Welcome back to Zone 5

Most Missouri gardeners are aware that the majority of our state is within USDA Plant Hardiness Zone 5. This means that our long-term average minimum winter temperature falls between -10 and -20 degrees F. Following several milder-than-average winters, the first full winter of the new millennium was more normal. However, despite having the second coldest December since 1895, the minimum temperature we experienced in Boone County was -5 degrees (Zone 6). So, it appears that we still have some dues to pay to average out our last few winters.

Trees and shrubs commonly show various symptoms of injury after a "normal" Missouri winter. These may include leaf browning of evergreens, shoot dieback, dead buds or cambium, and trunk cracking. Causes of winter injury include desiccation of leaves and stems by cold, dry winter winds and freezing of non-hardened plant tissues during an early winter cold snap or following a warm spell in midwinter. While it is often difficult to determine with certainty which wintry conditions caused the damage, the symptoms usually provide clues.

Click on photo to see larger size Photo credit: Chris Starbuck Winter desiccation injury to pine needles is often worst on the windward (northwest) side of the plant.

Click on photo to see larger size Photo credit: Chris Starbuck Often tips of needles turn brown with bases remaining green.

The main type of injury evident after the winter of 2000-2001 is leaf/needle browning of evergreens. Some evergreen Euonymus shrubs suffered sufficient desiccation injury to foliage to cause complete defoliation. Southern magnolias and hollies in unprotected locations have more leaf browning than they have had for the past 5 years. Many pines, junipers and yews exhibit yellowing or browning of needles. Much of this injury probably happened in December and early January when cold winds removed excessive moisture from the foliage. Through a phenomenon called sublimation, water can be lost from frozen leaf tissue by changing from the frozen to the vapor phase without going though the liquid phase. Or, browning can occur when non-frozen evergreen foliage loses water to the atmosphere while the stems and roots are frozen and unable to transport water. Often the injury is most severe on the windward side of the plant and wind protection reduces desiccation injury.

Click on photo to see larger size Photo credit: Chris Starbuck Winter desiccation injury to late-season growth of yew.

Click on photo to see larger size Photo credit: Chris Starbuck Close-up of winter injury to yew foliage.

In some cases injury to evergreens such as yews and boxwoods is primarily on the shoot tips. The usual explanation for this pattern is that new growth following late summer pruning did not have time to develop cold tolerance before the onset of cold temperatures. In cases where entire plants show browning, the injury may be due to exposure of newly planted shrubs to winter desiccation before they were able to develop an extensive root system. In other cases, browning may be observed on plants near streets and sidewalks, indicating salt spray to the foliage or root absorption of salty runoff.

In general, the winter injury evident now looks much worse than it is. As new growth resumes, those leafless Euonymus shrubs will quickly regenerate leaves and the brown needles on pines, yews, and junipers will become less obvious. Use a little touch-up pruning after growth begins to cut back dead branch tips to the point where new shoots are emerging. By midsummer, our first real winter of the millennium will be but a faint memory.

Chris Starbuck, Woody Ornamentals, UMC