Frost protection works by holding in heat, not by adding it. During the day, sun warms the soil. On a clear night that heat radiates away into the sky, and plant leaves can drop below freezing even when the air a few feet up stays slightly above it. A cover slows that loss, keeping a little of the soil's warmth around the plants.

That makes frost protection very effective on some nights and almost useless on others. This guide explains how to tell the difference, which plants are worth covering, and which methods help.

The short answer

  1. Know what you are protecting. Tender crops such as tomatoes, peppers, beans, squash, and cucumbers are injured by any frost. Hardy crops such as kale, collards, spinach, and cabbage shrug off light frosts.
  2. Cover before sunset, all the way to the ground. Use frost fabric, old sheets, or blankets, anchored at the edges, so the cover traps heat rising from the soil.
  3. Keep plastic off the leaves, and keep fabric dry. Plastic touching foliage and wet fabric both conduct cold.
  4. Uncover in the morning once temperatures rise, so the sun can warm the soil for the next night.
  5. Harvest instead when a hard freeze or a windy cold front is coming. Covers help most on still, clear nights a few degrees below freezing.

Frost or freeze: know which night you face

Colorado State University Extension separates cold nights into two kinds, and the difference decides whether covering is worthwhile.

  • Radiation frost happens on calm, clear nights. With no clouds to hold heat in, it radiates away from the ground and plants. Early-fall and late-spring radiation frosts are usually only a few degrees below the point where plants are damaged, so covers save plants on these nights.
  • Advective freeze arrives with a cold front and wind. The whole air mass is cold, the wind strips heat from covers, and temperatures can fall well below the damage point. Protection is much less reliable. For tender crops, harvesting is often the better choice.

Forecast clues help. Clear skies, light wind, and a low in the low 30s °F mean a radiation frost that a cover can handle. A strong cold front with wind and a low in the 20s °F is a freeze that will likely beat it.

Low spots in the garden frost first. Cold air is dense and drains downhill, pooling in hollows and against fences and walls. Plants on a slope or near the house often escape a frost that blackens plants in a low corner.

How cold each crop can take

Vegetables fall into three broad groups by cold tolerance. Plants that have hardened through a spell of cool weather tolerate more cold than soft, newly set transplants.

Cold tolerance of common vegetables, from Purdue and Illinois Extension. Temperatures are approximate and depend on how well plants have hardened.
GroupWhat it takesExamples
TenderInjured or killed by any frostTomatoes, peppers, eggplant, beans, squash, cucumbers, melons, sweet corn
Frost-tolerantLight frost; usually damaged in the upper 20s °FCauliflower, radishes, young cabbage and onion transplants
Very hardyMid-20s °F; usually damaged in the low 20s °FKale, collards, spinach, cabbage, Brussels sprouts, carrots, beets

In practice, that means:

  • In fall, a first frost is the end of the season for tender crops unless you cover them, and a light cover can buy them another week or two of harvest. Very hardy crops often improve after a frost and need protection only as nights head into the low 20s.
  • In spring, tender transplants set out early are the plants most often lost to a late frost. Frost-tolerant and hardy crops planted weeks before the last frost date usually come through light frosts on their own, once hardened off.

Covers that work

A cover works best when it reaches the ground on all sides, so it traps the heat rising from the soil around the plants. A sheet tied around a plant's stem, or draped over the top only, leaves the heat source outside.

Frost fabric (floating row cover). Lightweight spun fabric laid directly over plants or over hoops. Extension fact sheets list roughly 2°F of protection for the lightest insect-barrier weights, around 4–6°F for medium weights, and up to about 8°F for heavy frost blankets. Heavier fabric also blocks more light, so it is a night cover rather than one to leave on for weeks. Check the manufacturer's rating for the fabric you buy.

Old sheets and blankets. They work well as long as they stay dry. Colorado State notes that fabric that soaks up water can make things colder through evaporative cooling. Lay them loosely over the plants, support them on stakes or hoops if they would crush the foliage, and weigh down the edges with bricks, boards, or soil.

Cold frames and plastic tunnels. A plastic cover on a frame gave 3°F to more than 6°F of protection in Colorado State trials, and can stretch the season by two to six weeks at each end for cool-season crops. They heat up fast in the sun, so open or vent them on sunny days.

Buckets, boxes, and cloches for single plants. An overturned bucket or cardboard box over a tomato plant works on the same principle. Put it on in the evening and take it off in the morning.

Water jugs as heat stores. Jugs of water set inside a tunnel or cold frame soak up heat during the day and release it slowly at night, adding a little protection inside an enclosed space.

Measure what your cover doesPut a minimum thermometer under your cover at plant height and another in the open. After the first cold night you will know how many degrees your setup buys you, and on which nights to trust it.

Use the soil's heat

The soil is the heat source a cover relies on, so help it store as much as possible.

  • Uncover during the day. Sheets and blankets left on block the sun from rewarming the soil. Take them off in the morning once the temperature is above freezing, and put them back before sunset if another frost is coming.
  • Water dry soil a day ahead. Moist soil absorbs and holds more heat than dry soil. Don't soak soil that is already wet, and don't water in place of covering.

What backfires

  • Plastic touching the leaves. Plastic conducts cold, and leaves touching it are often damaged where they touch. If you use plastic, hold it off the foliage with stakes or hoops.
  • Wet fabric. A blanket soaked by rain or dew can leave plants colder than no cover at all.
  • Covering only the top of a plant. Wrapping the canopy and leaving the base open lets the soil's heat escape.
  • Leaving covers on in the sun. Plastic especially traps heat and can cook plants quickly once the sun is up.
  • Garden sprinklers. Commercial growers protect crops by running sprinklers continuously through a freeze, with carefully engineered systems. Intermittent home sprinkling can make damage worse.

Containers and houseplants

Plants in pots have no ground heat to draw on, and their roots are far less cold-hardy than the tops. Move containers of tender plants into a garage, shed, or porch on frost nights, or cluster them against the house under a cover.

Houseplants that spent the summer outdoors should already be inside before nights drop into the 40s °F. See when and how to bring houseplants indoors.

The morning after

  • Leave frozen plants alone until they thaw. Frozen leaves and stems are brittle and easily damaged by handling.
  • Wait a few days before judging the damage. Frost-killed tissue turns limp, dark, or water-soaked as it thaws. Hardy plants that looked limp at dawn often recover fully by afternoon.
  • Harvest what's left of tender crops. Pick ripe and near-ripe tomatoes, peppers, and squash from lightly damaged plants. Green tomatoes can ripen indoors.
  • Hold off on pruning shrubs and perennials until you can see what actually died. Pruning too soon can remove tissue that would have survived.

Plan around your frost dates

Frost protection works best as part of a plan, not a scramble.

  • Know your average first and last frost dates, and treat them as probabilities rather than promises. The seasonal planner explains how to read frost dates and pick a safer date for plants you cannot replace.
  • Keep covers ready before you need them. Cut fabric to fit each bed, and store hoops, clips, and weights where you can reach them in the dark.
  • In spring, match planting to cold tolerance. Plant hardy crops weeks before your last frost date and wait for warm soil for tender ones. The seed starting calculator works out both from your last frost date.
  • In fall, decide what's worth saving. A cover that buys two more weeks of tomatoes is worthwhile; the October guide covers the rest of the season's frost work.

Winter protection for perennials, shrubs, and bulbs is a different job. There the goal is to keep roots evenly cold, not warm. See protecting against the thaw, not the cold in the fall checklist.

Sources and further reading