Institute of Food and Agricultural Sciences
Hendry County Horticulture News
When Jack Frost Comes Nipping at Your Door
As we have seen repeatedly over the past few weeks, winter temperatures in Florida are often low enough to cause cold injury to tropical, subtropical, and occasionally temperate plants not adapted to Florida climatic conditions. Freezing conditions occur annually in north and central Florida, while below freezing temperatures are rare but not unusual here in south Florida.
Freezes can be characterized as radiational or advective. Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of objects into the environment. These surfaces can become colder than the air above them due to this rapid loss of heat or long wave radiation. When the air is moist, a radiant freeze results in deposits of ice or frost on surfaces. Dry radiational freezes leave no ice deposits but can cause freeze damage. Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces.
Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss occurs during an advective freeze, the conditions are quite different from a radiational freeze. Plant protection during advective freezes is more difficult.
The ability of plants to withstand freezing temperatures is affected by temperature fluctuations and day lengths prior to a freeze. A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures. A sudden decrease in temperature in late fall or early winter usually results in more damage than the same low temperature in January of February. Short durations of warmer temperatures in midwinter can deacclimate some plants resulting in bud break or flowering. Deacclimated plants are more prone to freeze injury.
Cold injury can occur to the entire plant or to plant parts such as fruits, flowers, buds, leaves, trunks, stems or roots. Many plant parts can adapt to tolerate cold, but fruits and roots have little ability to acclimate or develop cold tolerance. Cold injury to roots of plants in exposed containers is a common occurrence and usually is not evident until the plant is stressed by higher temperatures.
One type of winter injury is plant desiccation or drying out. This is characterized by marginal or leaf tip burn in mild cases and totally brown leaves in severe cases. Desiccation occurs when dry winds and solar radiation result in the loss of more water from the leaves than can be absorbed and/or transported by a cold root system.
Homeowners can take steps to help acclimate plants to cold temperatures and to protect plants from temperature extremes. These steps range from selection of a proper planting site to alteration of cultural practices.
Plants in containers can be moved into protective structures where heat can be supplied and/or trapped. Containers that must be left outdoors should be protected by mulches and pushed together before a freeze to reduce heat loss from container sidewalls.
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 since the plants die back to the ground annually.
Coverings protect more from frost than from extreme cold. Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury by reducing radiant heat loss from the plant and the ground. Foliage in contact with the cover is often injured because of heat transfer from the foliage to the colder cover. Some examples of coverings are: cloth sheets, quilts or black plastic. It is necessary to remove plastic covers during a sunny day or provide ventilation of trapped solar radiation. A light bulb under a cover is a simple method of providing heat to ornamental plants in the landscape.
Ornamental plants can be protected 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. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Water must be evenly distributed and supplied in ample quantity to maintain a film of liquid water on the foliage surfaces. Over irrigation may water soak the soil resulting in damaged root systems.
Severe pruning should be delayed 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 if a high level
of maintenance is desired.
Florida homeowners can enjoy a vast array of tropical or semitropical plant materials in their landscapes. Tropical and subtropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. A combination of tender and hardy plants should be planted in order to prevent total devastation of the landscape by extremely cold weather.
Good luck and good gardening.
Gene McAvoy is the horticulture agent with the Hendry County Extension Service. Direct your horticulture questions to PO Box 68, LaBelle, FL 33975, e-mail - firstname.lastname@example.org or phone 863-674-4092 or 863-983-1598. You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle. Office hours are from 8:00 - 5:00.
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