How to protect plants from a late frost

Last checked October 25, 2026

Your tomatoes are in, and tonight's forecast says frost. Here's what works, how much it helps, and what doesn't work, according to university extension services.

Know which kind of cold is coming

The National Weather Service uses two main alerts in the growing season:

• A Frost Advisory means temperatures may drop to between 33 and 36°F, with widespread frost possible. Unprotected tender plants may be damaged.
• A Freeze Warning means forecasters are confident temperatures will drop below 32°F within the next day, with significant damage likely to unprotected plants.

Colorado State University Extension describes two kinds of frost. A radiation frost happens on a calm, clear night, when the ground's heat escapes into the open sky. Early and late in the season these frosts are usually only a few degrees below what plants can take, so covering them is well worth it. An advective frost comes with a cold front blowing in. Temperatures can fall far below what plants can survive, and protection may not be enough.

A calm, clear night is the classic frost night, and frost can form even when the forecast low is in the mid-30s: the official thermometer sits about 6.5 feet up, and on those nights the air at plant level is colder, as the National Weather Service office in Tallahassee explains.

Cover plants the right way

A cover works by trapping the heat the soil soaked up during the day. That's why the details matter.

• Reach the ground. University of Georgia Extension calls it a myth that a blanket over a plant keeps it warm. To protect a plant, the cover must reach all the way to the ground and be anchored, so the soil's heat stays under it. Weigh the edges down with rocks, bricks or soil, which also keeps the wind from lifting it.
• Keep it dry. Old sheets and blankets work well as long as they stay dry, says Colorado State. A wet cloth cools as the water evaporates and can make the plant colder.
• Keep plastic off the leaves. Plastic touching leaves can make cold damage worse. Hold it up on stakes, hoops or a frame.
• Take it off in the morning. Remove sheets, blankets and plastic in the daytime so the sun can warm the soil again for the next night. Plastic left on in sunshine can cook the plants underneath.

How much protection each cover gives

These figures come from extension trials and guides. They're the protection under the cover compared with outside.

• Floating row cover (the white, gauzy garden fabric): about 2°F for a lightweight grade and 6 to 10°F for a heavyweight one, according to University of New Hampshire Extension. Row cover lets light through, so it can stay on for days.
• Clear plastic on a frame: 3°F to over 6°F in Colorado State's trials in Fort Collins. Open it at least a slit on sunny days: under a closed cover, an hour of sun can push the temperature past 130°F.
• A space blanket on top of a plastic-covered frame, shiny side down: in Colorado State's trials this kept plants from freezing on a night that fell to 0°F after a sunny day. Take it off each morning.
• Christmas lights under the plastic: in the same trials, one string of 25 mid-size (C-7) bulbs per four-by-five-foot frame, on from dusk to dawn, gave 6°F to over 18°F of protection.
• Water-filled plant protectors, the rings of plastic tubes that stand around a single tomato: protection down to the mid-teens, says Colorado State, as long as the plant hasn't outgrown them.

For a hard freeze, University of New Hampshire Extension suggests doubling up: two layers protect better than one when temperatures drop below 28°F.

The watering question

You may have heard that watering before a freeze protects plants. University of Georgia Extension calls this half true. Soil that is well watered can keep the air just above it as much as 5 degrees warmer, but only if the whole area around the plant is watered, and the plant is close to the ground. Farmers who protect crops by running sprinklers all night are doing something different; that takes continuous overhead watering and isn't practical in a home garden.

Pots and baskets

Containers are easier: carry them into a garage, shed or porch for the night, and bring them back out in the morning.

Where frost hits first

Cold air is heavy, and on still nights it slides downhill and collects in low places. Virginia Cooperative Extension warns that a low, unsheltered garden is likely to be a frost pocket, while gardens on the upper side of a slope and sheltered from wind are less likely to get an early frost. If you're choosing where to put tender plants, choose the higher, sheltered spot.

Before the first fall frost

In fall, the plan is different: you're trying to finish a harvest. Virginia Cooperative Extension's advice when frost is forecast:

• Pick all ripe tender crops, such as tomatoes, summer squash, melons, eggplant, cucumbers, peppers and okra. They can't take frost.
• If the forecast is for a mild frost, no lower than 30°F, cover tender plants that still carry plenty of unripe fruit, using baskets, burlap, boxes, blankets or buckets. Warm days after the frost will still ripen some fruit, as long as the plants are covered each frosty night.
• Don't rush to pick cool-season crops such as cabbage, cauliflower, broccoli, spinach and Brussels sprouts. They can take some cold, and their flavor may improve after a frost.

Plan ahead with your dates

Your frost dates tell you when to be ready. On your ZIP code's page, the "early year" fall date is when 1 fall in 10 has already had its first frost: a good day to get covers out of the shed. In spring, the "late year" date is when you can usually put them away.

Sources

• Spring Frost/Freeze Information, National Weather Service, Aberdeen, South Dakota. Checked October 25, 2026.
• Frost and Freeze Information for the NWS Tallahassee Area, National Weather Service, Tallahassee. Checked October 25, 2026.
• Frost Protection and Extending the Growing Season (CMG GardenNotes #722), Colorado State University Extension. Checked October 25, 2026.
• Using Row Covers in the Garden, University of New Hampshire Extension. Checked October 25, 2026.
• Extending the Crop Season: Unheated Spaces (C1027-14), University of Georgia Cooperative Extension. Checked October 25, 2026.
• Fall Vegetable Gardening (426-334), Virginia Cooperative Extension, Virginia Tech. Checked October 25, 2026.
• How to Use the Maps, USDA Agricultural Research Service. Checked October 25, 2026.

More guides: What your last frost date really means · Hardiness zones, and what changed on the 2023 map · Starting seeds indoors: when and how · What to plant, month by month