Factors Affecting Susceptibility to Cold Injury
When selecting plants, it is important to understand that St. Johns County’s humid subtropical climate has a USDA hardiness zone of 9a, which means the lowest average temperature is between 20°F to 25°F. If you live in other parts of the state, you can research your areas hardiness zone by viewing the USDA Plant Hardiness Zone Map. Look to the plant label when selecting plants to find out if they are suitable for your hardiness zone. If not, you will need to offer protection when temperatures dip. Tropicals, such as bird of paradise and papayas, are not adapted to withstand temperatures below freezing, and many will suffer injury when temps drop below 50 degrees Fahrenheit. Subtropical plants, on the other hand, can acclimate to survive freezing and below freezing temperatures. Exceptions are plants that have been recently planted and are not yet established, which may experience injury in freezing temperatures.
Cold Weather Terminology
Frost occurs when water vapor freezes on a surface, and can happen even when temperatures are above freezing. Freezing is when temperatures dip below 32 degrees Fahrenheit. When temperatures dip below 28 degrees Fahrenheit, we refer to that as a hard freeze. Fortunately, we can use the same techniques to protect our plants during a frost as during a freeze.
Other Factors Affecting Cold Hardiness
Weather patterns and day length can affect plants ability to withstand a freeze. When temperatures gradually decrease over time, the plants ability to acclimate to colder temperatures in increased. The opposite is true when temperatures suddenly take a dip during the fall, which can cause more damage than the same low temp would in mid to late winter. Cold injury can affect the whole plant, or selective parts of the plant (flowers, fruits, buds, leaves, stems, roots, trunk). Flowers, fruits and roots have the least ability to adapt of all plant parts. Desiccation, or drying out, is another common type of winter injury. This happens when the combination of solar radiation and dry winds cause more water to be lost from the leaves than can be absorbed or transported by the cold root system.
Prevent Cold Injury with Preemptive Planting
A cultural practice that can help protect your less cold tolerant plants is to plant them in an area of the landscape that has a warmer microclimate, such as under a tree canopy or close to a protective structure such as a fence or building. Also keep in mind that cold air settles in low areas, making this an unwise place to plant tender plants. Similarly, plants in poorly drained areas are more susceptible to cold injury because the roots are likely to be weak and shallow. However, watering right before a freeze can offer protection, as moist soil will absorb more heat from the sun than dry soil and will re-radiate the heat over night, resulting in a slight elevation in temperature.
Frost Protection Methods
So you have planted slightly outside of your hardiness zone and a freeze is on its way. No need to despair as frost protection methods such as mulching and covering can impart some level of protection. Mulching protects the root system and offers radiant heat to low growing plants. Mulching can make a big difference in whether or not your perennials survive the cold winter because it imparts protection exactly where the plants need it- in the root zone.
Covering plants is an option that offers more protection from frost than from extreme cold. Keep in mind that frost covers need to extend all the way to the ground to trap the radiant heat. Frost cover options include blankets, quilts, black plastic, or “frost cloth” that can be purchased commercially. Smaller plants can be protected with a plastic gallon jug cut to fit over the plant. If leaving on for extended periods of time, remember to remove plastic covers or at least provide ventilation on sunny days to prevent scalding foliage.
If all of this is just too much for you to deal with, just keep in the mind the basic Florida-Friendly Landscaping principle of “right plant, right place” when designing and planting your landscape. Selecting plants that require the same properties your property naturally has to offer (light, soil pH and texture, moisture level, etc.) will minimize the amount of inputs (irrigation , fertilizer, pest control, and time spent coddling your plants) required to maintain a healthy and attractive landscape.