Frost Protection Methods: Row Covers, Cloches and When to Act

Frost protection methods such as row covers and cloches work best when you know when to act, usually on a clear, still evening.

Protect plants when the forecast low is 3°C (37°F) or below on a clear, still night, because leaf surfaces can run 2 to 3°C colder than the air at head height. Cover by late afternoon with fleece held off the foliage, water the soil earlier in the day, and uncover once the frost has melted. This guide covers how to read the threat, fleece weights, cold frames and cloches, and the popular tricks that don’t work.

It sits in the Seasonal Gardening guides and ties directly to Spring Garden Tasks (late frosts) and Autumn Garden Tasks (the first frost and lifting tender plants). For the long cold season itself, see Winter Garden Care.

Frost protection at a glance

Dates matter as much as degrees. Hardy crops can go out 2 to 4 weeks before your average last frost, tender crops 1 to 2 weeks after it. In autumn, expect the first damaging frost within about two weeks either side of your average first frost date. The Northern Hemisphere calendar months you see in guides flip by about six months in the Southern Hemisphere, so always count from your own frost dates.

Read the night, not just the forecast

Radiation frost, the common kind, needs a clear sky, light or no wind and dry air. Heat leaves the ground as infrared and nothing returns it. Cloud cover acts like a blanket, which is why a cloudy night at 2°C is usually safe and a clear one at 4°C may not be.

• Look at the sky at dusk. Clear and calm with temperatures already falling quickly means frost is likely even if the forecast says 3°C.
• Read a min-max thermometer at plant height, not on a wall. Readings in the open are often 1 to 3°C below the official forecast, and hollows can be colder still.
• Note frost pockets. Cold air is dense and flows downhill like water, pooling against solid hedges and fences at the bottom of a slope. The same border may be frost-free 10 m uphill.
• Expect wind frost differently. A cold wind from the north or east (advection frost) damages plants even at temperatures above 0°C, and fleece must be pegged hard.

Fleece: weights and what each can do

Horticultural fleece is spun polypropylene sold by weight in grams per square metre (gsm). Heavier fleece insulates more and lets in less light. Treat the numbers below as rough, because wind, soil moisture and how well you seal the edges change results.

The gains don’t add up neatly, so two layers of 30 gsm are close to one 50 gsm sheet but easier to peel back on mild days. Fleece works mainly by trapping the warmth the soil releases, so it must reach the ground on all sides and be weighted with stones or soil, not just draped. Hold it off tender growth on hoops or canes to stop frozen leaves fusing to it.

Cloches, tunnels and cold frames

Glass and rigid plastic cloches protect single plants. A cold frame, a bottomless box with a lid of glass or twin-wall polycarbonate, can run roughly 5 to 10°C warmer than outside in sun and a few degrees warmer on a still night. It needs ventilating on any day when the inside would pass about 20°C: prop the lid 5 to 10 cm open for most spring days and remove it fully on warm afternoons. On very cold nights, add an old blanket or a sheet of fleece over the glass, and close the lid before dusk to hold the day’s heat. A cold frame is also the best way to harden off seedlings before planting out (see How to Transplant Seedlings).

Water before frost, and other heat-storage tricks

Moist soil holds and conducts more heat than dry, loose soil. On the afternoon before a forecast frost, water beds and pots, ideally by 2 pm so leaves have dried by evening. The effect is modest, perhaps 1 to 2°C near the surface, but it’s free. Bare, firm soil radiates its stored heat better than soil under thick mulch, so pull mulch back from tender seedlings for the night and replace it afterwards. Mulch is still valuable around dormant roots, where it slows freezing and thawing (see Mulching Mastery).

A frost-night routine

1. By early afternoon: water soil and pots if dry, and harvest anything ripe that frost would spoil, such as basil, courgettes and tomatoes.
2. By late afternoon: place hoops or canes, then lay fleece or a blanket over them to the ground. Peg or weight every edge.
3. Move pots: bring tender pots into a porch, garage or against the house wall, raised off the ground on bricks or feet.
4. At dawn: check the thermometer. If frost is still on the covers, wait until it melts, then remove fleece unless more cold is due.
5. After a frost: don’t cut frozen growth back for several weeks. Dead leaves insulate crowns, and the true extent of damage only shows once buds break.

