How to Protect Plants from Frost (UK Guide)

A frost doesn't have to mean the end of your plants. Most of the frosts a UK garden gets are light ones on clear, still nights, and a sheet of fleece, a cloche or the lid of a cold frame put on before dusk is enough to see tender plants through them. The trick is knowing which cover suits which job, and putting it on at the right moment.

This guide explains how frost damages plants, when to protect them, and how fleece, cloches, cold frames and bubble insulation each work, with the mistakes to avoid. If you want to know when frost typically reaches your area, start with the first frost checker.

How frost damages plants

Plants are mostly water. When the temperature in and around the leaves drops below freezing, ice forms and damages the cells. When it thaws, the damaged parts collapse: leaves go dark, limp and glassy, then turn black or brown. That's the blackened look courgettes, dahlias and beans have the morning after the first frost.

How much a plant minds depends on how hardy it is. Hardy plants, like kale, leeks, garlic and most fruit bushes, are built to survive freezing. Tender plants, like tomatoes, courgettes, chillies, beans, dahlias and pelargoniums, come from warmer climates and are damaged or killed by the first frost. Many plants sit in between: a light frost scorches them, a hard one kills them. The RHS rates every plant on a scale from H1a to H7 by the coldest winter temperature it can take. Hardy or tender? explains the scale and how to read a plant label.

The point for protection is simple. Hardy plants need no help. Borderline plants need a cover on the coldest nights. Tender plants you want to keep need to be moved somewhere frost-free for the winter.

When to protect your plants

Frost forms on particular kinds of night, so you can see it coming.

Clear skies. In the day the soil soaks up warmth from the sun. After dark it gives that warmth back. Cloud holds some of it in; a clear sky lets it all escape, and the ground and plants cool quickly.

Still air. Wind keeps mixing in slightly warmer air from above. When it drops, the coldest air settles at ground level, around your plants.

After a cold day. A bright, chilly autumn day with a northerly wind, followed by a clear, calm evening, is the classic frost set-up.

When those come together, cover tender plants, even if the forecast low is just above freezing. Leaves and the ground cool below the air temperature a little above them, so a ground frost can form when the forecast says it won't.

The other half of the timing is your area. A region's typical first-frost window tells you when to have covers ready. First frost dates in the UK, by region has the table, with the Met Office's frost-day averages for each region, and the first frost checker counts down to yours.

Frost pockets

Cold air is heavy. On a still night it flows downhill and collects in the lowest ground, so gardens at the bottom of a slope, in a valley or in a dip get frost earlier in autumn, more often, and later into spring. A solid fence, wall or hedge across a slope can dam the cold air and create a frost pocket behind it. If that's your garden, plan around the early end of your region's frost window, and grow your most tender plants in the warmest spot: up the slope, or against a house wall.

The covers, and when each one works

There's no single best frost protection. Each cover suits a different job, and most gardens end up using two or three of them.

Horticultural fleece

Fleece is a thin, white, spun fabric that you lay over plants. It lets light, air and rain through, so you can leave it on through a cold spell without the plants sweating or going pale, and it's light enough to rest on most plants without crushing them.

Best for: beds and rows of plants, groups of pots, half-hardy plants on their coldest nights, and young plants in spring that went out a little early. Also inside an unheated greenhouse, draped over plants on hard nights.

How to use it:

• Lay it loosely over the plants so it touches the ground all round. The warmth comes from the soil, so the edges need to reach it.
• Weigh the edges down with bricks, stones or pegs, or tuck them into the soil, so wind can't whip it off.
• For tall plants, push in a few canes and drape the fleece over them so it doesn't flatten the tips.
• On a harder frost, fold it double. Two layers trap more than one.
• Take it off in the morning on a mild day so the plants get full light, and put it back that evening if another frost is due.

Fleece is cheap, folds away small and lasts several seasons if you dry it before storing it. Its weaknesses are wind, which lifts it unless it's pinned well, and very hard or long frosts, which it can't hold off on its own.

Cloches

A cloche is a rigid cover for a single plant or a short row: a glass or plastic bell, a tunnel of plastic over hoops, or the cut-off top of a large plastic bottle. Because it's rigid, it stays in place in wind and doesn't touch the leaves.

Best for: single tender plants you want to get through an autumn cold snap, rows of winter salads, and warming soil before you sow in spring. A row of cloches over winter salads keeps them picking for longer.

How to use it:

• Put cloches on before dusk on frosty nights. In a cold spell they can stay on, but open the ends or lift them on sunny days so the plants don't overheat or turn mouldy in the damp.
• Close the ends of a tunnel cloche on cold nights; an open-ended tunnel becomes a wind tunnel.
• Water around the outside edge: rain runs off the cloche and soaks in sideways.

A cloche on its own gives a little protection. On a hard night, lay fleece over the top as well.

Cold frames

A cold frame is a low box with a clear lid: a mini greenhouse at ground level. It's the best place for pots, trays and young plants that need shelter but not heat.

Best for: overwintering hardy and borderline plants in pots, cuttings taken in autumn, autumn-sown sweet peas and broad beans, and hardening off seedlings in spring.

How to use it:

• Close the lid on cold nights and open it on mild days for air. A closed frame in winter sun warms up quickly, and damp, still air encourages mould.
• On a hard frost, lay fleece or an old blanket over the lid at night as well, and take it off in the morning.
• Stand pots on a layer of gravel or slats rather than bare soil, so they drain.

