Cold Frames, Row Covers and Low Tunnels

Row covers, low tunnels and cold frames each buy you a few degrees, not a different climate. Extension figures for floating row cover run from about 2°F of frost protection for lightweight fabric to 6–10°F for heavy fabric. Low tunnels are often credited with only 2–4°F. Used well, that is enough to start cool-season crops a couple of weeks early, carry greens into late fall, and get through a borderline spring night. The bigger risk with all three is usually overheating on a sunny day, not cold.

How much protection each tool gives

Figures differ between sources, and the differences are worth seeing side by side.

Why the spread? Fabric weight is one factor. Covering materials and construction vary too: a low tunnel can be covered in light fabric, heavy fabric or plastic. Conditions matter as well. Auburn's freeze protection guide separates calm, clear-sky radiation frosts from windy arctic freezes, and says protection methods are of "limited value" against the windy kind.

Plan on the low end of each range. If the forecast is 30°F and your tomatoes are under light fabric, you are relying on a degree or two.

Row covers

Floating row cover is lightweight spunbonded fabric laid directly over crops or supported on hoops. It lets in light, air and water. The University of Maine reports that it can raise daytime air temperature around plants by up to 10°F. It "may last two to three years" if stored properly.

Tips from the sources:

• Weight is a trade-off. UNH notes that even a lightweight insect-barrier grade can block up to 10% of light, and heavier grades add more frost protection. Match the weight to the job: light fabric for insects and mild nights, heavy fabric for real cold.
• Keep it off the leaves where you can. Clemson's Home & Garden Information Center recommends leaving space between a hooped cover and the plant tops, so frost doesn't form where fabric touches leaves.
• Pull it for pollination. UNH advises removing covers from insect-pollinated crops like squash, cucumbers, tomatoes, peppers and eggplant once female flowers bloom.
• Watch the heat. UNH reports temperatures under row cover can run as much as 15°F above outside air and recommends removing covers above 90°F.

Row covers also keep insects off crops. That use is covered in Row Covers for Pest Control.

Low tunnels

A low tunnel is row cover or plastic stretched over hoops. WVU Extension's beginner guide gives practical specs: hoops 4–5 feet apart for PVC and 5–6 feet apart for metal, with 4- or 6-mil agriculture-grade plastic. The University of Alaska Fairbanks describes 9-gauge wire hoops, 6 feet long for a 4-foot bed, standing 16–20 inches high and spaced every 4–5 feet.

WVU recommends venting cool-season crops when the tunnel gets above 55°F. It says low tunnels can give peppers, tomatoes and sweet corn a two-week early start. WVU also notes their limits: Swiss chard dies below 15°F even under cover.

Cold frames

A cold frame is a bottomless box with a clear lid, sitting on the soil. It gives you a sheltered spot for hardening off, fall greens and early spring sowing.

Construction guidance from extension sources:

• Width: 4 to 5 feet at most, "about as wide as is convenient to reach across," per NC State.
• Orientation: south-facing, or south to southeast (NC State, Maine, Clemson). SDSU Extension also mentions west-facing as an option.
• Lid slope: about 10 to 15 degrees, per SDSU.
• Glazing: old windows, polycarbonate, polyethylene film or acrylic all work (SDSU).

Venting is where cold frames succeed or fail. Clemson says to vent on sunny days to keep temperatures below 60°F for winter greens. SDSU describes cool-season crops doing well at air temperatures of 50–80°F, and says to open lids on warm, sunny days and close them in the late afternoon. Those two figures aren't in conflict. Clemson's is a conservative ceiling for winter greens, and SDSU's is the broader comfort range. A cheap thermometer inside the frame is the simplest way to learn how quickly yours heats up.

Cold frames are also a good tool for hardening off transplants. University of Maryland Extension calls them "an excellent environment" for it.

Warming the soil first

Air protection is only half the job. Seeds respond to soil temperature, and soil warms slowly in spring. The University of Alaska Fairbanks measured soil 8 to 10 degrees warmer under clear plastic mulch than under no mulch, and 3 to 6 degrees warmer under row covers. In Alaska work with these systems, it reports zucchini yields two to five times greater, and cucumber yields eight times greater with mulch and row cover combined. Those results come from a very short-season climate. Gains in milder regions may be smaller. Utah State says plastic mulch lets most crops go in one to two weeks earlier than usual.

A few degrees of soil temperature can cut germination time sharply. See the emergence table in Soil Temperature for Germinating Vegetable Seeds.

Extending into fall and winter

Season extension works in fall as well as spring. Hardy greens and roots under a cold frame or tunnel can keep producing after the first frost. Clemson lists spinach, kale, collards, mustard, Swiss chard, radishes, turnips, beets and carrots as good candidates. For South Carolina, it suggests sowing from mid-October to early November for December harvests. Timing will be earlier farther north. Use your fall freeze probabilities, explained in Your Last Frost Date Is a Probability.

For the complete spring and fall sequence, including seed starting, lights and hardening off, go back to Seed Starting and Season Extension.