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Extension News: Protecting plants from the cold

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Bethany O’Rear

Regional Extension Agent Bethany O’Rear

Question: We installed a new landscape early last fall. I absolutely love the interest that it has added to our home. How can we protect our investment from cold damage?

Answer: Great question and quite timely, as our weather can go from relatively mild to extremely cold in a matter of days. Most plants should be fine, but you will need to closely watch any marginal plants that are subject to cold damage.

With very cold temperatures, it is impossible for marginally cold tolerant plants to acclimate to these extremes. This is especially true of most sub-tropical plants and half hardy perennials. If you are a gardener who likes to push the hardiness zone to extremes, you will be saddened this spring when those marginal plants disappear from your garden. If you are unsure which zone you garden in, follow this link for the USDA hardiness zone map: www.usna.usda.gov/Hardzone/index.html

The news is better for more cold hardy species of woody plants. In these plants, cooling temperatures initiate the accumulation of sugars, modification of proteins and changes in cell membrane permeability — all of which increase the plant’s cold hardiness. While most plants require both short days and lower temperatures to develop full cold hardiness, other plants harden only in response to low temperatures, regardless of the day length.

For woody landscape plants, low temperature injury, often called freeze damage, can be caused by intra- or extracellular ice formations within the plant. When intracellular ice is formed, crystals originate inside plant cells. This type of ice formation would be extremely rare in Alabama’s hardy plants and it is unlikely to occur even during an unusually cold spell.

The other more likely type of freeze damage occurs when extracellular ice forms during normal cold winter conditions. This means that water moves out of plant cells as temperatures approach 32 degrees to prevent freeze damage and then back into cells for hydration when the temperature rises above freezing. This type of freeze damage is not lethal to most woody plant species that have been properly acclimated and are cold hardy to the zone where you live. Injury can occur, however, if the cells are dehydrated for relatively long periods of time, or subjected to very low temperatures that they cannot tolerate.

For the more cold hardy woody plants, the freezing and subsequent rapid thawing can actually be more damaging than a sustained cold period. It would be better for the plants to thaw slowly to avoid bark splitting. Since you have newly planted shrubs or young trees with exposed trunks, you may consider wrapping them before they start to thaw to prevent this rapid thawing action.

Normally, we don’t have a problem with root damage but a little extra protection may prove useful should we receive prolonged cold temperatures. Applying a layer of mulch, 2 to 3 inches deep, will aid in maintaining a more even soil temperature and retaining soil moisture as well. Plants that benefit from this practice include perennials, rock garden plants, strawberries and other shallow-rooted species.

 Apply bark products, compost, pine needles, straw, hay, or any one of a number of readily available materials from the local garden center. Also, pine boughs or Christmas tree remains can be propped against and over evergreens to help protect against damage from rapid thawing mentioned earlier. Place this cover on the southern exposed side of the plant where the sun strikes the tree causing rapid thawing.

Only time will tell how much damage plants have sustained but keep an eye on marginal woody plants this spring because they may be more susceptible to borer and beetle attacks after winter cold injury. I hope these tips are helpful. Happy gardening!

For more information on this topic or other horticulture related topics, please contact Bethany O’Rear, Regional Extension Agent, at (205) 612-9524 or email bao0004@aces.edu.