Many plants don't show injury or damage until a week or more after exposure to near-freezing temperatures. Read on to learn more . . .
Those of us who yearn for the tropics, but live in climates that are not quite truly tropical, are often filled with trepidation when the weather forecasters talk of coming cold or the dreaded cold front. This is especially true for people who live in climates that are mostly warm year round. One of the worst parts of this is thinking that your plants made it through, only to see leaves turning yellow and dropping off a week or more after the cold event took place. My experience with cold events in south Florida has shown me how different plants react. My objective here is to share my experiences with the conditions that are either beneficial or detrimental to a plant facing exposure to near-freezing, as well as some steps you can take to give your plants the best chance of coming through in good shape.
Too Much Warmth is Not Cool!
Perhaps the most important factor affecting the response of a plant to cold events is the weather leading up to the cold event. A series of unseasonably warm days and nights, followed by a sharp and extreme drop in temperatures to near freezing at night is the worst possible scenario. This is because even tropical plants benefit from a process known as hardening off; in this case, I'm referring to a plant being exposed to cooler temperatures over a period of time to help it acclimatize to the lower temperatures. Even hardy plants can be injured if they are not properly hardened off. Warm temperatures over a period of a few weeks will negate any hardening-off that your plants have attained. Therefore, if a cold snap sets in right after, you'll be more likely to see significant damage to your plants, even if the temperatures don't reach freezing. On the other hand, if the weeks prior to a cold event are cool (upper 40s to mid 50s F at night and no more than low 70s F daytime), the hardier plants will come through much better, and even the more tender ones will sustain less damage.
The wind is your friend
Surprisingly, wind is your friend when a cold event is imminent. This is because the wind mixes up the air layers, preventing the colder air from settling down to the ground to cause freezing temperatures at ground level. You might not think so if you, like me, are out in the wee hours spraying your plants with warm water to keep frost off while the wind is hitting you at 15 or 20 miles per hour, but your plants don't feel it the way you do. For them, it is more important that the leaf surface not reach freezing than whether the wind is whipping around. On a still, cold night, frost protection measures must be implemented much earlier because even if your thermometer at eye level reads 38 degrees F, the ground at your feet can be freezing already. Wind delays this from happening, unless the cold event is actually a hard freeze event, which is a horse of another color entirely.
When a cold event happens, even if you don't see any noticeable damage the next day, damage may still have occurred. This is especially true for tropical plants that are not considered hardy. Some immediate signs are water-soaked spots on leaves, or an overall wilted look to the plant when the root zone is clearly moist enough for the plant not to wilt. Most of my plants are aroids, so I have seen a range of responses from this group of plants in the wake of a cold event. More tender philodendrons and alocasias may appear fine the day after, but as days pass by, leaves begin showing yellowed zones (see picture at left), water-soaked areas, and a bending down below their normal position on the plant. One reason why a cold-stressed tropical plant looks wilted is that the cold temperatures depress the plant's ability to absorb water. This is also how temperate plants get scalded in the winter, even though they are hardy to below freezing temperatures. With the roots frozen, the plant cannot absorb water to replenish what is lost on an unseasonably warm air day.
In my experience, the most pronounced cold damage response, besides outright leaf burn, is the tendency of damaged leaf petioles to curve downward away from the growing point. I have referred to this in the past as flattening out. This is because, in extreme cases such as Alocasia robusta, the plant looks as though it has been flattened from the top down. Some plants will recover from this condition to a greater or lesser degree, depending upon the weather in the following weeks after the cold event and the innate hardiness of the plant. In all cases of cold damage, one is well advised to be careful not to overwater the plants as they are much more susceptible to rot when in this weakened condition.
A final note is that the rate at which the ambient temperature rises after the cold event affects how individual plants respond. Too rapid a warmup can lead to more severe damage than a very gradual warmup because the top of the plant begins to become more metabolically active while the roots are still too cold for activity. This leads to the appearance of wilting and dessication damage in otherwise moist plants. A plant whose root zone is kept warm even while the air temperatures dip will be less damaged that one in which the soil temperature is cold while the air temperature gets warm.