may take most of the next growing season for the plant to recover from this “windburn” damage.
Winter Garden Maintenance
So what can you do to minimize some of these winter stresses to your plants?
Mulching. Once the soil has frozen in early winter, mulch around the base of shrubs and trees with an organic material to a depth of 6 to 8 inches. The insulating effect of the mulch will help prevent heaving and keep the plants in a state of dormancy during occasional periods of warmer weather. Snow itself acts as an excellent insulator if it remains for long periods. For groundcover plants, use a material that will allow air circulation and will not mat down. This prevents low-growing perennials from rotting underneath the mulch. Pine needles, wood shavings, and dry oak leaves work well for this purpose. Remember to remove mulches early in the spring to allow the soil to thaw before bud break so that the roots can once again take up water.
Pruning. Snow can present problems, particularly structural damage to shrubs and trees. Large branches with a lot of surface area are the most susceptible to breakage from heavy snow or ice. Inspect your plants for the need for preventive pruning and complete that pruning early in the winter season if possible. Otherwise, try to remove buildup of snow from branches or plants that may break under too heavy a load. If any branches should break, prune off the damaged material as soon as possible to minimize additional stress to the plant.
For other pruning, it is best to wait until late February or March, but before bud break. Pruning before or during the coldest part of the winter may increase the stress to the plant from greater desiccation at the cuts. Pruning in the fall before complete dormancy may stimulate new growth, which would be highly susceptible to freezing and desiccation.
Wind protection. Setting up wind blocks can protect smaller shrubs and trees vulnerable to damage from wind and cold. Techniques include wrapping with burlap, placing cardboard or other stiff material around the plant, and setting up straw bales on the windward side of the plant.
Watering. One other factor to keep in mind throughout the winter is the moisture content of the soil. Even though dormant, plants continue to transpire through the winter, though at a lower rate. If little precipitation occurs, drought stress can kill plants as readily in winter as in summer. At the minimum, your trees and shrubs should receive water about once a month. If there is no snow or rain (and if there is a point when the ground is not frozen) try to give your plants a good soak well into the root zone every month or so during the winter.