Plant by plant: who needs help

Fruit trees need a different approach. Fleece over a small tree is fine during flowering, but remove it in the morning so pollinators can work, and don’t use plastic over blossom because it keeps flowers wet and overheated.

What does not work

• Plastic sheeting touching leaves. It conducts cold and builds ice crystals on the inner surface.
• A single thin bedsheet in a hard freeze. It helps on a light frost only. Below -4°C you need real insulation or you should move the plant.
• Evening overhead watering. Wet leaves freeze and get worse damage. Commercial orchards use continuous sprinklers that keep ice forming all night, which releases heat; a garden hose can’t match that and a gap in the spray makes things worse.
• Candles and open flames. They heat almost nothing and risk fire. If you use string lights inside a cloche, they must be outdoor rated and on a residual current device (see Outdoor Electrical Safety).
• Covers left on in sun. Under glass or plastic, temperatures can pass 30°C within an hour on a bright winter day and cook foliage.

Troubleshooting frost damage

How frost protection works for your plants

Frost kills tender tomato, courgette and dahlia, and it damages blossom on apricot, peach and cherry. Fleece traps heat and protects plants by several degrees, while a cloche or cold frame warms seedlings and holds humidity. Mulch around the roots cools the soil more slowly and protects crowns of lavender and fuchsia.

Cold air flows downhill and settles in hollows, so a frost pocket needs hardier plants, and a south-facing wall warms and shelters fig and citrus. Watering before a frost helps, because wet soil holds more heat than dry. Pots of citrus and olive need moving under cover and a dry spot prevents root rot. When should you cover plants? Cover them in the evening and remove fleece in the morning, since trapped moisture can encourage botrytis. Your extension service issues frost warnings and local dates you can use.

How frost protection methods work: when to act and what to cover

A row cover holds heat and protects tomato, pepper and bean seedlings down to about -2°C, while a cloche shelters lettuce and spinach. Frost damages tender dahlia and geranium first, so you should cover them on clear, still evenings. Mulch protects roots and reduces heat loss from loam, and compost around rose crowns holds warmth too. Water the soil before a frost, because moist clay and loam hold more heat than dry sand. Fleece of 30 g suits most borders, and a double layer adds about 2°C of cover. Citrus and olive in pots need a move to shelter below 0°C. Wind causes the worst damage, so a hedge of yew or box blocks it and protects tender plants. Remove covers by mid morning, because trapped heat causes botrytis and weakens lettuce. Frost kills fig and banana leaves, but straw and mulch protect the crown. Your extension service or the rhs publishes local last frost dates. The winter garden care guide explains longer protection.

You can test a cover on a calm night, and you’ll see which tender plants need more shelter.

Common mistakes

• Covering after dark. The ground has already lost its stored heat, so there’s little to trap. Cover by late afternoon while the soil is still warm.
• Trusting the airport forecast. It describes open, level ground. Your garden may be 3°C colder in a hollow, so use a thermometer and your own records.
• Using fleece without ground contact. A cover that hangs 20 cm above the soil lets the trapped warmth escape. Seal the edges.
• Planting out by the calendar. A warm April can be followed by a frost in May. Wait for your last frost date plus a safety margin for tender crops.
• Cutting back frost damage at once. You remove the insulating layer and risk a second frost killing the exposed stems. Wait until growth restarts.

For the opposite extreme, read Heatwave and Drought Garden Care.

Tender palms need extra shelter, as described in Palm and Tropical Trees Care.

For how fleece, cloches and frames compare, see Best Frost Protection Fleece & Covers.

Next steps

Use Succession Planting to time sowings so a late frost costs you one batch, not all of them. Keep a log in Planning for Next Year, and work through the Year-Round Checklist to make sure fleece and frames are ready before the first cold night. Back to the Seasonal Gardening guides.