Season Extension Desk: row cover, cold frames, and frost-night covers

Light frost is about 32F. A hard freeze is below about 28F. Pick tender crops first. Cover light-frost greens with row cover, a low tunnel, or a cold frame. Leave hardy kale and roots in, and cover them only if you want a longer harvest. Degree gains come from extension charts, not promises.

Plan frost nights with your zone first frost date: pick tender crops, cover light-frost greens, leave hardy kale and roots in. Row cover degree gains, cold frames, low tunnels, and a checklist.

How much warmth each cover adds

  • Light row cover (about 0.45 oz per sq yd) (About 2F): Insect barrier and light frost insurance. Not enough for a hard freeze on tender crops.
  • Medium row cover (0.5 to 1.0 oz per sq yd) (About 4 to 6F): Workhorse for fall greens and brassicas through early freezes.
  • Heavy row cover (1.5 to 2.2 oz per sq yd) (Up to about 8F): Overwintering spinach and kale. Cuts a lot of light.
  • Low tunnel (fabric or plastic on hoops) (Same as the fabric, or more with clear plastic): Hoops keep cover off leaves. Vent clear plastic on sunny days above about 50F.
  • Cold frame (Often about 5 to 10F on clear nights): Vent every sunny day. Close before hard freezes.
  • Cloche, milk jug, or pot cover (A few degrees for one plant): Quick one-night save for a few plants. Remove by mid morning.

Pick first; cover only buys a night or two

Damaged at about 32F. Harvest usable fruit and leaves before dusk. A light cover may ride out one light frost, not a hard freeze.

Tomatoes, peppers, eggplant, basil, cucumbers, squash, beans, corn, okra, melons, watermelon, sweet potatoes

Cover these on frost nights

OK near 28 to 32F with cover. Medium row cover, a low tunnel, or a cold frame is the real season extension here.

Lettuce, Swiss chard, beets, peas, cabbage, broccoli, cauliflower, kohlrabi, celery, bok choy, arugula, radishes, cilantro

Leave in; cover optional for longer harvest

Takes hard frost into the low 20s F. Flavor often improves after frost. Cover or mulch to keep picking deeper into winter.

Kale, collards, Brussels sprouts, spinach, leeks, carrots, parsnips, turnips, parsley

Frost-night checklist

  • Check the forecast low each evening once nights near 40F
  • Sort beds: pick tender, cover light-frost, leave hardy
  • Cut row cover and gather hoops, pins, and rocks before frost season
  • Water dry beds in the afternoon before a frost night
  • Cover light-frost beds before dusk; vent or remove covers by mid morning

Sources

Shop Seeds Now: cool-season greens. Some outbound seed-house links may be affiliate links.