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"Protecting Plants from Frosts and Freezes"

By Skip Richter, Travis County Extension Horticulturist

Frosts and freezes mark the transition of our gardening seasons here in central Texas. Each year as cold weather threatens, residents go to great (and often strange) lengths to safeguard their plants from cold damage. Some techniques work well, and others don't. Judging from the calls we receive at the Extension Office, there is some confusion when it comes to how to protect plants from cold injury.

This is about the time when we normally experience our first freeze of the fall/winter season. We had a light frost a while back, but not enough to really shut things down in the garden and landscape. In order for plants to develop maximum hardiness they need the weather to become progressively colder over time. Warm days followed by a really cold snap are a recipe for serious plant injury.

Here in Texas we live between the northern zones where there is a REAL winter season each year and the sub tropics where freezing weather rarely if ever arrives. As a result, our landscapes include many plants that are marginally hardy and need some help to make it through an unusually cold winter.

Plants in containers are especially susceptible. Plant roots have little ability to acclimate or develop cold tolerance. While the ground stays well above freezing on a bitter cold night, the soil in a container gets almost as cold as the air temperature. Even though container plant's above ground parts may survive a cold snap the roots may suffer injury. Such injury is often not evident until several months later when warmer temperatures begin to place increased demands on the plant.

To protect container plants, group them closely together in a protected location up against the home. If very cold temperatures are forecast, cover them with a blanket for the night.

The best way to protect in-ground plants from the cold is to cover them. There are some thick types of plant cover fabric available in local garden centers and by mail order that can provide a few degrees of protection on a cold night. Blankets also work well but are heavier and may require some supports to avoid crushing tender bedding plants, especially when weighted down by rain.

A sheet of plastic placed on top of a blanket can help hold the warm air underneath.   However, plastic can burn any leaves it touches on a cold night. By placing it over a cloth cover, this is avoided. If more than one night of protection is needed, remove the covers during the day to allow the sun to warm the soil and then cover them again late in the day.

Blankets keep us warm because they help contain the heat that our bodies produce. Plants do not produce heat for the cover to hold in. The heat we are trying to contain is in the soil. Therefore the covers should go over the plants and to the ground, rather than be wrapped around the plant and tied around the trunk. Those "landscape lollipops" don't get much, if any, protection.

Two other handy items are a mechanic's light or string of outdoor Christmas lights. These can be placed under the covers to give added heat. Just take the obvious precautions to avoid fire hazards and electric shorts. Also take care not to allow a hot light bulb to contact and damage plant tissues such as the trunk or branches. Use lights beneath a cover to protect valuable but marginally hardy plants like a Satsuma orange tree or a kumquat bush.   They can also make the difference for an in-ground bougainvillea on a really cold night.

When a freeze is forecast, give plants a good watering a day or so in advance. Drought stressed plants are more susceptible to cold injury. The moist soil is also a good "heat sink", absorbing heat during the day and radiating it out slowly on a cold night. Combined with a cover it can make a small but important difference.

Of course we must take care not to overwater, creating a waterlogged soil condition. Soil dries out much more slowly in winter. Soggy soil excludes oxygen from the roots, often resulting in root loss and attack by root rotting fungi.

Sprinkling the foliage and branches of plants prior to a freeze does NOT help protect them, contrary to some folk's opinions. In fact it can do more harm than good.

Finally, use leaves to mulch perennial plants. A thick blanket of leaves can help protect marginal perennials such as ginger, esperanza, and firebush.