Weather 101 - Frosts and Freezes in Southern Climes

It's pretty cold down here in Naples, Florida this morning (February 5, 2009). Not record-breaking cold, but cold enough for various freeze and hard freeze warnings across the area. Freezing warnings mean that below freezing temperatures are expected for up to 3 hours; hard freeze warnings mean that temperatures of 28 degrees or lower are expected for 4 or more hours.

Frost/Freeze Climatology

Frosts (deposition of ice crystals on plants even if air temperatures are slightly above freezing) and freezes are not common in south Florida (where I live), extreme south Texas (especially the Lower Rio Grande Valley) and in the Central Valley of California. Central Florida is more prone to such events, but, again, it's not common. So, when unusual cold strikes these rich agricultural regions, it becomes a significant weather event.

California, surprisingly, has extended periods of record-breaking cold in its inland agricultural regions about every 5 years. There were significant freezes in California in 1990, 1998, 2002, and 2007. Central Florida has a slightly lesser frequency (large agricultural losses in 1983, 1996, 1997 and 2001). South Florida and south Texas have the lowest incidence due to their more southern latitudes and proximity to water bodies (either ocean, Gulf of Mexico and/or larger rivers).

Agricultural Aspects

Regardless of location, major frosts and freezes can wreak havoc on agricultural production, our garden plants, electric and natural gas distribution, and in some places, even affect large-scale water distribution and water distribution within our homes.

The longer the cold temperatures persist and/or the more extreme the temperature readings, the more significant the event. When it comes to agriculture, however, even a few hours with below freezing temperatures (depending upon the plant or the crop) can prove lethal or destructive.

In this content piece, I will be focusing more on the agricultural impacts. I will post other pieces that better address the meteorological factors that bring on such events and various societal impacts linked to freezes.

Some plants are better able to withstand colder temperatures. Larger, non-tropical trees, evergreens, and even some perennials (e.g., colder climate plants such as pansy, ornamental cabbage, kale) don't mind winter's chill. But, many plants, especially those acclimated to tropical or semi-tropical zones and many plants linked to vegetable and tropical fruit production are simply too cold-sensitive. On some plants, only the fruits and vegetables die with below freezing temperatures. For others, the whole plant succumbs. Critical temperatures are not uniform across the spectrum of plant varieties.

Taking Action

Regardless, whenever below freezing temperatures are expected, you can look to the National Weather Service and local media meteorologists to tell you about it. In this technological era, it is rare (practically non-existent) for a freeze to sneak up on anyone.

When a frost or freeze threatens, you have to take some action to ensure the well-being of your home garden or your agricultural plantings. Some actions are easy (e.g., bring in sensitive potted plants). I did that yesterday morning, armed with the knowledge that a freeze was likely. You can also cover plants with anything that blankets them from contact with cold air. This is nothing different than we humans do to keep warm. However, for plants, we are trapping ground heat and moisture, not the continuous supply of body heat. For this reason, wind chill is not a factor in determining the risk of frost or freeze.

According to Mary Ann M., a local resident of the Estates area in east Naples, "there are lots of things you can do to protect plants." With more than an acre of land, filled with ornamental and tropical fruit-bearing plants, Mary Ann has had to deal with cold often enough to know her stuff.

Mary Ann and her husband offer the following advice:

...Again, the easiest action is to take in sensitive potted plants (Figure 1).

...Beware of using plastic. Plastic has high heat conductivity, allowing cold to permeate the plastic through contact, killing any plant matter that touches the plastic (Figure 2).

...Beware dark plastic. If you leave it on too long after sunrise, sunlight can overheat the plastic, burning the plants below.

...Instead, cover plants with lightweight blankets or sheets. These will trap heat, without crushing the plants.

...The best protection seems to come from placing boxes over the plants and then covering the boxes with blankets. This allows for a larger trapped air pocket and keeps a large separation between plants and outside air. Several years ago, Mary Ann and her husband fought 22-degree readings this way. Only the plants protected by combined boxes and blankets survived.

...Watering the ground (not the plants) early in the day before an expected freeze will allow for daytime heating; further, wet soil retains heat better than dry soil (this is linked to heat capacity). It also provides a source of evaporation under any protection. As humidity increases under any protective covering, it acts to temper the drop in temperature.

...As a last ditch effort, sprinklers can be used to protect citrus or other sensitive fruits. As water freezes, heat is given off; meanwhile the icy covering separates the colder air temperatures from the surface of the fruit.

...In light wind, clear sky situations, warmer air remains atop a colder layer near the ground (Figure 3). With this inversion layer (the opposite of the normal vertical temperature profile), large fans can be used to mix the air, bringing warmer temperatures down to the ground. Heaters (including smudge pots) can also be used to warm the air and create an insulating layer of cloud-like matter. Recognize however, that the latter adds pollutants to the air.

You can get further information about "Passive Frost Protection of Trees and Vines" thanks to an informative online document posted by The University of California's Cooperative Extension Service (Division of Agriculture and Natural Resources).

We are now halfway through this winter (and hopefully the worst winter has to throw at us), but more cold weather will eventually come our way. So, keep atop weather forecasts, plan to take relatively simple actions and then continue to enjoy your plants and/or fruits and vegetables.