What temperature to cover plants for frost

Cover tender plants when the forecast low reaches 36°F (2°C) on a clear, calm night. Frost can form on leaves when the air is a few degrees above freezing. At 32°F (0°C), tomatoes, peppers, basil, squash, and dahlias take damage. At 28°F (-2°C) or below, most garden plants freeze. Cover plants in late afternoon, before the air cools. Use frost cloth, old bedsheets, or upturned buckets. Hold the cover down at the ground with rocks or bricks. Remove it the next morning once the air warms above freezing.

Short on time? Check the forecast low every afternoon in fall. If it says 36°F or lower, cover the tender plants. If it says 28°F or lower, cover them and also harvest anything you can.

Frost, freeze, and hard freeze

These three words mean different things in the garden.

A frost is ice that forms on plant surfaces. It can happen when the air reads 33°F to 36°F (1°C to 2°C), because leaves lose heat to the sky and run colder than the air around them. The National Weather Service issues frost advisories for this range and freeze warnings for 32°F (0°C) and below. Clear skies and no wind make frost most likely.

A freeze is when the air itself drops to 32°F or lower. Water inside plant cells turns to ice and the cells burst. Leaves turn dark, limp, and wet-looking by the next day.

A hard freeze is 28°F (-2°C) or lower for several hours. It kills most plants that are not cold hardy, and it freezes the top inch or two of soil.

Cloudy nights hold heat, so a cloudy night at 34°F rarely causes damage. A clear, still night at 34°F often does.

Which plants need cover

Group your plants by how much cold they take. That tells you what to cover first.

Tender plants die at 32°F. This group includes tomatoes, peppers, cucumbers, squash, beans, and basil. Dahlias, zinnias, cosmos, and impatiens belong here too. Cover these at the 36°F forecast, or pick the crops and move on. My post on when to dig up dahlias covers what to do after frost blackens the tops.

Half-hardy plants handle a light frost. Lettuce, broccoli, cabbage, carrots, and beets fit here. Pansies and potted mums do too. Cover them when the forecast drops to the upper 20s°F (about -2°C or lower).

Hardy plants take a hard freeze. Kale, spinach, leeks, and garlic need no cover for most fall nights. Newly planted garlic benefits from a layer of straw or shredded leaves. My guide on how to grow kale explains why a frost often makes kale taste sweeter.

Plants in pots and new transplants lose heat faster than established plants in the ground. Cover them first. Better yet, move pots against the house wall or into a garage for the night.

What to use as a cover

Frost cloth is the best choice. It is a light fabric sold as frost blanket, row cover, or garden fleece. It lets air and moisture pass through and adds a few degrees of protection. Heavier weights give more protection. Check the package for the rating.

Household items work for one night. Old bedsheets, blankets, towels, and burlap all do the job. Cardboard boxes and five-gallon buckets suit small plants. Flip a bucket over a pepper plant and weigh it down with a brick.

Plastic needs extra care. Clear plastic traps heat in the day, but it can burn leaves in the sun. Where it touches a leaf at night, it can pass cold straight through and freeze that spot. If plastic is all you have, hold it off the plants with stakes or a hoop frame. Remove it at sunrise.

How to cover plants the right way

Start in the afternoon. The soil has stored heat from the day, and the cover traps it. If you wait until after dark, the heat is already gone.

Water the soil in the morning or afternoon if it is dry. Moist soil holds more heat than dry soil. Do not water at night, since wet soil in a freeze can harm roots.

Drape the cover over the plant and let it reach the ground on all sides. A cover that stops at the leaves lets the ground heat escape. Pin the edges with rocks, bricks, or soil. Use stakes, tomato cages, or hoops to hold the fabric off the foliage. Fabric that rests on leaves can pass cold into them. If you need to stake tall plants, my post on tomato cages shows which types work as a frame.

For a whole bed, set hoops every three feet and lay the cloth over them. Clip the cloth to the hoops, then weigh the edges.

When to take the cover off

Remove covers the next morning when the air rises above freezing and the frost on the cloth has melted. Leave a cover on too long in warm sun and plants overheat under it.

If a cold stretch lasts several days, lift the cover on mild afternoons so the plants get light. Put it back on before sunset.

Do not trim frost-damaged leaves right away. Dead leaves insulate the plant against the next cold night. Wait until the weather settles, then cut back what is dead.

When covering will not save the plant

Covers add only a few degrees of protection. Frost cloth is no match for a night in the low 20s°F (-6°C to -4°C) or a week of hard freezes. Tender crops will not survive that.

Harvest before a hard freeze. Pick all peppers and ripe tomatoes. Green tomatoes ripen indoors at room temperature. Cut basil and bring it inside. Dig carrots and beets if the ground may freeze solid, or cover the row with a thick layer of straw.

For plants you want to keep, such as rosemary in a pot, bring them indoors. My post on rosemary in winter explains how to keep it alive until spring.

Many gardeners cover the garden for the first frost or two, then pick everything and stop. Most tender crops are done by then. The leftover work is on the November gardening tips list.