By Ann Marie VanDerZanden
Department of Horticulture
Iowa State University
The long, mostly dry Indian Summer is drawing to a close. To our benefit, the many bright sunny days have been beautiful and have allowed the vibrantly colored leaves to linger longer on trees. But since this weather pattern has been associated with limited rainfall, it is even more important this year to spend a little extra time getting your evergreens ready for winter.
Until the ground freezes, evergreens, and even deciduous trees and shrubs, need regular watering to ensure they are fully hydrated going into the dry winter. Plants that go into the winter drought stressed will suffer the most. These plants may not survive winter or if they do survive may have substantial die-back of stems as well as a number of dead or damaged leaves.
All evergreens, and particularly broad leaf evergreens such as boxwood and rhododendron, are susceptible to winter desiccation. Winter desiccation also is referred to as winter burn or winter browning. Desiccation occurs when the evergreen's foliage loses moisture due to the bright winter sun and harsh winter winds. In winter the plants are not able to absorb enough additional moisture from the soil to replace the water that evaporates from the foliage and stems. In addition to making sure the plants are well watered before the onset of winter there are a few other measures homeowners can take to protect the plants.
Anti-transpirants (Wilt-Pruf for example) are available at many garden centers and nurseries and can be sprayed on plants once winter arrives. These products create a thin film over the leaves and stems and reduce the amount of water that can evaporate from these tissues. They often must be reapplied during the winter as they tend to break down over time and lose their effectiveness.
Other protection measures include loosely wrapping the evergreen with burlap, or protecting the windward side of the evergreen with some type of screen (easily made with burlap and stakes). In the future you can choose a site that is protected from winter winds and direct afternoon sun to locate new evergreens you add to your landscape.
In addition to desiccation damage, evergreens such as arborvitae and junipers are susceptible to breakage from snow build-up and ice storms. This time of year you can loosely wrap these plants with rope or twine to keep all of the branches tied together to prevent splits and breakage. During the winter be aware of where these plants are when snow is being removed so snow does not pile up on top of the plant. You also can gently shake the shrubs to remove snow so it does not continue to accumulate as it is falling.
Even if all of your best efforts to protect the evergreens don’t work perfectly, there still may be hope for the plants. In the spring after the plant is actively growing, you can prune out and reshape the plant as necessary. You may need to do some additional pruning over time to reshape the plant and increase your winter protection measures for next year.
Ann Marie VanDerZanden, Horticulture, (515) 294-5075, firstname.lastname@example.org