Winter Challenge: Keeping Tropical and Sub-Tropical Landscapes Alive in Cold Weather

The trick to keeping tender plants safe from cold weather is knowing the limitation of your plants and your options to protect them from cold weather.

Freezing Limits

Most plants grown in Pinellas County require temperatures below 28° for several hours to be damaged. Why? It is the freezing of water in the plant cells that kill plants – not the actual cold weather. The water in the plant cells freezes at 28° - not 32° - because the water in plant cells contains sugars and salts that act like anti-freeze.

The exception is frost. Frost, by a process called sublimation, can quick freeze leaves and stems covered with frost – at sunrise and not before – even when the actual air temperature is above 32°. How? At daybreak, the sun’s first rays of sunlight strike the frosted surface causing the thin layer of ice to go from solid to gas very quickly sucking all the heat from the plant. The leaf temperatures drop from 32° (the temperature of the frost) to 28° - and freezing the water in the plant cells.

So, the limits to monitor are:

a) Temperatures below 28° for 2 or more hours or

b) Frost warnings

Options for Preventing Cold Damage

Water

The primary protection against cold damage is your irrigation system. Irrigation water is above 32° - otherwise it would be ice. Normally well water is above 60°. Irrigating a landscape with 60° water can dramatically warm a landscape and prevent the 28° air temperatures from damaging plants.

But what of the ice that could form? A layer of ice on leaves and stems actually works like a wet suit works for divers. A thin layer of warm water remains between the leaf surface and the ice coating the leaf. The ice coating acts like insulation protecting the leaf from the colder air. The bottom line is the leaf temperature covered with ice will be no less than 32° - not below 28°. Also the risk of the ice becoming so heavy as to break limbs is low – remember the irrigation water is warm and will melt the ice not add more ice.

Optimum times to water are during the hours when temperatures are below 28°. For protection from frost, simply water your landscape just before sunrise to remove the frost from the landscape.

Covering Plants

A second option is covering your plants. This is very effective but not as practical as using your irrigation system. Covering works by trapping the heat from the soil and plant in the covering keeping temperatures above 28°. You must use a covering that “breathes” and never use plastic sheeting. For best effect the covering should act like a tent over the plant extending beyond the trunk and sealed at the soil surface. The down side of covering is that it is very labor intensive to install and remove, requires large fabric blankets and must be secure enough not to be removed by strong winds.