How to Protect Plants from Frost

Most frost damage is preventable with a few dollars of fabric and an evening's attention. Here is when to act, which plants need protecting, and the methods that actually hold heat in.

When to act

Frost is most likely on clear, calm nights. Without cloud cover, the ground and leaves radiate heat straight into the sky; without wind, the cold air settles undisturbed around your plants. Plants can frost even when the forecast low is a few degrees above freezing.

A practical trigger: protect tender plants whenever the forecast low is 36°F or below on a clear, calm night. If the National Weather Service issues a Frost Advisory or Freeze Warning for your area, act that evening. Cloudy or breezy nights at the same temperature are much less dangerous.

Which plants need protection

Plants differ widely in how much cold they take. These ranges are approximate; young seedlings are more vulnerable than established plants.

• Tender: damaged at 32°F. Tomatoes, peppers, eggplant, basil, cucumbers, squash, pumpkins, melons, beans, corn, sweet potatoes, and most summer annuals like zinnias. Protect these every time.
• Semi-hardy: shrug off a light frost (about 28–32°F). Lettuce, Swiss chard, beets, carrots, potatoes (foliage may be damaged), peas, cauliflower, and cilantro.
• Hardy: survive hard frosts into the mid-20s°F or lower. Kale, cabbage, broccoli, Brussels sprouts, spinach, turnips, onions, and garlic. Kale, spinach, and carrots often taste sweeter after a frost.

Nine methods that work

1. Cover plants in the evening

Frost cloth (also sold as row cover or garden fabric), old bed sheets, and lightweight blankets all work by trapping heat rising from the soil. Put covers on before sunset, while the ground still holds the day's warmth. Let them drape all the way to the ground and weigh the edges down with rocks or soil. A cover that only sits on top of the plant, open at the bottom, lets that heat escape.

Lightweight row cover adds a few degrees of protection; heavier frost cloth adds more. Check the rating on the package, and double up on a hard-freeze night.

2. Keep plastic off the leaves

Plastic sheeting conducts cold, so leaves touching it can freeze right through it. If plastic is all you have, put it over a frame of stakes or hoops so it doesn't touch the foliage, or lay a sheet over the plants first and the plastic on top.

3. Take covers off in the morning

Remove covers once temperatures climb above freezing. Left on in the sun, they can overheat plants underneath, especially plastic.

4. Water the soil during the day

Moist soil absorbs and releases more heat than dry soil. Water the beds in the morning or early afternoon before a frost night so they can store heat through the day. Avoid soaking the foliage in the evening.

5. Use cloches for individual plants

For a few seedlings, an overturned bucket, flowerpot, or a milk jug with the bottom cut off makes a quick cloche. Vent or remove them in the morning so the plants don't cook.

6. Add thermal mass

Water holds heat. Water-filled plant protectors (the ring-shaped "walls of water") or a few dark jugs of water set beside plants under a cover soak up heat during the day and give it back at night. This works well for early tomato transplants.

7. Move containers

Pots lose heat from all sides and freeze faster than plants in the ground. Move them into a garage or shed overnight, or at least against a south- or west-facing wall under the eaves.

8. Add a little heat, safely

Old-style incandescent string lights under a cover give off enough warmth to help on a borderline night. LED lights produce almost no heat and won't help. Keep any heat source away from fabric, use outdoor-rated cords, and never leave an open flame or space heater under a cover.

9. Build longer-term shelter

Low tunnels (row cover or plastic over wire hoops) and cold frames turn one night's protection into weeks of season extension. They let you plant hardy crops earlier in spring and keep salad greens going well past the first fall frost.

Plant smart to avoid the problem

• Avoid the lowest part of the yard for tender crops. Cold air pools there.
• A spot beside a south-facing wall, fence, or paving stays warmer at night.
• Time tender transplants by your safe last-frost date, not just the average, if you won't be watching forecasts.
• Harden off seedlings before planting; acclimated plants tolerate cold snaps better than ones straight from indoors.

After a frost

Don't rush to prune. Frost damage often looks worse the first morning than it turns out to be, and blackened leaves shield the healthy growth beneath them from the next cold night. Wait a few days, then trim away clearly dead tissue once you can see what recovered.

Tender annuals with blackened, mushy stems won't recover; replant if the season allows. Hardy crops usually bounce back within days.

The first fall frost

The first fall frost is often followed by weeks of mild weather. Covering tender crops through that first cold night or two can buy you several more weeks of harvest. Before a hard freeze, pick everything tender: peppers, squash, basil, and tomatoes. Green tomatoes that have reached full size will ripen indoors at room temperature.