By Sarah Browning and John Fech, UNL Extension Horticulture Educators
In fall, warm dry days with cool nights usually lead to great fall leaf coloration, but dry late summer and fall conditions have a downside, too. Extremely dry conditions result in root death, and a corresponding death of branches and foliage. But perhaps more important, even moderately water-stressed plants are more susceptible to attack by secondary pests, such as insects or disease pathogens.
Young or newly planted trees and shrubs are most susceptible to drought injury, because they have not yet established the extensive root system needed to draw moisture from the soil during dry conditions.
And finally, trees and shrubs that are not fully hydrated going into winter are prone to winter desiccation, a common type of winter injury that occurs when the amount of water lost by plants exceeds the amount picked up by the roots.
What is Winter Desiccation?
All trees lose water during normal metabolic processes, even in winter. In summer, when woody plants have a full canopy of foliage, large amounts of water are lost through their leaves. During winter months, photosynthetic processes are slowed, especially for deciduous plants that drop their foliage, but they still lose water through exposed bark, twigs and buds. Evergreen trees and shrubs, as well as broadleaf evergreen plants like holly or mahonia, are very prone to winter desiccation because they lose water at a higher rate through their foliage.
Warm, sunny days or windy conditions during winter increase the amount of water lost. If the soil is frozen or soil moisture is low due to dry winter conditions, plant roots may be unable to pick up enough water to meet its needs. Evergreen needles dry out and die, either partially or completely, but they may hold their green color until warmer temperatures arrive in spring, thus delaying the onset of browning symptoms.
Winter wind accompanying dry periods can accelerate water loss from the needles, resulting in more severe needle damage or death on the side of the tree facing the prevailing wind.
Each spring many homeowners find dead, brown foliage on their evergreen plants, particularly arborvitae and boxwood, but sometimes also pine, spruce, fir, juniper and yew. The extent of the symptoms can vary from brown needle tips on one side of the plant, to one or two branches, to the whole tree. Injury is found on the outer portion of the branches and is often most severe on the side of the tree facing the wind or a source of radiated heat, such as a south or west-facing brick wall or street.
Repairing Summer Drought StressSince Nebraska frequently experiences dry summer, fall and early winter conditions, deep-watering trees in fall can mean the difference between healthy, vigorous trees or stressed, struggling trees next spring.
Begin by applying a 3-6 foot diameter ring of mulch around the base of trees and shrubs, with 3-4 inches of an organic material, like coarse wood chips, to help conserve soil moisture. Don't pile the mulch up against the tree's bark though, keep it at least 3 inches away.
During fall, plants should receive one inch of water per week, either through irrigation or rain, to enable them to fully rehydrate following summer's dry conditions. So if rain doesn't come, plan to water trees. A deep soaking every 2 weeks is adequate for most trees in unirrigated landscapes. Apply water deeply, moistening the soil to a depth of about 12-18 inches, until the ground freezes. Use a long-bladed screwdriver or piece of rebar to check the depth of water penetration. Once you've reach dry soil, it will be much harder to push the probe into the ground.
If using a sprinkler, let it run in a low pattern in one area until the top 12-18 inches of soil is moistened then move it as needed to water the entire area underneath the tree's canopy.Or coil a soaker hose several times around the tree from the trunk to the dripline, and let it run until the soil is moistened.A watering basin 2-3 inches deep and 3-4 feet in diameter, constructed around the base of young trees will hold water until it can percolate into the soil.Or 5-gallon buckets, with holes in the base, can be used to irrigate small trees.
An Added Layer of Protection
Finally, on plants with a history of winter injury, apply an anti-desiccant monthly throughout the winter. These products form a protective layer on leaf and stem surfaces slowing the rate of winter water loss. They can be particularly beneficial for broadleaf evergreen plants such as holly and boxwood, or fall planted trees & shrubs.
One such product called Wilt-Pruf, is a liquid pine oil polymer that forms a clear, flexible coating on plant leaves and stems, but does not interfere with normal plant respiration or photosynthesis. Use the winter application rate and spray plants thoroughly AFTER they have hardened off and become dormant, usually late November or early December. Read and follow all label directions carefully, particularly the re-application recommendations, to avoid plant damage and maintain the product's effectiveness throughout the winter.
Plants that are repeated affected by winter desiccation should be evaluated for underlying problems making them susceptible to winter injury, or relocated to a more protected location.