The first frost of autumn rarely announces itself. A clear, still evening after a mild day, the temperature slides quietly past zero in the small hours, and by morning the dahlias are black and the courgettes have collapsed. Most frost damage happens on just a handful of nights a year — and most of it is avoidable if you know which nights to worry about, which plants are actually at risk, and what to do in the half-hour before dark. This guide covers how to protect plants from frost: how frost forms, why some corners of the garden freeze first, how to keep pots from freezing solid, and what to do the morning after if frost gets there first.
- Danger nights clear sky, little wind, under about 4 °C at dusk and falling
- First frosts usually October–November inland · later near coasts and in towns
- Cover by late afternoon, before the ground loses the day's warmth
- Most at risk tender plants, plants in pots and anything in a low, sheltered hollow
How frost forms — and why still, clear nights are the dangerous ones
Most garden frosts are radiation frosts. During the day the soil soaks up warmth; at night it radiates that warmth back into the sky. Clouds act like a lid and send some of it back down, and wind keeps mixing in slightly warmer air. On a clear, calm night neither happens, the ground and everything on it cools fast, and the air just above the soil can drop several degrees below the official reading.
That is why weather forecasts talk about two kinds of frost. A ground frost means the surface — grass, soil, low leaves — has dropped to 0 °C or below. An air frost means the air at head height has too, which is a harder, more damaging frost. In autumn and spring a forecast of 3 or 4 °C with clear skies very often means a ground frost at plant level, which is all it takes to kill a courgette or a young zinnia.
Ground frost
The surface freezes; the air above may not.
- Common on clear nights forecast at 1–4 °C
- Kills tender seedlings, bedding and low leaves
- A single layer of fleece is usually enough
- Gone within an hour or two of sunrise
Air frost
The air itself is below zero, often for hours.
- Forecast as 0 °C or below
- Blackens dahlias, freezes pots, damages half-hardy shrubs
- Needs a double cover, shelter or moving under cover
- A hard frost (−5 °C or lower) can last all day
Frost pockets: the cold corners of your garden
Cold air is heavier than warm air, so on a frosty night it flows downhill like water and collects in the lowest places. The bottom of a slope, a sunken patio, the foot of a garden that ends in a solid fence or hedge — these are frost pockets, and they can be several degrees colder than a spot only a few metres higher. If you have one tender plant to place, put it halfway up the slope, not at the bottom.
The warmest spots are the opposite: beds against a south- or west-facing house wall, which soaks up heat by day and gives it back at night, and anywhere under the canopy of a tree or a roof overhang, which blocks the radiation to the open sky. A hedge across the bottom of a slope traps cold air above it; a gap at its base lets the cold drain away. Walk the garden on the first frosty morning and notice where the white lasts longest — that is your map for next year.
Know what actually needs protecting
Hardiness is the first question, not fleece. Tender plants — dahlias, pelargoniums, chillies, tomatoes, courgettes, basil — are damaged at or just below 0 °C and need to be lifted, brought in or harvested before the first frost; our guides to overwintering tender plants and overwintering chilli plants cover that in detail. Half-hardy plants survive a light frost but not a hard one; this includes many salvias, penstemons, young autumn-sown seedlings and half-hardy annuals such as strawflowers, which is why they are only planted out once spring frosts are over. Hardy plants need nothing at all: leeks, kale, sprouts, parsnips and most perennials sail through. Some even improve — frost turns starch to sugar, which is why frost makes parsnips sweeter and Brussels sprouts nuttier.
If you buy plants, the RHS hardiness rating on the label is a useful shortcut: H1–H2 need frost-free conditions, H3 tolerates a light frost (down to about −5 °C), and H4 and above are hardy through an average British winter.
The night-before frost checklist
- Check the forecast at teatime Clear skies, light wind and 4 °C or less at dusk: act. Cloud and a breeze usually mean you can relax.
- Water dry soil during the day Moist soil stores and releases more heat than dry soil. Water in the morning or at midday, at the base, never over the leaves in the evening.
- Cover before the warmth escapes Fleece, an old sheet or cardboard over hoops or canes, in late afternoon. Plastic must not touch the leaves — it conducts cold straight through.
