Protecting Plants From A Late Frost

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Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

Have you had your first frost?

I haven’t had a frost at my house, yet. I know that because my Celebrity tomatoes that I planted last spring are still alive and have a few green tomatoes growing on them.

Historically, our first frost in Angelina County is the middle of November. And that date was three weeks ago!

There are two types of frosts that can occur. One is when a cold front sweeps into an area. Called an advective frost, winds are typically gusty, clouds may occur and the cold air layer may reach more than a mile high. One seldom sees the first frost of the season under these conditions.

Our typical first frost is a radiation frost. This occurs under a clear sky and calm winds much like we’ve had this past week. Clear skies and calm winds allow radiant heat from the Earth to rise to the upper layers of the atmosphere and colder air to collect below.

This is also known as an inversion. The atmospheric conditions are inverse or opposite of normal daytime conditions when air temperature decreases with height. In an inversion, cold air collects near the ground while warmer air lies above this trapped cold layer.

On cloudy nights, the cloud cover acts like a blanket on the Earth, trapping radiant heat from the ground. Study the forecast and you’ll note the cloudless nights are typically the colder nights.

The topography of the land and your landscape also affects where frost will occur. On any given property, there may be temperatures on different sides of the house, under protective trees, on south or north facing slopes, or in low-lying spots.

Cool air settles at the bottom of slopes because it is heavier than warm air. Frost pockets will then form in valleys where cool air is trapped. Higher altitudes also have colder temperatures.

This is the concept that can confound me at times. Our house is at the top of a small hill. I know I’ll escape the cooler air that settles to my neighbors around me. Yet, I realize my “hilltop” home isn’t high enough for me to have the cooler air at higher altitudes.

Nevertheless, you are apt to see frost damage at the bottom of slopes and on the (high) hilltops, while the hillsides are frost-free.

But rather than just talking about the weather, there are several things that gardeners can do to minimize the effects of the first radiation frost. Keep a careful eye on the weather forecast too. We know cool, clear nights with low humidity, often following a cold front, are signs of an impending frost.

But what can you do to protect your plants?

Irrigate before the frost. A moist soil can hold four times more heat than a dry soil. It will also conduct heat to the soil surface faster than a dry soil, aiding in frost prevention.

In a study performed years ago, the air temperature above a wet soil was 5 degrees F higher than that above a dry soil and the difference was maintained until 6 a.m. the next morning.

Covering plants can give you 2 to 5 degrees F protection. The covers can be laid right over the crop, or can be supported on stakes. The difference being that protection is less wherever the cover touches the plant. Any material can be used to cover the plants, however woven fabrics are better insulators then plastics or paper.

The best time to apply covers is in the late afternoon after the wind has died down. Remove covers the next morning before the sun hits them.

I will let my tomato plants suffer from the eventual frost. I’ve enjoyed a good run of them. At best, I’ll pick what is left before the frost and see if they are far enough along to ripen in the kitchen.

Next week we’ll talk about those plants who can survive frost and cooler weather.

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