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Yardsmart: How textile covers protect plants from frost

Maureen Gilmer
Tribune News Service

Whether you’re growing a fall food garden or tending your backyard tropicals, frost changes everything. When it arrives, and how cold those first early mornings are, dictate how long plants will survive outdoors. First frost is a figment of the early morning, and the coldest point is always that hour just before dawn when the freeze comes.

If you study the lay of frost on your yard or property, you’ll find it may not occur uniformly. At night, cold air is like water flowing downhill to pool in low lying areas — a phenomenon called “cold air drainage.” Plants in these spots will experience earlier and more serious frost damage. So even if these low spots are more protected from the elements and warmer in the evening, they are indeed colder in the early morning after warm air rises and is replaced by cold moving downslope during the night hours.

Another place frost may be absent is beneath shrubs and trees because the tree canopy traps and holds warmer air through the dawn hours. Gardeners traditionally move their tender potted plants under trees or patio covers to protect from a light frost. This is the example that inspires specialized textile covers that hold warm air next to the plant all night long.

These modern fabrics share similar characteristics that make them ideal for protecting plants in situ. First the material must be lightweight in order to avoid crushing plants and distorting the foliage. Second they must be porous enough to allow rain to come through and reach the soil. Third they must be dense enough to keep warmth inside unlike sheet plastic that offers no insulation, blocks oxygen transfer, and supports copious condensation on the inside surfaces.

For individual plants or those in pots that need just a few nights’ protection, the best choice for protection is the Planket. In the past I have used bed sheets and other fabrics to protect my plants from brief frost, but these are heavy and difficult to anchor against wind here. Plankets come in many sizes and shapes with handy drawstrings that allow you to toss it over the plant, pull the cord around the bottom and tie it off. You’ll find the Planket sold in most retail home improvement and discount stores, but to learn more and see the range of products check the website at

From comes a similar product called a “frost bag,” which is a large bag made of row cover material that allows water and light to pass through while insulating the plant from frost. Unlike the Planket, which cuts off light to the plant, frost blankets allow up to about 80 percent light transmission so plants stay active in marginally hardy areas. It’s the perfect way to help those late green tomatoes ripen outdoors or protect your larger potted plants such as dwarf citrus.

Row cover fabrics by Agribon are designed for food crops for a low cost mini-greenhouse for anyone who wants to harvest deep into the winter. Row covers are designed for linear applications of crops. Fabric can be laid right on top of low plants or use a series of hoops covered with light weight row cover fabric for taller ones.

Agribon fabrics are rated by the degree of frost protection offered. Agribon AG-19 is for light frost, adding four degrees of protection. Agribon AG-30 is used for medium frost protection up to six degrees. AG 50 is heavy frost protection for freezes down to 24 degrees, and AG 70, the thickest typically used for heavy frost protection.

The ability to protect plants from frost can be the key to growing a winter garden in marginally warm zones. Keep your row cover fabric, frost bags or Plankets on hand so when NOAA predicts the first frosts, lay them out the night before and sleep late in the cold morning knowing nothing outside will be damaged.

Maureen Gilmer is an author, horticulturist and landscape designer. Learn more at