Desiccation
During the winter evergreen plants continue to lose water by transpiration. Water loss is greatest during periods of strong winds and on bright, sunny days. Even though the air temperature is cold the temperature inside the leaf can be warm. The brighter the sun the higher the rate of transpiration. Desiccation occurs when water is leaving the plant foliage faster than the roots can replenish lost moisture. Root absorption is reduced or prevented when the soil is cold or frozen. The foliage of plants, such as camellia and boxwood, may turn yellow or orange due to mild desiccation or excessive sunlight. More severe injury is commonly seen as discolored, burned evergreen needles or leaves. In severe cases the branches can also become dehydrated and die. Damage is normally worst on the side of the plant facing the wind or sun or near a reflective surface (white house, concrete paving, snow cover).

Prevention - Grow plants that are known to be winter hardy in your area. When planting broadleaf evergreens that are known to be easily injured, such as some cultivars of gardenia, azalea, camellia, and daphne, select a location on the north, northeast, or east side of a building or other barrier where they will be protected from prevailing winds and intense winter sun. These exposures will also delay spring growth, thus preventing injury to new growth or flowers from late spring frosts. The worst location would be the south side of the landscape with no shade and exposure to windy conditions. Avoid planting tender plants in low spots that create frost pockets and sites that are likely to experience rapid fluctuations in temperature.

Special precautions can be taken to protect plants during the winter. Antidesiccant compounds are sold in many garden centers. However, research has shown that these compounds degrade rapidly and are of little value to homeowners.

Shade plants from direct winter sun and wind. Plants that freeze slowly and thaw slowly will suffer less damage. Build a frame over the plants, cover with a material, and seal to the ground. Small evergreens can best be protected by using wind breaks to reduce the force of the wind and to shade the plants. Windbreaks can be created by attaching burlap or canvas to a frame on two sides of the plant.

Watering before a severe freeze can reduce desiccation damage. A 3-inch layer of mulch will reduce water loss and help maintain uniform soil moisture and temperatures around roots.

Care after damage - Wait before pruning because even dead-looking plants may still be alive. The extent of winter damage can best be determined after new growth starts in the spring. Damaged plants may be delayed in starting spring growth. Broadleaved evergreens showing leaf damage will usually produce new leaves if branches and vegetative leaf buds have not been too severely injured. Damaged leaves may drop naturally or be removed by hand.

Prune all dead twigs or branches back to within 1/4 inch above a live bud or flush with the nearest live branch. Do not remove branches that reveals a green layer underneath when scraped with a knife. They should eventually develop new growth.

Freezing
Fall growth
- New growth stimulated in early fall by late summer fertilization may not have had time to harden off sufficiently to survive sudden drops to below freezing temperatures. Ice crystals rupture cell walls. This damage will show up as discolored leaves and dead branch tips.

To prevent damage, avoid late summer or early fall fertilization while plants are still actively growing. Avoid late summer pruning which stimulates new, tender growth and reduces the supply of nutrients available to the plant through the winter.

Split bark - A sharp temperature change between day and night can freeze water within branches or even the trunk of a tree, causing it to explode or split open in a symptom called frost cracks. If not severe, these cracks seem to close when warm weather arrives, but the wood fibers within may not grow back together. This is sometimes called southwest injury because it is commonly found on the southwest side of young shade trees where warm afternoon sun creates further extremes in the day and night temperatures.

Spring flowers or foliage - Alternating or unseasonably warm winter temperatures can prematurely stimulate the opening of flowers or leaf buds in the spring which might be killed by freezing night temperatures. Many spring-flowering shrubs (quince, forsythia, camellia) flower over an extended period of time with only a small portion of their flowers open at one time. The open flowers may be killed but the unopened buds normally go unharmed. If the leaf buds are killed, the plant will often be delayed in resuming spring growth but will eventually put out new growth from dormant buds.

Plants in shaded locations are less likely to have begun early season growth and will receive some protection from the tree canopy (reducing radiation heat loss from the soil). Plants in low elevations are more likely to experience cold injury since warm air rises and cold air settles. Often plants with a southern exposure are more subject to premature spring growth than plants with a northern exposure.

Watering plants several hours before a freeze can provide some protection. A well-watered soil will absorb more solar radiation than dry soil and will re-radiate heat during the night.
Some protection can be provided by covering the plants with burlap, sheets, plastic, or blankets. Be sure the covering does not touch the plants --- foliage in contact with the covering can sustain injury from the heat transfer between the colder covering and the foliage. The covering should extend to the ground in order to retain heat lost from foliage and the ground. A light bulb placed under the cover will offer additional protection. While plastic is ideal for trapping moisture and heat radiated from the soil it can create additional problems unless it is removed during the day or covered to prevent temperature buildup. The extra heat at best will speed up growth of buds making them more susceptible to a late frost. In worst cases the tender new foliage can be cooked from the high temperatures that build up under the plastic.

Winter injury may also occur because of desiccation (drying out). Plants need water during the winter. When the soil is frozen, the movement of water into the plant is severely restricted. On a windy, sunny, winter day, transpiration continues, and broadleaved evergreens can become water deficient in a few minutes; the leaves or needles then turn brown.

Wide variations in winter temperatures can cause premature bud break in some plants and consequent bud-freezing damage. If temperatures drop too low during the winter, entire trees of some species are killed by the freezing and splitting of plant cells and tissue.

Winter's low temperatures will have some adverse effect on several types of landscape plants, but permanent damage to the shrubs is unlikely, according to Ross Penhallegon, horticulture agent with the Oregon State University Extension Service.

During prolonged cold snaps, broadleaf evergreens such as rhododendrons, camellias, and evergreen azaleas are likely to develop brown, scorched-looking leaves, especially on the windward side. The brown leaves are caused by the plant's inability during sub-freezing weather to replace
moisture dried out of the leaves by the wind.

Conifer evergreens, including arborvitae and cypress, also will show a browning of the needles.

The brown leaves cannot be revived. However, a mid-spring application of fertilizer to stimulate new growth may help cold-damaged trees and shrubs. Mulching around the base of landscape plants will help prevent the ground from freezing and help the plants absorb water.

Camellias, rhododendrons and other broadleaf evergreens will likely have fewer flower blooms this coming spring if the developing buds freeze. To get an idea of the extent of freezing damage, cut forming buds open. If they are green, the flower is still okay. If they are brown or black, the flower bud has been killed.

Cold damage can occur on plants that are grow out of climate or that are exposed to rapid drops in temperature. Some plant parts are more susceptible than others.Tropical indoor plants are prone to low temperature injury both in the home and while in transport. Cold injury symptoms usually show up as a blackening of the tissues and typically don't appear until a week or more after the injury has occurred. In the fall cold hardiness first occurs in the terminal buds. The last tissue to achieve cold hardiness are the branch crotches. Roots are more sensitive to cold injury than the above ground parts of a plant. This can be a problem for container plants in the winter. In the spring , the buds lose their cold hardiness first. Late frosts often damage the fruit buds of peaches, cherry, apricots, and strawberries.

The damage from cold will kill the least hardy tissue with a recognizable pattern. Frost damage to conifer needles will uniformly kill all needles of a given age from the tip of the needle to a given point.frost cracks will occurs on the main trunk of plants on the southwest side. The bark separates from the wood due to extremes in daily temperature.


Consumer Horticulture | Weather

© Erv Evans, Consumer Horticulturalist
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