How to cover plants for frost, and when to take the covers off

How a frost cover actually works

A cover does not make heat. It holds in the warmth the soil gives off at night. Under a clear sky that heat rises and escapes, and a leaf can cool to freezing while the air around it is still slightly above. Iowa State University Extension explains that coverings trap the radiant heat from the ground, which keeps frost from forming on the leaves and lowers the risk of the plant freezing.

That is why timing and shape matter more than thickness alone. In an Iowa State Extension article, horticulturist Sherry Rindels writes that covering plants the night before with a sheet, blanket or tarp traps the warmth from the soil over the plants and will usually protect them into the upper 20s. A cover that stops well short of the ground traps less of that warmth.

Covers have a limit. Iowa State's current guidance is that once temperatures drop below about 28°F, covering is typically no longer effective, and that damage can still occur above 28°F if below-freezing temperatures last five hours or more.

What to use as a frost cover

Most of what works is already in the house. Iowa State lists sheets, blankets, towels, tarps, frost fabric and row cover material. Oregon State University Extension adds burlap and cardboard containers for individual plants. Plastic is the weak option: Rindels notes that plastic used as a covering usually doesn't work as well as fabric.

Garden row cover is a white, lightweight fabric of spun-bonded polyester or polypropylene that lets air, light and water through. The University of Maryland Extension sorts it by weight: lightweight grade (about 0.45 ounce per square yard) gives roughly 2°F of frost protection, medium weight (0.5 to 1.0 ounce) gives 4 to 6°F, and heavyweight (1.5 to 2.2 ounces) gives up to 8°F. Maryland says it can be cut to size and reused for two to three years.

For a single cold night, an old bedsheet over a few stakes does the job. For repeated use, Maryland notes that medium-weight row cover is useful for spring and fall crops and recommends a support frame for medium and heavyweight grades.

How to put covers on, step by step

1. Check the forecast in the afternoon. Iowa State advises acting once forecast lows dip into the mid-30s °F.
2. Set up supports before you need them. Iowa State suggests stakes, posts, PVC pipe, patio furniture, sawhorses or wire loops to hold the covering up so it does not touch the foliage. Maryland describes simple low tunnels 15 to 30 inches high with hoops spaced 2 to 4 feet apart.
3. Drape the cover all the way to the soil and weigh down the edges. The University of Maryland Extension anchors row cover with sod pins, boards, bricks, sandbags, rocks or soil along the sides and at each end.
4. Cover in the evening, before the temperature falls. Rindels recommends covering the night before, and Iowa State advises closing cold frames and tunnels before temperatures drop below freezing; the same timing suits a sheet.

Pots, containers and hanging baskets

Containers are easier to save than beds. Iowa State calls bringing plants indoors the easiest way to protect them, and suggests moving potted tropical plants and annuals into a heated room, a porch or a garage overnight.

If pots have to stay out, Oregon State University Extension recommends setting them on the ground and covering them with evergreen boughs, straw, leaves, old blankets, sheets, burlap or row cover, wrapping the pots in bubble wrap, and not leaving containers hanging.

Some patio plants are worth keeping through winter. The University of Wisconsin–Madison Division of Extension describes overwintering tender geraniums by moving potted ones indoors, trimming them back by one-third to one-half and setting them in a sunny spot, noting that they fare better in cooler rooms of 60 to 65°F.

When to take the covers off

Iowa State's rule is simple: remove the covering the next day once temperatures rise above freezing. Cold frames and grow tunnels work the same way, opened during the day and closed again before temperatures drop below freezing.

Leaving covers on in the sun can cause its own trouble. The Wisconsin Extension horticulture program notes that heavyweight covers can raise daytime temperatures under them by 10°F or more, and that tomatoes and peppers may drop their blossoms if it gets above 86°F under the cover. Maryland adds that weeds grow faster under row cover too, so lift it now and then to check.

If another cold night is due, fold the covers beside the bed rather than putting them away. Light row cover tears easily, and Maryland suggests storing it in sealed containers out of season because mice will nest in it.

Common covering mistakes

Letting the cover sit on the leaves. Iowa State advises propping coverings up so they do not touch the foliage, and Maryland notes that fabric laid directly on plants can rub and injure stems and leaves in windy weather.

Using plastic as the main cover. It is better than nothing, but Iowa State's Rindels found it usually works less well than sheets or blankets.

Covering too late or too short. The point of a cover is to trap heat coming up from the soil, so put it on in the evening and let it reach the ground rather than stopping halfway down a tomato cage.

Counting on covers in a hard freeze. Below about 28°F, Iowa State says covering typically stops working. Harvest instead: Iowa State Extension suggests picking mature, greenish-white tomatoes before the first frost, wrapping each in newspaper and storing them in a cool, dark place at 60 to 65°F to ripen. The hardest part is often simply knowing which night to act. FrostCall sends one line at 8 PM on evenings when a crop on your list is at risk, "cover" for garden crops and "bring them in" for potted plants, and stays silent on other nights.

Frequently asked questions