Season extension: row covers, cold frames and low tunnels

On this page: Season extension chart · Row cover weights · Row covers · Low tunnels · Cold frames · Containers and small spaces · Where sources differ

What a cover can and cannot do

Every method here works the way Iowa State describes the cold frame: sunlight warms the space under the cover by day, and that heat is released at night. The gain is a few degrees, not a heated greenhouse. Missouri Extension says the air inside a cold frame "generally is not more than 5 to 10 degrees above the outside temperature," and Utah State calls the protection from a low tunnel "minimal." That is enough to carry cool-season crops through light frosts, or to get warm-season transplants out a little earlier, as long as you start from your own average frost dates.

Utah State's targets show what the covers are aiming for: cool-season crops need air of at least 40°F (ideal 50–80°F) and soil of at least 45°F; warm-season crops need air of at least 50°F (ideal 70–85°F) and soil of at least 60°F. ATTRA, the sustainable-agriculture service of the National Center for Appropriate Technology (NCAT), says these techniques can "extend the growing season by weeks or even months."

Season extension chart: degrees, frost limits and weeks gained

Each figure is given as its source states it. Utah State rates protection in planting weeks: low means planting 1–2 weeks before the suggested date, medium 2–4 weeks and high 4–8 weeks.

Row cover weights: light versus warmth

Heavier fabric holds more heat but lets less light through. ATTRA sums it up: "lightweight versions serve primarily as insect barriers, while heavier fabrics provide frost protection." Insect-barrier use is covered in the flea beetle and cabbage worm guides.

UMN warns that on warm days a heavy cover "could retain too much heat and kill the plants underneath."

Floating row covers

Row covers are spun polyester or polypropylene fabric, "permeable to light, water, and air" (UW). Lay them loosely over the crop or over hoops that lift the fabric off the plants, and hold the edges with lumber, bricks, sandbags or smooth rocks rather than garden staples, which can tear it (UGA). ATTRA notes the limits: left on too long, fabric can rub foliage and hold back upright growth, and it may need to come off for pollination. For frost alone, WSU suggests the heavy grade on frost nights, removed after the temperature rises.

Low tunnels

A low tunnel is a temporary row of hoops, usually rebar, electrical conduit or PVC pipe, about 4 feet tall and 3 to 6 feet wide, covered with fabric or plastic (USU). Plastic captures more heat, but it also keeps out rain and air, so it must be pulled back daily to vent (UGA); UConn warns that on sunny days the heat inside "can rise to harmful levels." NC State's rule for row covers and low tunnels over crops such as tomatoes and peppers: open them during the day if it is expected to exceed 90°F under the cover for several hours. ATTRA lists the trade-offs: low height makes weeding and harvesting harder, and tunnels withstand high winds and snow less effectively.

Cold frames

1. Size it to reach. Iowa State suggests about 12 inches high in front and 18 inches at the back, and no more than 3 feet deep. NC State gives 4 to 5 feet as about as wide as is convenient to reach across. Missouri sizes beds to a standard 3 by 6 ft glass sash, sloping 1 inch per foot from back to front.
2. Face it south. Iowa State and Missouri face frames south; Maine says south to southeast; Utah State, south or west. Set the north side against a house, garage or fence to cut heat loss (Iowa State).
3. Choose a lid. An old storm window, window sash or shower door, or rigid plastic or fiberglass panels, mounted on hinges (Iowa State). Missouri also lists 4 or 6 mil clear polyethylene, replaced yearly unless greenhouse grade; a double layer with an air space holds heat better.
4. Hold the heat. Paint the inside walls black; bank the outside with straw bales or soil to damp temperature swings (Iowa State).
5. Vent every sunny day. Open the lid when the inside air passes 85°F and close it before sunset (Iowa State). ATTRA notes that temperatures can spike on sunny days and damage plants.

Cold frames suit cool-season crops such as lettuce, kale, cabbage, cauliflower and broccoli (UMN). In Iowa, a frame lets lettuce, spinach and radishes go in two to four weeks before their usual early-April planting, and fall sowings in late August or early September can be harvested "well into November in most growing seasons" (Iowa State). Frames also make hardening off tomatoes and peppers easier, since plants need not be carried indoors each night (Iowa State); see seed starting indoors for the rest of that process.

Season extension in containers and small spaces

• Move the pots. Plants in pots "can be moved inside a greenhouse or into an enclosed porch" when the cold comes (UVM). More on pot sizes is in container sizes for vegetables.
• One plant, one cloche. A glass jar or a translucent milk jug cut in half can be placed over an entire plant, with venting as the temperature rises (WSU); UW's version is a clear juice jug with the bottom cut out. For a short cold snap, UVM covers smaller plants with an inverted cardboard box resting on the ground and removes it in the morning.
• Portable frames. Lightweight commercial cold frames can be moved to different sun exposures as the seasons change, or set on concrete blocks or bricks to add height for taller plants (UW). Iowa State suggests building a seasonal frame with pins or eye hooks so it comes apart for storage.
• Raised beds. Bend ½-inch PVC pipe and push the ends into the soil inside the bed frame to make hoops (UGA). UVM suggests adding a cold frame to a raised bed of cold-hardy greens and root crops.
• Water-filled protectors. Maine Extension calls water tubes "practical only for small plantings of high-value crops"; UMN uses water-filled protectors to set tomatoes, peppers and eggplant out three weeks early (see container tomatoes).

For sun, weight and wind limits on a balcony, see balcony gardening.

Where sources differ

To turn weeks gained into dates, start from your frost dates, then use the planting calendar for spring and the fall count-back on succession planting for late sowings.