Season extension: cold frames, row cover and hoop houses
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
A few degrees and a few weeks at each end of the season are easy to gain with covers. None of it needs heat or electricity.
Floating row cover
Lightweight spun fabric laid over hoops or straight on the crop. Lets in light, water and air. Light grades give about 2–4°F of frost protection and keep out flea beetles, cabbage worms and carrot rust fly; heavy grades give 6–8°F but cut more light.
The cheapest, most flexible tool here. Weigh the edges down and leave slack for plants to grow into.
Cold frames
A bottomless box with a clear sloped lid, facing south. Good for hardening off seedlings, overwintering hardy greens, and starting spring sowings weeks early. Vent it on sunny days — it can overheat fast even when the air is cold.
Low tunnels and hoop houses
Low tunnels are row cover or greenhouse plastic over hoops across a single bed. A walk-in hoop house (high tunnel) is the same idea at scale. Plastic adds more warmth than fabric but must be vented and does not let rain through.
Stacking methods multiplies the effect: a hardy crop, under a low tunnel, inside a hoop house can crop through a cold-winter month it otherwise could not.
How much cold each crop can take
tomato, bell pepper, chili pepper, cucumber, zucchini, winter squash, pumpkin, bush bean, pole bean, sweet corn, eggplant, okra and 14 more
lettuce, swiss chard, broccoli, cauliflower, potato, bok choy, celery, globe artichoke, fennel, celeriac, endive & escarole, napa cabbage and 1 more
pea, cabbage, carrot, beet, radish, onion, arugula, turnip, kohlrabi, mustard greens, green onion, radicchio and 5 more
spinach, kale, garlic, leek, brussels sprouts, collard greens, parsnip, rutabaga, asparagus, rhubarb, horseradish, mache and 3 more
What to grow under cover
Cold-hardy crops that keep going in low light: spinach, kale, mâche, claytonia, arugula, scallions, carrots left in the ground. Growth nearly stops below about 10 hours of daylight, so plant to near-full size before midwinter and harvest through it.
Protecting plants on a single cold night
Water the soil during the day if frost is forecast - moist soil holds and releases more heat. Cover plants in the late afternoon to trap the warmth still in the ground, and let the cover reach the soil on all sides, because the heat comes from below, not from the plant.
Old sheets and blankets work in a pinch. Keep plastic from touching leaves, since it conducts cold straight through, and take covers off the next morning so plants do not cook in the sun.
Venting: the most common mistake
A closed cold frame or plastic tunnel on a sunny day can climb well past 80°F even when the air outside is freezing. Open lids or roll up tunnel sides once the inside passes about 70°F, and close them in mid-afternoon to hold heat for the night.
Automatic vent openers - a wax-filled cylinder that expands with heat - lift a cold frame lid without power or attention and are worth having if you are away during the day.
Pollination under cover
Squash, cucumbers and melons need bees to set fruit. If they are under row cover to keep pests off, uncover them once the female flowers appear, or the plants will flower without producing - see poor fruit set on squash and cucumbers.
Common questions
How much frost protection does row cover give?
Lightweight row cover gives roughly 2-4°F of protection, and heavier grades about 6-8°F, but they let through less light. Protection is best on still, clear nights when the cover traps heat radiating from the soil.
Can I use bedsheets or plastic to cover plants from frost?
Bedsheets and blankets work for a night or two. Plastic works too if it does not touch the leaves - where it touches, the cold passes straight through. Remove any cover in the morning so the plants do not overheat.
When should I use a cold frame?
In spring, to harden off seedlings and start hardy crops two to four weeks early. In fall, to keep hardy salad greens like spinach, mâche and lettuce going well past the first frosts. Vent it whenever the sun is out.
Do I need to remove row cover for pollination?
Only for crops that need insects to set fruit - squash, cucumbers, melons and pumpkins. Leafy crops, roots and brassicas can stay covered all season, which also keeps flea beetles and cabbage worms off.