Judging from some of the recent news headlines, one might get the impression that southern Florida is setting records for cold weather. Nothing could be further from the truth. The winter of 2009-2010 was the coldest since 1940, and there was another cold outbreak in December 2010, but since then we have had unusually warm winter seasons.
Reaction to cold nights around the middle of December 2017 and again during the first week of the new year has been a little bit frantic in some quarters, but the truth is that we simply have become spoiled by our abnormally warm recent winters. Low temperatures in southern Florida this winter have come nowhere near record readings. Nevertheless, there has already been enough cold weather in 2017-2018 to cause damage to tropical plants, and additional cold weather is likely to invade our area from time to time over the next couple of months. Accordingly, the staff at Richard Lyons’ Nursery wishes to address the subject of dealing with the effects of cold weather on our tropical plants.
In deciding what to do to protect plants from cold weather, you should first become acquainted with the cold-hardiness of each of your species. 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. That approach avails you of people’s experience locally. But you can also supplement their input by way of an Internet search for 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 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 the humidity is high. (Following passage of the cold front in mid-December 2017, even though lows in urban areas were in the 40s, temperatures in parts of the farming area dropped to 36 degrees, with frost.) It is not uncommon even on a breezy night for winds to cease right 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 in our region, the duration of temperatures under 32 degrees become important for the well-being of those plants which have enough hardiness to tolerate some frost. Consider citrus, for example. The trees themselves can handle considerable cold, but their fruit is less hardy. Cold can be both beneficial and detrimental to fruit. A certain amount of chill 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.
Even if your plant exhibits no outward signs of cold damage, don’t assume that it came through unscathed. Palms in particular may look fine even though they have suffered internal damage to the bud that may lead to the plant’s death months later. To be on the safe side if you own a species known to be cold-sensitive, pour or spray a fungicide (diluted according to label instructions) directly into the bud at the spot where new leaves emerge, and administer a second dose 7-10 days later. Alternatively, you may apply 3% hydrogen peroxide, the strength found in drugstore formulations, into the bud. But avoid the temptation to treat your palm with both types of chemicals; they are not compatible.
Maintain standard watering habits. While periods of rainy weather may relieve you of the need to provide supplemental water, you should stay alert for the sudden onset of dry weather that requires intervention on your part.