If you live in north or central Florida, you may be aware that tropical, subtropical and recently planted, unestablished plants are more susceptible to cold injury from typical freezing temperatures (such temperatures are rare in south Florida). Tropical and summer annuals cannot withstand temperatures below 50°F (10°C), while subtropical plants can harden and adapt to the Florida’s climate. Properly conditioned temperate plants can survive in freezing weather.
Types of Freezes – Radiational and Advective
Radiational freezes or frosts occur on calm, clear nights when heat escapes from the surfaces of plants into the environment. These surfaces can become colder than the air surrounding them because of rapid loss of heat and long-wave radiation. In moist conditions, the surfaces develop ice and frost deposits. While ice deposits don’t form during dry radiational freezes, the freeze does cause plant damage. Plant damage from a radiational freeze can be reduced by decreasing radiant heat loss from plant and soil surfaces.
Advective freezes occur when cold air masses move quickly from the north, causing a sudden temperature drop and windy conditions. Radiant heat loss also occurs during an advective freeze, but conditions are different from those associated with a radiational freeze. Protecting plants during advective freezes is more difficult than protecting them during radiational freezes.
How Cold Weather Affects Landscape Plants
The ability of plants to handle a freeze depends on two things: temperature changes and day lengths before the cold event. A slow decrease in temperature over time allows the plant to adapt to cold temperatures better than it can when temperatures drop suddenly. This is why a sudden temperature decrease in late fall or early winter usually results in more damage than the same low temperature in January or February.
Freeze injury can affect the entire plant or certain plant parts, such as fruits, flowers, buds, leaves, trunk, stems, or roots. While many plant parts are able to tolerate cold, flowers, fruits, and roots, however, don’t have that ability.
One type of winter injury—plant desiccation or drying out—causes leaf tips to turn brown in mild cases and whole leaves to discolor in severe cases.
What to Do before a Freeze
Homeowners can take steps to help their plants adapt to cold temperatures and to protect plants from temperature extremes. These steps range from selecting proper planting sites to changing cultural practices.
- Mix cold tender plants with hardy plants. This will make it less likely the whole landscape will devastated by extremely cold weather.
- Identify microclimates in your landscape when choosing the planting site for cold-sensitive plants. For example, tender plants should not be planted in low areas where cold air settles. Poorly drained areas result in weak, shallow roots that are susceptible to cold injury.
- Add a tree canopy to the landscape to provide shade. Tree canopies trap radiant heat loss from the ground to the atmosphere and thereby raise air temperature beneath them. Plants in shaded areas usually go dormant earlier in the fall and remain dormant later into the spring. Shading also decreases winter desiccation and, in some woody plants, reduces bark splitting.
- Create microclimates out of fences, buildings, and temporary coverings. The height, density and location of a windbreak will affect the level of wind speed reduction at a given site.
- Make sure plants are in good nutritional health. Nourished plants tolerate cold temperatures better and recover from injury faster than plants with poor nutrition.
- Apply fertilizer at the correct rates and times and only when needed. Because fertilizer can pollute water, some municipalities have adopted ordinances that regulate the creation, sale, and application of lawn or landscape fertilizers. Check with your county’s UF/IFAS Extension office for recommendations and rules.
- Water landscape plants before a freeze to protect plants. A well-watered soil will absorb more solar radiation than dry soil, re-radiating heat during the night and slightly raising low night temperatures in plant canopies. (Note that prolonged saturated soil conditions damage the root systems of most plants.)
- Avoid late summer or early fall pruning. This can result in growth flushes more prone to cold injury.
- Routinely check for pests. Healthy plants can survive cold better than plants weakened by disease, insect damage, or nematode damage. Contact your county’s Extension Office for information on pest identification and recommended management.
Methods of Protection
Different plants require different protective measures. Here are ways to provide the proper protection to several common plants:
For Plants in Containers
Transfer them to protective structures where heat is available, if needed. 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 become damaged if crowded together for too long.
For Low-Growing Plants
Place mulches around these plants to protect the roots and prevent loss of radiant heat. Heat radiating from soil surfaces warms the air above the soil or is carried away by air currents, which protects low-growing plants on calm cold nights.
The root system is all that needs to be protected because some perennials die back naturally in winter and others survive freezes and quickly regenerate new foliage from the roots.
For Grafted Plants
Grafted or budded plants, like some gardenias and many fruit trees, can be protected by insulating the trunks with commercial tree wraps or soil mounded one- to two-feet high. 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 can better protect plants from frost than from extreme cold. These tools must extend to the ground to trap radiant heat and may need to be secured with rocks, bricks, soil, etc., if it’s windy. The cover should not rest on foliage, as it may suffer from injuries due to contact. Some examples of coverings include commercial frost protection fabrics such as frost cloth, bed sheets, quilts, or black plastic.
Jugs and Temporary Greenhouses
Gallon milk or water jugs serve as great protective shields to house small plants. Simply cut the flat bottom off and place them over the plants.
Temporary greenhouses made of wood framing and plastic sheets can protect valuable plant specimens.
Alternative Heat Sources
A light bulb or string of Christmas lights under a cover is a simple method of offering heat to plants in the landscape. Remove plastic covers during a sunny day, or provide ventilation of trapped heat.
Switching off in-ground irrigation systems before freezing temperatures occur can protect plants from freeze. Though sprinkling plants with water before a freeze can helps keep leaf surface temperatures near 32°F (0°C), home irrigation systems are not designed to supply water in quantities large enough to maintain a film of liquid water on plant surfaces, and water restrictions prohibit this overuse of water in much of the state.
What to Do after the Freeze
- Check for plant water needs following a freezing period. They may have gone through a great loss of moisture during a windy advective freeze. Plants will lose water vapor on a sunny day after a freeze, but sometimes their roots are too cold to function properly.
- Apply water (if needed) to allow plants to thaw, rehydrate, and dilute fertilizer salts that might otherwise burn plant roots. Water in the soil of containerized plants can actually freeze, which prevents water from reaching roots.
- Inspect wood for cold injury by lightly scraping the bark with your fingernail and examining the color of the cambium layer (food conducting tissue) just underneath. (Green tissue means the plant is still alive at that point. Black or brown coloration signals dead or injured tissue.)
- Prune branches that are at the point of discoloration. Branch tips may be damaged, while older wood may be free of injury. If in doubt, wait to prune until new growth appears to make sure live wood is not removed. Dead, unsightly leaves may be removed as soon as they turn brown after a freeze.
Cold injury may look like a lack of spring bud break on a portion or all of the plant, or as an overall weak appearance. After a harsh cold event, some plants may be very slow to recover, so give plants the recovery time they need to regain their healthy state.
Adapted and excerpted from:
S. P. Brown, D. L. Ingram, and T. H. Yeager, “Cold Protection of Landscape Plants,” UF/IFAS Extension Environmental Horticulture Department (rev. 06/2014)