Cold Protection Strategies for Gardens in Southwest Florida

Southwest Florida is a unique area for growing many tropical and subtropical plants, both edibles and ornamentals.  A wide range of plants can thrive in our warm and humid climate.  However, many areas of Lee County experience significant cold weather during January and February that can damage or potentially kill sensitive plants.  There are a number of practical ways to reduce cold damage to our plants and gardens.  Here are several strategies available to gardeners.  

The first simple and effective approach is to select cold tolerant plants and avoid installing sensitive plants in the first place.  Start by getting to know the historical low temperatures in your area or neighborhood.  You can also observe the mature and thriving plants in your area, indicating which plants are growing successfully.  If you don’t see many tropical plants, such as Mango or Jackfruit, that may be a clue that such species could be damaged in your area.  Plant nurseries often provide some guidance to help gardeners make wise choices, and a huge amount of information is available online.  However, nothing compares to first hand information, such as talking to an experienced gardener in your area.  They have likely seen what grows well over the years and whether cold sensitive plants are a viable option or a pipe dream.  

A second strategy for protecting your plants is to modify the microclimate of your property.  This isn’t a fast option, rather it can be accomplished over a number of years.  This can involve planting a windbreak using trees, shrubs, or bamboo to reduce the impact of cold or drying winds. Fast growing trees can also be planted to develop a protective canopy layer within a garden or food forest, which helps to insulate the temperature in your garden on clear, calm nights.  Microclimate is also affected by nearby structures that block wind or radiate heat during the night.  Planting sensitive plants or trees in protected pockets of a property can help improve their chances on a cold night.  Planting near a large body of water can also be helpful, however, small ponds or creeks do not usually have a significant impact on temperature.  Avoid planting in depressions or low areas, as these will collect cold air as it settles in the landscape.  Employing as many of these strategies as possible will have the greatest impact on a cold night.  Be aware, however, that microclimate effects are diminished greatly during windy nights.

Another strategy that can help your plants resist cold is to discourage excessive late fall or winter growth.  Avoid heavy applications of fertilizer after October as late season growth can increase cold sensitivity.  Also avoid any pruning of sensitive plants or fruit trees in the fall, which could promote a flush of tender growth during winter.  A new flush of tender leaves is much more vulnerable to cold than mature growth.  While tropical plants are not capable of going dormant during the winter, we can reduce their sensitivity to cold by not encouraging vigorous growth as we approach the winter months. 

A critical but sometimes neglected strategy is to simply keep up consistent irrigation during the cold season.  The soil should be kept moist, but not waterlogged, which could encourage disease.  Irrigation is especially important in the days running up to a cold weather event.  Dry soil holds much less heat than moist soil, which means that there will be less heat to radiate upward during a cold night from dry soil.  Wet soil is able to capture the heat energy from the sun, which will in turn be given off during a clear and calm night, providing heat to nearby plants.  

Frost blankets are also available and effective as a way to protect sensitive plants.  There are commercially available products used for agriculture and nursery crops and some frost blankets are also available online or at garden centers to homeowners.  These products are most effective for young and small plants that are relatively close to the ground.  The blanket should be pulled over the plants before evening in order to trap the heat energy that was stored in the ground during the day.  This heat will then warm the plants during the night, preventing them from frost or freezing.  The blankets or frost cover should meet the ground on all sides of the plant in order to capture the heat beneath the material.  If heat can easily escape from the cover, the effectiveness of the frost blanket will be greatly reduced.  This strategy works very well for young crops, nursery plants, or small vegetable gardens.  Frost blankets can typically raise the temperature beneath the cover around 5-8 degrees, depending on the quality of the product.  Covering young fruit trees is possible, with extra support and often some creativity, but it is not practical to cover large trees.  For very sensitive or very special trees, a heat lamp or small space heater can be added to provide an internal heat source.

A more radical and intensive strategy for protecting larger or more mature trees is to use overhead water.  This category of freeze or frost protection is more knowledge intensive and the potential for doing harm to the trees is much greater than the previous strategies.  Water must be applied consistently from the point when the air temperature reaches a wet bulb of 32 degrees, and then can only be cut off when the temperature rises to the same wet bulb of 32.  The wet bulb is simply the temperature of a wet or evaporating surface.  This indicates the temperature that the tree will experience when you turn on the water.  Turning on the water too late, after the temperature has descended to a wet bulb of below 32 can result in evaporative cooling and can quickly damage the trees.  Water must be applied in an adequate amount to the surface of the plant to keep the leaf surfaces from freezing.  As water itself freezes, heat is given off and this heat is what protects the plant and prevents it from freezing.  However, as this happens, ice can begin to form on the leaves and branches of the trees building up significant weight on the structure of the trees.  It is not unusual for large limbs and branches to break and crash from trees after many hours of running overhead water during a freeze event.  This method is also water intensive and should be reserved for commercial agriculture and plant nurseries.

Another strategy to protect plants from cold is through the use of various types of heaters.  Propane space heaters, smudge pots, or a barrel fire can provide a lot of heat, as well as smoke which can add a layer of insulation over a property or farm during the night.  The rising heat of one or several fires can help to create a convection effect which helps to mix upper warm air layers with the cold layers closer to the ground.  These methods require a significant investment in the heater itself, but more importantly require a fuel source and care while in use to prevent fires.  Smudge pots were once used extensively in Florida citrus groves, but their use was discontinued because of pollution concerns.  A barrel fire is effective, but must be tended all night in order to protect plants until temperatures rise with the rising sun.  In general, heaters are effective but are costly, require constant management and are resource intensive.

As you consider which of these strategies to employ for your garden, be sure to study and learn more about the unique features of your property and location.  Make a habit of recording minimum temperatures during the colder months of the year, share information with your neighbors and compare notes.  Tap into local resources such as the University of Florida IFAS and FAWN, the Florida Automated Weather Network.  These are good resources for historical and real time weather data for areas throughout the county.