Do’s and Don’ts of Fall Maintenance for Woody Ornamentals

At this time of year we in Missouri should keep in mind that two catastrophic freeze events have occurred on the date of November 11. On 11-11-11, many Midwestern hunters froze to death when a giant storm dropped the temperature from 82 degrees to 13 degrees F within 24 hours. Twenty-nine years later, during the Armistice Day blizzard of November 11, 1940, temperatures dropped from near 60 degrees F over most of Missouri to about 10 degrees within about a threehour period and then continued to drop to below zero by the morning of November 13 in some parts of the state. A combination of warm fall temperatures until that point and the precipitous temperature drop caused massive damage to woody plants all over the Midwest, destroying most of the fruit trees in Missouri in a single night. While these are extreme events, they illustrate that woody plants are particularly vulnerable to freezing injury at this time of year. We horticulturists should do whatever is in our power to encourage our woody friends to develop dormancy and the cold hardiness associated with it.

Cultural practices that can help plants get though the winter include irrigation, trunk wrapping and intelligent mulching. It is important that newly planted trees and shrubs, especially evergreens, not be allowed to go into the winter with a dry root zone. Before putting the hoses away, make certain to recharge the soil moisture around these plants so they do not desiccate during exposure to cold, dry winter winds. Wrapping the trunks of newly planted trees is important for two reasons. First, it helps reduce moisture loss from tender bark and, second, it can help prevent winter sun scald. Sun scald occurs on clear, sunny days in late winter when the afternoon sun shines directly on the bark on the southwest side of the tree, warming it, increasing cambial activity and reducing freezing tolerance. Then, when the sun sets, the bark temperature may drop well below freezing within a few hours, resulting in death of cells in the bark. This type of damage provides easy access to the bark by stemboring insects and fungal, decay organisms. Wrapping the trunk with a light-colored material, such as the kraft paper used by landscapers is an excellent practice for preventing trunk scald. Do not, however, wrap a trunk with a dark-colored material such as weed barrier fabric. This increases the temperature of the underlying bark and provides no signi. cant insulation to prevent bark freezing. Mulching is a good practice to help get small plants through the winter. It can keep surface roots above freezing and prevent "heaving" of roots out of the soil with repeated freezing and thawing. But we can help plants develop dormancy sooner in the fall if we delay mulch application until the soil surface begins to freeze. This practice drops the soil temperature sooner, resulting in slowing of growth as we near the mid-November danger zone. Do not apply a mulch layer thicker than about 3 inches, since this may cause the soil to hold too much water during wet periods in winter and spring, leading to root and crown rot.

Practices that should be avoided in late summer and early fall are excessive nitrogen fertilization and severe pruning, both of which stimulate growth and interfere with development of dormancy. While fertilizer applied in October and November may not stimulate late growth, the nitrogen applied will be subject to loss by leaching.

Fall is the worst time of the year to do major pruning, in which more than 20 percent the plant is removed. It is common to see dieback of stem tissues directly below pruning cuts or of new growth that comes in response to late summer or early fall pruning. Even perennials are more likely to survive the winter if the dead tops are not removed. Cutting back to green tissue tends to promote growth and reduces accumulation of dried leaves and snow that may insulate the crown. When in doubt, it is usually best to delay major pruning of woody plants until February or March. In the case of spring- flowering shrubs, pruning after . owering will allow for a good bloom display and provide maximum time for development of . ower buds for the following season.

Chris Starbuck, Woody Ornamental Horticulture, MU, (573) 882-9630