- Weigh down the edges A cover that lifts in the night lets the cold air flow in. Bricks, planks or a little soil on the edges.
- Huddle and lift the pots Group containers against the house wall, on pot feet, or move them into the shed, garage or greenhouse.
- Uncover the next morning Once the frost has lifted, remove sheets and cardboard so plants get light. Fleece can stay on for a few days if more cold is forecast.
Fleece buys you roughly 2 °C per layer — enough for a ground frost, not for a hard freeze. For setting up longer-term protection with cloches, low tunnels and cold frames, see our guide to extending the growing season.
Protecting plants in pots
A plant in a pot is far more exposed than the same plant in the ground. Soil in a border is insulated by the earth around it; compost in a pot is surrounded by cold air on every side and can freeze right through, killing roots that would have been fine in the ground. Waterlogged compost freezes harder and expands, which is also what cracks terracotta.
- Raise pots on feet or bricks so water drains away and the base does not sit in a puddle that freezes.
- Group them together against a sheltered house wall. A cluster of pots protects the ones in the middle.
- Wrap the pot, not the plant. Bubble wrap, hessian or an old blanket around the container keeps the roots from freezing; the top growth of hardy plants does not need covering.
- Move the tender ones in. An unheated greenhouse, porch or garage is enough to keep a pot above freezing on most nights.
- Keep compost on the dry side in winter — damp, not wet.
Our guide to container gardening from seed has more on choosing pots and compost that cope with winter.
What to do after frost damage
If frost catches you out, the next morning matters. Do not touch or shake frozen plants: ice crystals in the leaves tear the cells. Let them thaw slowly — a rapid thaw in bright morning sun does more damage than the frost itself, so throw a sheet over plants that catch the first sun, such as camellias against an east wall, until the frost has gone.
On shrubs and perennials, leave the blackened growth in place until spring. It looks terrible, but it shields the buds lower down from the next frost. Cut back to healthy growth in April once the risk of hard frost has passed; you will often find new shoots coming from the base. Check autumn-sown seedlings and new plantings after a hard frost too: frost can lift them out of the soil ("frost heave"), so firm them gently back in with your fingers. Frosted lettuce or chard should be left to thaw on the plant before picking. Tender annuals that have turned to mush are finished — compost them, and collect any ripe seed first.
Common mistakes
- Trusting a forecast of 3 °C. On a clear, still night that often means a frost at ground level.
- Covering too late. A cover put on at 10 pm traps cold air, not warm. Cover in late afternoon.
- Plastic sheeting on the leaves. Where it touches, the leaves freeze anyway. Use fleece, or keep plastic off the foliage with canes.
- Watering in the evening. Wet leaves freeze faster. Water early in the day, at the base.
- Forgetting the pots. Roots in a container freeze long before roots in the ground.
- Pruning off frost damage straight away. The dead growth is protection. Wait until spring.
Frequently asked questions
At what temperature should I cover plants?
Cover tender plants whenever a clear, calm night is forecast at about 3–4 °C or lower. Thermometers and forecasts measure air above the ground; at plant level it can be 2–4 °C colder.
Is it better to cover plants with plastic or fleece?
Fleece. It lets air and moisture through, gives about 2 °C of protection per layer and can rest lightly on most plants. Plastic traps condensation and conducts cold wherever it touches leaves; if you use it, keep it off the foliage and remove it in the morning.
Should I water plants before a frost?
If the soil is dry, yes — during the day, at the roots. Moist soil holds more heat and releases it during the night. Avoid wetting the leaves late in the day.
Will plants recover from frost damage?
Hardy and half-hardy plants usually do: leave the damaged growth until spring, then cut back to healthy buds. Tender plants such as basil, courgettes or dahlias above ground do not — though a dahlia tuber below ground is often still fine if you lift it promptly.
A few metres of fleece, a pile of bricks and ten minutes at teatime will see most gardens through the first frosts of the year — and the hardiest crops will taste better for them. Fill the beds with plants that relish the cold: kale, leeks, Brussels sprouts, corn salad and winter purslane. Browse the full vegetable seed collection, read our autumn gardening tips, or explore the rest of our growing guides.
