AFFECTS OF WINTER ON THE LANDSCAPE
04-08-2000
As the weather warms and spring moves into full swing many gardeners will be attempting to forecast the amount of injury they will have in their gardens. In diagnosing any plant injury, there are two areas to examine -- symptoms and signs.
A symptom is a plant's response to injury. Symptoms of winter injury to plants and foliage include reddening, yellowing, browning or blackening, drooping, crisping or curling to name a few. On the other hand, a sign is the actual presence of the causal agent, such as an insect or disease organism. The signs are the evidence. Determining winter injury can be difficult because there will be no signs to match the symptoms.
Low-temperature injury can be manifested by several symptoms. First, extreme cold will cause the leaves, twigs, stems and even roots to freeze. It has been demonstrated that roots do not have basic genetic hardiness to low temperatures. Essentially, this type of cold causes a disruption and breaking down of the cell contents and tissues. Damage can also occur to normally hardy plants that have not hardened off for one reason or another.
Symptoms of low temperature injury are severe wilting, complete browning of the leaves and stems, and general collapse of above-ground plant parts. Flower buds on rhododendron may be killed by these conditions.
Some winter injury is actually drought injury. On sunny days with drying winds, water is lost from the leaf and stem surface. Because the soil is frozen, the plant cannot replace the lost water. Soil around shallow-rooted plants may also dry out which causes an even greater moisture deficiency in both the roots and top of the plant.
Under normal conditions plants gain hardiness by a process called acclimatization or hardening in autumn. This is caused by environmental signals such as gradually cooling temperatures and/or decreasing day length. Maximum hardiness is achieved in mid-winter and the plant gradually loses hardiness from mid-winter to spring. An unusual cold snap in the fall, before the plant is fully hardy, may injure plant tissue, as well as late spring freeze after the plant has deacclimatized.
This dehardening process is generally considered to be a slow process that takes place over a number of weeks in later winter and early spring. However, under certain conditions is may occur more rapidly as in the case of extended periods of above average warm temperatures. The same situation can occur on localized portions of the plant when bright sunshine heats up dark plant parts to well above the ambient temperature.
When the solar radiation is removed, either by clouds or sunset, the elevated temperature in the leaf will drop rapidly. If the ambient temperature is below freezing, the plant may not have the ability to reacclimatize itself to its former degree of hardiness, and death of the plant tissue results.
In general, the most damage will be:
• on tissue farthest away from the veins, such as leaf margins, tips and tops of the plants.
• on the side of the plant most exposed to sun or air flow.
• on the leaf tissue that is perpendicular to the sun; sometimes the outline of an overhanging leaf will leave a green "shadow" on the sunburned leaf below.
Shrubs and trees growing in containers above ground level are susceptible to root damage. This type of damage is often not noticed until spring, when little or no new growth takes place.
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**Dr. Robert Nuss is a horticulturist at Penn State. He coordinates all extension horticulture programs. He has bachelor’s, master’s, and doctoral degrees in ornamental horticulture and has been on the Penn State faculty since 1966.