Watering Trees and Shrubs

John Wilson

If you had trouble with winterkill or dieback on your trees and shrubs last year, here are some suggestions on how to reduce the problem this year. If you recall, the fall a year ago was much like we are experiencing so far this year. Very little rain which was great for harvest, but not so great for trees, shrubs and perennials in your landscape.

We kind of had the “perfect storm” by having very few storms over winter. Dry soils in the fall followed by little snow cover or moisture melting into the soil caused the dieback that many people experienced last spring.

Well, my crystal ball isn’t good enough to predict what kind of winter we’re going to have, but so far our fall hasn’t put a lot of moisture around the roots of our landscape plants, so it would be a good idea to water young trees, shrubs, and evergreens before the soil freezes. It’s important for trees and shrubs to go through fall and into winter with a moist soil.

Roots do not go dormant as quick as stems and branches. While the tops of plants go dormant or stop growing sometime during fall, roots continue to grow throughout the fall and even into December if soil temperatures allow.

To encourage fall root growth, provide adequate moisture up until the soil freezes. Check the soil around your trees. If the top few inches are dry, moisture is needed. For trees and shrubs, moisten the soil to a depth of eight to 12 inches while taking care not to overwater.

Keep in mind roots are less cold hardy than stems. Roots surrounded by moist soil are less likely to suffer cold temperature injury because moist soil holds more heat than dry soil. Frost penetration is deeper and soil temperatures are colder in sandy or dry soils.

With newly planted trees, cracks in the backfill soil can allow cold air to penetrate to roots, reducing fall root growth and killing new roots. Check for soil cracks and fill these with soil. Providing adequate moisture will prevent soil cracks from forming.

When twigs and stems die in a tree or shrub, we are aware it’s happening. When roots die, we cannot see the dead roots and are not aware roots are dieing. This could be one explanation why one tree establishes quickly while another is slow to establish or dies.

Plants going into winter with adequate fall moisture are also less likely to suffer damage from winter drying. Plant tissue, particularly the green leaves of evergreens, can lose moisture during winter. Most moisture is lost on warm, sunny, and windy winter days.

Moisture lost from plants during winter cannot be replaced by the roots, either because the soil is frozen or because roots do not function at soil temperatures below 40 degrees Fahrenheit. This is why fall moisture, either from rain or irrigation, is important.

When woody plants go into winter water stressed, their tissue can be killed by winter drying. This is easy to see on evergreens whose needle tips, or entire branches turn brown in late spring. Some evergreens survive and new growth covers up the damaged growth. Other evergreens are killed by winter dessication.

The leaf and flower buds on deciduous plants, those that drop their leaves, as well as small twigs may be killed by winter dessication. This can result in sparse flowering or leafing. Plants may be forced to produce secondary buds which uses stored food within the plant and may lead to stress.

Keep the soil of trees and shrubs moist up until the soil freezes. Mulch trees and shrubs with a two to four inch layer of wood chip mulch to help conserve soil moisture. Keep the mulch one foot away from the trunk to avoid voles making a home in the mulch and gnawing on the trunk. It’s also important mulch layers are not too deep. Roots that grow into the mulch will be killed by cold winter temperatures, further stressing a tree.