Row covers, low tunnels, and frost blankets
How to use fabric and low tunnels for a few degrees of frost protection, insect exclusion, and wind, without cooking the bed.
Buy the fabric for one job. Colorado State University’s frost-protection note says floating row covers can give up to about 10°F of protection, and that the amount depends on the material and the grade.
The mountain vegetable fact sheet is more specific: lightweight spun-polyester row cover can protect down to 24°F depending on thickness, lets sun and rain through, and does not need venting. A product sold as a frost blanket protects further and can add another week or two at each end of a mountain season.
Plastic on hoops can add about a month at each end, and wind or snow will wreck it if you put it out too early or leave it too late. Insect-weight cloth is not that frost blanket. If the label does not state a temperature, do not invent one. The parts list and how the prices compare is in season-extension fabrics.
Three jobs, three fabrics
Frost. You want a cover on the night the forecast crosses your crop’s limit, and off again when the sun returns. Heavier polypropylene frost blankets, the ones sold for that purpose, are the right tool. They are not pretty and they do not belong on the bed for six weeks straight, because they cut light.
Lay them over hoops so the fabric is not freezing to the leaves. Pin every edge. A Front Range gust will put an unpinned blanket in the street, and the plants will be uncovered at 2 a.m., which is the entire point of failure.
Insects. A lightweight row cover, translucent enough to leave on, keeps flea beetles off arugula and cabbageworm off broccoli if it goes on the day you sow or transplant and the edges stay buried or weighted. This is the cover described on the cool-season crop page. It is a poor frost tool by itself. Use it as the everyday layer and add a blanket over the hoops on the cold night.
Heat for a warm-season start. Clear plastic on hoops is a low tunnel. It is how a Front Range gardener gets summer squash and tomatoes through a cold May, and how a mountain gardener might steal two weeks for a bush bean.
It gathers more heat than fabric and it gathers it fast. Vent it. Slit the plastic, roll the sides, or open the ends every mild day.
University of Idaho’s season-extension introduction is written for high-altitude gardens where a frost can show up in midsummer and the arrival of the season cannot be scheduled. A tunnel you cannot vent is not part of that strategy. It is a trap.
Walls of water, the water-filled teepees sold for tomatoes, are a single-plant version of the same idea.
They work on the Front Range for the last two weeks of risk. They are fiddly in wind, they are not a substitute for a short-season cultivar, and they come off once nights settle. A cold frame is the rigid version, better for a small salad bed and worse for a forty-foot row of beans.
Hoops and edges
Hoops can be nine-gauge wire, PVC, or metal electrical conduit, pushed over the bed or into sockets on a raised bed. Space them close enough that snow or a wet blanket does not sag onto the plants.
In a hail climate, the same hoops may later carry hail cloth. Build one set of supports and change the skin. Hail cloth and frost blanket are not the same roll of fabric.
Hail cloth is a woven shade that stays all summer. Frost blanket is a temporary night layer. The hail page keeps that distinction, because people buy one product and ask it to do both.
Edges are the engineering. Boards, sandbags, or soil bury the hem. Rocks blow off or tear the fabric. A cover that flaps will abrade leaves and then rip.
Check the wind forecast before you install a new sheet of plastic. A low tunnel on the day a chinook is forecast is a kite with plants under it.
When it goes on, and when it comes off
Tie the cover to your station’s dates, not to enthusiasm. Hardy crops can be sown in the open two to four weeks before the median last 32°F date if the soil is workable, which is CSU’s planting-guide window.
A cover lets you push that earlier, or protects the sowing you made on time from a night that overshoots the median. Very tender crops still should not live under a closed tunnel until after your late date unless you are committed to daily venting. The method is in reading a spring frost.
Remove or open covers once the risk has passed, so light and pollinators get in. A bean or tomato that blooms under a closed sheet will not set a crop if insects cannot enter and the air is too hot. Squash and cucumbers under insect fabric need the fabric off at flowering unless the cultivar sets without pollination.
Brassicas that are heading can lose the insect cover once the leaves are tough and you are willing to look for caterpillars, or they can keep it if the fabric is truly light. Feel the difference between a cover that is protecting and a cover that is hiding a pale, stretched plant. The second one comes off.
In fall, a frost blanket returns for the first cold nights and can keep a Front Range tomato or pepper ripening through an early scare. It will not hold a tropical plant through a 20°F night. At mountain elevations, the fall use is mostly for greens and for hardening the last of the brassicas, not for saving a melon.
After a cold night
If you covered and the plants still look burned in the morning, leave them alone until they thaw. Water the soil, not the foliage, and do not fertilize a damaged plant to “help it recover.” That is the same restraint CSU educators describe after hail. Tissue that is going to live will show green growing points in a few days. Tissue that is black and soft will not, and a dose of nitrogen will not change the physics of ice.
A cover that was pinned down is also a cover that kept wind off. On the Front Range and the high plains that side effect is sometimes worth as much as the degrees. A new transplant under fabric for a week hardens into the climate instead of taking the full sandblast on day one. Then take it off during the day so the plant learns to stand up.
Checklist
- Use lightweight fabric for insects, heavier blanket for frost, plastic only where you will vent.
- Keep fabric off the leaves with hoops on frosty nights.
- Pin every edge. Plan for wind before you plan for cold.
- Open plastic whenever the sun is on it and the day is mild.
- Take insect cover off crops that need pollinators, once they flower.
- Time the effort to your median and late frost dates.
- Do not leave a frost blanket on for a month and then wonder why the plants are yellow.
- In hail country, consider hail cloth on the same hoops for the summer months.
A cover is a few degrees and a calmer wind. The crop still has to fit the season. Run the days-to-maturity sum in that article before you buy the cultivar you are about to tuck in.
Sources
- CSU Extension, Frost Protection and Extending the Growing Season, up to about 10°F depending on material and grade.
- CSU Extension, Vegetable Gardening in the Mountains, row cover down to 24°F depending on thickness, frost blankets, and plastic tunnels.
- CSU Extension, Vegetable Planting Guide.
- University of Idaho Extension, BUL 936, season extension for high-altitude, short-season gardens.
- CSU CO-Horts, hail-damage notes, on not pushing fertilizer onto damaged plants.