Thanks to cable television and the Internet, we now have unprecedented exposure to data on the climate. Surely we’re living in the Golden Age of forecasting! Unfortunately, the professionals aren’t always in agreement on long-term projections. This winter, for the period December through February, one source predicted below-average temperatures, while another foresaw above-normal conditions. In light of that yawning gap in prognostication, the prudent homeowner should heed the advice of wise old growers who caution that, regardless of what sort of weather has been occurring, it takes just one night of frigid temperatures to seriously affect tropical plants.
What, then, should you do to protect your plants from cold weather? First, acquaint yourself with the cold-hardiness of each of your plants. If you didn’t ask the seller at the time you bought the plant, then talk to neighbors or friends who have grown the species that you have. This approach is probably the best, since it avails you of people’s experience locally. But you can also supplement that information by way of an Internet search entering the plant name along with a term such as ‘cold-hardiness.’ The steps you take to learn the cold tolerance of each of your species will allow you to rank the relative sensitivity of your plants and help you avoid wasting time on those that don’t need special care.
Next, pay attention to the details of the forecast. Frost is a major concern during periods of frigid weather, because it can cause the fluids within plant cells to expand and rupture cell walls. A breezy night can be protective, because it prevents the formation of frost on leaf surfaces. However, at some level of intensity, winds can themselves cause damage; accompanied by low humidity, they can desiccate leaves. On the other hand, if winds are expected to be calm, it is possible for frost to form at a temperature above 32 degrees, particularly if humidity is high. And it is not uncommon even on a breezy night for winds to cease just before dawn, and that can be devastating, because just a few minutes of calm can allow the formation of damaging frost.
On the rare nights when a freeze is expected, the duration of temperatures under 32 degrees becomes important for the well-being of those plants which have enough hardiness to tolerate some frost. Consider citrus, for example. The trees themselves can tolerate considerable cold, but their fruit is less hardy. Cold can be both beneficial and detrimental for fruit. A certain amount will cause it to sweeten, but if subfreezing temperatures persist for more than six hours, fruit cells begin to sustain damage.
Once you know the relative cold-hardiness of the plants in your possession, you can take steps to prevent, or at least minimize, damage. Obviously, you will have more choices at your disposal if you are dealing with containerized plants, at least those small enough to be moved. An easy and effective tactic is to relocate them beneath the cover of a large, dense-canopied tree. The tree’s crown keeps heat that is stored in the soil from radiating away as quickly as it would from open locations. Just a few degrees’ worth of extra warmth may be enough to protect your plant. If your containerized plants are too heavy to move, you may nevertheless possess a slight advantage if they are on higher ground on your property, since cold air drains to the lowest spots as it invades an area. In pancake-flat southern Florida, ‘higher ground’ is a fairly subtle concept, so even if you have placed your large containers in sites that are only a foot or two above the surrounding soil, that can help them escape the worst damage.
You may also put a cover over a plant, but be aware of the limitations. A cloth cover can help a bit, but it permits heat loss through the fabric. A plastic cover can be more effective, but you must avoid allowing it to come into contact with plant surfaces, since the plastic will act as a conductor of cold into the plant.
If sub-freezing temperatures are predicted and your containerized plants are too numerous to move, you can employ the technique used by vegetable farmers: Icing. A layer of ice applied by a sprinkler can protect plant surfaces from the worst effects of cold. However, if you opt to go that route, you must be sure not to turn off the water too early. The best policy is to continue watering until the sun is high enough in the sky to melt the ice from plant surfaces.
Finally, you should give some consideration to what remedies to undertake following a damaging cold event. In most instances, the best approach is to assess the damage, but do nothing initially. In particular, don’t prune dead leaves from shrubs or trees; they tend to insulate the plant from further problems in any subsequent cold weather outbreaks. Wait until the chance of frost for the season is safely past. By that time, the plant may generate new growth that provides you the perfect natural indicator of where to prune away dead material.
But note that with respect to palms, there may not be reliable signs of serious cold damage. Within any palm stem or trunk there is only one growing point, known as the terminal bud. Sometimes cold can affect leaves insignificantly, but cause severe damage to the bud that will be manifested only months later following its slow disintegration inside the palm. If you are comfortable making chemical applications, you may wish to apply one of the fungicides that helps prevent budrot. If you prefer not to use agricultural chemicals, you may wish to spray 3% Hydrogen peroxide (H2O2), the strength found in drugstore formulations, to trickle into the buds at the place where new leaves emerge from the palm. However, do not use peroxide and a fungicide in any combination.
For information which may be useful to you in evaluating cold temperature threats, you should peruse the site http://fawn.ifas.ufl.edu/.