Bubble insulation

Horticultural bubble insulation is a large-bubble plastic sheet made for lining the inside of greenhouse glass. It slows the heat lost through the glass at night, so an unheated greenhouse stays a little warmer and a heated one costs less to run. It does block some light, which matters less in winter, when most plants under glass are barely growing.

Best for: greenhouses and cold frames from autumn to spring, and wrapping pots whose roots would otherwise be exposed.

How to use it:

• Fit it inside the greenhouse while the glass is dry, using clips or fixings made for the frame. Leave the vents free so you can still let air in on mild days.
• Wrap a layer round a large pot that's too big to move, from the rim to the ground. Leave the top of the compost clear so it can still drain.

The details of lining and heating a greenhouse, with a running-cost calculator for heaters, are in heating a greenhouse in winter. Pots have their own guide: protecting pots and containers from frost.

Which cover for which job

Other ways to soften a frost

Covers do most of the work, but a few habits make plants better able to cope.

Water dry soil in the daytime. Moist soil holds more of the day's warmth than dry soil and gives it off overnight. If the ground is dry before a forecast frost, water it in the morning or early afternoon. Don't water in the evening, and don't wet the leaves.

Mulch the roots. A thick layer of mulch, such as straw, bracken, compost or leaf mould, over the roots of borderline plants keeps the worst of the cold out of the ground. It's how gardeners in milder areas get dahlias, salvias and other borderline perennials through winter in the ground. Put it on after the soil has been wet by autumn rain, not when the ground is already frozen.

Stop feeding tender growth. Soft, sappy new growth is the first to be damaged. Stop feeding plants you want to overwinter by late summer, so the growth they have toughens up.

Use walls and shelter. A house wall, especially a south- or west-facing one, gives off some warmth at night and keeps off cold wind. Group pots against it. Plant borderline plants at the foot of it.

Keep the morning sun off. A frozen plant that thaws slowly takes less harm than one hit by bright early sun. If you can, grow borderline plants where the first morning sun doesn't fall on them, or leave fleece on until the frost has lifted.

In the greenhouse

An unheated greenhouse keeps off wind and rain and stays warmer than the garden outside, but it isn't frost-free. On a hard night the inside can still freeze. That's fine for hardy winter salads and autumn-sown broad beans, but not for tender plants.

To make an unheated greenhouse safer:

• Line the glass with bubble insulation in autumn.
• On frosty nights, drape fleece over the plants inside, then take it off in the morning.
• Hang a max/min thermometer to see how cold it really gets. Many greenhouses turn out to stay frost-free on most nights, and you can decide whether heating is worth it.
• Ventilate on mild days. Cold, damp, still air causes more winter losses under glass than frost does, through grey mould.

If you're keeping tender plants such as pelargoniums, fuchsias, citrus or chillies under glass, you'll need a heater on a thermostat set just above freezing. Heating a greenhouse in winter covers the choices and what they cost to run, and the greenhouse heater cost calculator does the sum at today's electricity price. For the greenhouse year as a whole, see greenhouse growing for beginners.

Pots and containers

Roots in a pot are far more exposed than roots in the ground. In the ground the soil around them insulates them; in a pot, cold comes in through the sides from every direction, and a pot can freeze solid. Plants that are perfectly hardy in a border can be killed in a pot, and frost can crack the pot itself.

The basics: move pots together against a house wall, lift them onto pot feet so they drain, wrap large ones in bubble insulation or hessian, and bring tender ones under cover. The full method is in protecting pots and containers from frost.

What not to do

• Don't cover too late. Covers trap warmth from the soil. Put them on before dusk, not at bedtime.
• Don't let plastic touch the leaves. Polythene and bin bags conduct the cold straight through, so leaves pressed against them still freeze. If plastic is all you have, prop it off the plants on canes, and take it off in the morning.
• Don't leave clear plastic on in the sun. Even in winter, plants under clear plastic can overheat and sweat on a bright day.
• Don't leave covers on for weeks in mild weather. Plants need light and air. Damp, dim conditions under a cover that never comes off invite mould.
• Don't knock frost off frozen leaves. Frozen leaves and stems are brittle and easily broken. Leave them to thaw.
• Don't plant out tender crops early in spring. A warm April doesn't mean the frosts are over. Wait until after your typical last frost, and keep fleece handy.

After a frost

If a tender plant has been hit, don't rush to cut it back. For annual crops like courgettes, beans and tomatoes, a frost is the end of the season: pick anything usable and clear the plants away. For perennials, the blackened top growth actually shields the crown below from the next frosts. Leave borderline shrubs and perennials alone until spring, when you can see which stems have new buds, and cut back to those.

Dahlias are the exception: the first frost that blackens their leaves is the signal to cut the stems down and lift the tubers for storage. Overwintering tender plants explains how, along with pelargoniums, fuchsias and chillies.

What you'll need

You can get through a UK autumn with very little: a roll of fleece, a few bricks or pegs to hold it down, and a sheltered spot by the house for pots. Cloches and a cold frame earn their place once you're growing winter salads or overwintering plants in pots. Bubble insulation is worth it for anyone with a greenhouse.

What next

• Check when frost typically reaches you with the first frost checker.
• Work through what to do before the first frost.
• Find out which of your plants need protecting in hardy or tender?
• Bring in the tender ones: overwintering tender plants.
• Keeping plants under glass? Read heating a greenhouse in winter.