Natural Injuries

Windburn Windburn can result when evergreens are exposed to strong prevailing winds throughout the winter months. The wind dries out the leaf tissue and the dehydrated tissue dies. Windburn causes a brown to black discoloration of the leaves on the windward side of the plant. Very often, leaves further into the plant or on the side opposite the wind show no damage. Broadleaved evergreens are highly suscep­tible to windburn because they have a great amount of leaf surface. Responding to dropping temperatures, many broadleaved evergreens roll their leaves in the winter to reduce the amount of exposed surface area (Figure 11-18).

Damage from temperature extremes Temperature extremes are inju­rious to plants that are at the limit of their hardiness (termed margin­ally hardy). Damage may range from stunting (when all of the previous season’s young growth freezes) to death of the plant if the severely cold weather is prolonged.

After an especially cold winter, certain plants may show no sign of injury except that their spring flower display is absent. This happens

figure и-is. This rhododendron has begun to roll its leaves as a means of reducing the exposed surface area during the winter. (Delmar/Cengage Learning)

if the plant’s flower buds have frozen while the leaf buds have not. The injury is common to forsythia and certain spireas in the northern states.

Unusually warm weather during late winter can also cause plant damage. Fruit trees may be encouraged to bloom prematurely, only to have their flowers killed by a late frost. As a result, the fruit harvest can be greatly reduced or even eliminated. Spring-flowering bulbs can also be disfigured if forced into bloom by warm weather followed by freezing winds and snow.

Sun scald Sun scald is a special type of temperature-related injury. It occurs when extended periods of warm winter sunshine thaw the above-ground portions of a plant, but the period of warmth is too brief to thaw the root system. Above ground, the thawed plant parts require water, which the frozen roots are unable to provide. Consequently, the tissue dries out and a scald results.

Sun scald is especially troublesome on evergreens planted on the south side of a building. It also occurs on newly transplanted young trees in a similar location. The thin young bark scalds easily, and the moisture content of the tissue is already low because of the reduced root system.

Heaving When the ground freezes and thaws repeatedly because of winter temperature fluctuations, turfgrass, bulbs, and other perennials can be subject to heaving and forced up from the soil. When their roots are exposed to the drying winter wind, they are killed.

Damage from ice and snow The sheer weight of snow and ice on plant limbs and twigs can cause breakage and result in permanent destruc­tion of the plant’s natural shape. Evergreens are most easily damaged because they hold heavy snow more readily than leafless deciduous plants (Figures 11-19A and B). Snow or ice falling off a pitched roof can split foundation plants in seconds (Figure 11-20). Plants that freeze before snow settles on them are even more likely to be injured because freezing reduces plant flexibility, causing twigs to snap rather than bend under added weight. Unfortunately, the older and larger a plant becomes, the greater the damage resulting from heavy snowfalls and ice storms. There have been accounts of severe winter storms destroying the entire street trees of cities.

Damage from animals Lacking other food in winter, small animals often feed on the tender twigs and bark of plants, especially shrubs. Bulbs are also susceptible. Entire floral displays can be destroyed by the winter feeding of small rodents. Shrubs can be distorted and stunted by the removal of all young growth. In places where the plant becomes girdled, it is unable to transport nutrients and will eventually die.