Part 3: Cold Protection of Landscape Plants


Methods of Protection

Plants in containers can be moved into protective structures where heat can be provided if necessary. Containers that must be left outdoors should be protected by mulches and pushed together before a freeze to reduce heat loss from container sidewalls. Plants may be damaged if crowded together for extended periods.

Heat radiating from soil surfaces warms the air above the soil or is carried away by air currents. Radiant heat from the soil protects low-growing plants on calm cold nights, while tall, open plants receive little benefit. Radiant heat loss is reduced by mulches placed around plants to protect the roots. For perennials, the root system is all that needs to be protected, because some perennials die back naturally in winter while others survive freezes and quickly regenerate new foliage from the roots.

Grafted or budded plants, like some gardenias and many fruit trees, can be protected by insulating the trunks with commercial tree wraps or one- to two-foot mounds of soil. This protects the trunk so that even if the branches freeze, the tree will be able to re-sprout from above the graft. Remove the wrap or mounded soil each spring.

Covers are better at protecting plants from frost than from extreme cold. Covers must extend to the ground to trap radiant heat and may need to be anchored with rocks, bricks, soil, etc., if is windy. Ideally, the covering should not rest on the foliage, as it may be injured by the contact. Some examples of coverings are commercial frost protection fabrics like “frost cloth,” bedsheets, quilts, or black plastic. Gallon milk or water jugs can be used to protect small plants. Simply cut the flat bottom off and place them over the plants. Valuable plant specimens can be protected with temporary greenhouses constructed of wood framing and plastic sheets. The addition of a light bulb or a string of Christmas lights under a cover is a simple method of providing heat to plants in the landscape. Remove plastic covers during a sunny day or provide ventilation of trapped heat.

Turn off in-ground irrigation systems before freezing temperatures occur. Nurseries and strawberry farmers protect crops during a freeze by sprinkling the plants with water. Sprinkling for cold protection helps keep leaf surface temperatures near 32°F (0°C), because sprinkling utilizes latent heat released when water changes from a liquid to a solid state (ice). Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Home irrigation systems are not designed to supply water in quantities ample enough to maintain a film of liquid water on plant surfaces, thus more harm than good usually results. Water soaks the soil resulting in damaged root systems plants break due to ice build up, and water is wasted. Furthermore, water restrictions do not allow this use in much of the state, and fines can be levied.

What to Do After the Freeze

Water Needs

Plant water needs should be checked after a freeze. Plants may have lost substantial moisture during a windy advective freeze. Plants will transpire (lose water vapor) on a sunny day after a freeze, but sometimes their roots are too cold to function normally. Water in the soil of containerized plants can actually freeze and be unavailable to roots. Apply water to allow thawing, rehydration of plants, and dilution of fertilizer salts that might otherwise burn plant roots.

Pruning

Cold injured wood can be identified by lightly scraping the bark with your fingernail and examining the color of the cambium layer (food conducting tissue) just underneath. Green tissue indicates the plant is still alive at that point; black or brown coloration indicates dead or injured tissue.Prune these branches behind the point of discoloration. Branch tips may be damaged while older wood is free of injury. If in doubt, delay pruning until new growth appears to ensure that live wood is not removed. Dead, unsightly leaves may be removed as soon as they turn brown after a freeze. Cold injury may appear as a lack of spring bud break on a portion or all of the plant, or as an overall weak appearance. After a particularly harsh cold event, some plants may be very slow to recover, so some patience is required.

More Information

For additional information on cold protection and coping with cold injury to plants, visit the UF/IFAS EDIS website at http://edis.ifas.ufl.edu.

via ENH1/MG025: Cold Protection of Landscape Plants.

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