Institute of Food and Agricultural Sciences
Hendry County Horticulture News
Cold Weather Can Threaten Landscape Plantings
Our normally balmy weather may lead gardeners, especially newcomers, to discount the risk of cold injury to landscape plantings. Winter temperatures in Hendry County may drop low enough to cause cold injury to tropical, subtropical, and occasionally temperate plants not adapted to Florida climatic conditions. Frost and freezes while uncommon do occur locally and with sufficient regularity that gardeners should be prepared to take preventative measures to avoid severe damage and even loss of valued specimens. Statistics indicate that most of our area will experience temperatures below freezing an average of 25 times over a twenty year period. Historically, frosts have occurred in Hendry County any where between the beginning of December to early March. The greatest risk of freezing temperatures occurs between mid December and mid February.
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. When the air is moist, a radiant freeze results in frost on surfaces. Dry radiational freezes leaves no frost but can cause freeze damage.
Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature. Windy conditions are normal during advective freezes. Due to the rapid drop in temperature and prolonged duration, plant protection during advective freezes is often more difficult.
The ability of plants to withstand freezing temperatures is affected by temperatures and day lengths prior to a freeze. A gradual decrease in temperature over a period of time increases the ability of plants to withstand cold temperatures. Short spells of warmer temperatures in midwinter can de-acclimate some plants making them more prone to freeze injury.
Cold injury can occur to the entire plant or to plant parts. Leaf and stem tissue will not survive ice formation inside the cells (result of rapid freeze) but many plants can adapt to tolerate ice formation between cells.
Another common type of winter injury is 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 inactive root system.
Homeowners can take steps to help acclimate plants to cold temperatures and to protect plants from temperature extremes. These steps range from proper plant selection and selection of a planting site to various cultural practices.
Plants are often planted past their northern limit in Florida. The simplest and most effective method of avoiding plant damage is to plant varieties that are adapted to your area. Information on the cold hardiness of different cultivars is available from a number of sources including gardening books, seed catalogs, your local plant nursery and the Hendry County Extension Office. The US Department of Agriculture has divided the entire country into plant hardiness zones based on minimum average winter temperatures. All of Hendry County falls between zones 9 and 10.
Temperature can fluctuate from one location to another, even within a residential landscape. These microclimates should be considered when choosing the planting site for cold sensitive plants. Fences, buildings, and temporary coverings, as well as adjacent plantings, can be used to protect plants from cold winds.
Plants grown with optimal nutrition will tolerate cold temperatures better and recover from injury faster than plants grown with inadequate nutrition. Late fall fertilization can result in a late flush of growth which is more susceptible to cold injury. Late summer or early fall pruning may also resulting in a late flush of growth
Watering landscape plants before a freeze can help protect plants. A well watered soil will absorb more solar radiation than dry soil and will re-radiate heat during the night. Beware, however, that prolonged saturated soil conditions will damage the root systems of most plants.
Covers that extend to the ground and are not in contact with plant foliage can reduce cold injury by reducing 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.
Ornamental plants can be protected during a freeze by sprinkling the plants with water. 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. Be aware that irrigation for several days may soak the soil resulting in damaged root systems and/or cause plant breakage due to ice build up.
If plants are damaged, 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 desired.
A combination of tender and hardy plants should be planted in order to prevent total devastation of the landscape by extremely cold weather. Tropical and subtropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. 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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