The Cactus Patch
Volume 10       February 2007      Number 2

Frost Protection and
Extending the Growing Season

by D. Whiting, C. Wilson, and C. O'Meara
Colorado State University Cooperative Extension


Blankets and Sheets
Grandma's old method of covering the garden with blankets and sheets works well as long as the fabric remains dry. If the fabric absorbs water, evaporative cooling can lead to colder temperatures adjacent to the blanket. To recharge the heat stored in the soil, remove the fabric in the daytime.

Floating Row Covers
Floating row covers are lightweight fabrics that lay directly over crops. Since they transmit light, they provide crop protection over an extended period of time without being removed. They provide 2 to 4 degrees F of frost protection, cut wind on tender plants, and screen out some insects. On insect pollinated crops, remove covers must be removed for pollination to occur. Floating row covers are popular in commercial vegetable production where crops planted in large blocks are easily covered with row covers. Many brands and fabric types are commercially available.

Plastic Covering on Frame
When plastic is used as a covering over a growing bed, it must be held up off the plants. Plants will freeze where the plastic touches them. Tunnel Gardening Gardening catalogs carry wire hoops for use in "tunnel" or cloche gardening. Hoops are placed at 3- to 5-foot intervals, depending on the wind exposure of the site. The wire hoops hold up a strip of plastic forming a tunnel-shape covering down the growing bed. Bury the edges of the plastic a few inches into the soil on all sides. On a raised-bed box made with lumber, staple the plastic to the sides of the box. Two-inch holes cut in the sides of the plastic tunnel at 2 to 3-foot intervals prevent over heating. This type of covering is popular with commercial tomato, pepper, and melon growers for an early start to the growing season. It provides 2 to 4 degrees F of frost protection, warmer growing temperatures inside the tunnel, and protects tender plants from cold spring wind. Tunnels are removed when warm weather arrives and the danger of frost is past.

Plastic Covered Cold Frame Made with Concrete Reinforcing Mesh

An easy cold-frame structure for a growing bed is made with 4-mil clear plastic (polyethylene film) draped over concrete reinforcing mesh. The structure is easily opened during warm days and closed for cold nights. It works well with a 4-foot wide, raised-bed garden system.

The frame is concrete reinforcing mesh, available at hardware and lumber stores. This stiff wire mesh typically comes 5 feet wide in 50 and 100-foot rolls. A 6-foot length is required to make a Quonset-type frame over a 4-foot wide growing bed. In trials, the low and spreading shape was ideal for trapping heat from the soil during a frosty night. Cover the frame with clear, 4-mil polyethylene plastic. It typically is sold in 10 foot by 25 foot rolls. For a 4-foot wide raised bed box, place a 31/2-foot wide section on each side, overlapping at the top. On a raised-bed box, staple the plastic to the sides of the wood box. In soil bed applications, bury the plastic a few inches along the sides.

During the day, the covering MUST be opened, at least a slit, to prevent overheating. With just an hour of sun, temperatures under a closed cover can quickly rise to over 130F! Cover must be opened at least a slit to prevent overheating. On cool days, open the top a crack to prevent excessive heat build-up. On a warm day, the plastic can slide down the side, ventilating and providing crops exposure to the outdoors. On freezing nights, close the cover completely. On warm nights, leave the covers open a crack. On stormy days with full cloud cover and no direct sun, the cover may remain closed. Not only will the covers provide frost protection, they also increase growing temperatures for early crop growth and provide protection from cold winds.

In trials in Fort Collins, Colorado, a plastic cover on a frame typically provides 3 degrees F to over 6 degrees F of frost protection. It works well for cool-season crops that are somewhat tolerant of frosty nights, and adds two to six weeks or more on both ends of the growing season. For warm-season tomato and summer squash crops that are intolerant of a frosty nip, adding a small light inside the cold frame provides even better frost protection.

Adding Space Blankets

On extra cold nights, placing an aluminum space blanket over the plastic on the frame significantly adds to the frost protection. With the aluminized side placed down (towards the plants), a space blanket reflects 99 percent of the heat. They are readily available where camping gear is sold. In trials in Fort Collins, topping a plastic-covered, concrete mesh cold frame with a space blanket prevented freezing when outside temperatures dipped to 0 degrees F following a sunny spring day. Remove the space blanket each day to recharge the soil's stored heat.

Lights for Additional Heat

Christmas tree lights For additional protection, add Christmas tree lights inside the cold frame. In Fort Collins trials, one 25-light string of C-7 (mid-size) Christmas lights per frame unit (4 feet wide by 5 feet long) gave 6 degrees F to over 18 degrees F frost protection. Lights were hung on the frame under the plastic and turned on at dusk and off at dawn. Christmas lights work better than a single, large light bulb in the center by eliminating cold corners and edges.

Space Blanket with Christmas Tree Lights

For the gardener really wanting to extend the growing season, try Christmas lights plus a space blanket. One 25-light string of C-7 (mid-size) Christmas lights per frame unit (4 feet wide by 5 feet long) with a space blanket on top gave 18 degrees to over 30 degrees frost protection in Fort Collins trials.


The wall-of-water is a cone-shaped ring of connected plastic tubes filled with water that surrounds a single plant, like a tomato, pepper or summer squash. This device works on the chemistry principle of heat release in a phase change; there is a significant amount of heat released as water freezes (changes from the liquid phase to the solid or ice phase). A wall-of-water provides frost protection typically down to mid-teen F temperatures. It also provides wind protection for tender plants and growing temperatures may be slightly warmer inside a wall-of-water. They are helpful to get a few extra weeks head start on vine ripe tomatoes. However, an extra early tomato may out-grow the protection and the tops may be nipped back by frost. Both cold air temperatures and cold soil temperatures are limiting factors in early crop production. When using a wall-of-water to start early crops, warm the soil with black plastic mulch.

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