How to protect plants from frost, from fleece to an upturned bucket

When frost is forecast, cover tender plants in the evening and uncover them the next morning. Horticultural fleece is the easiest cover, but an upturned pot, a cardboard box or an old bedsheet does the job for a night or two.

The rest of frost protection is timing: knowing which nights are risky, and keeping tender crops indoors until after your last frost date.

Which nights are risky

Frost forms on clear, still nights, especially after a sunny day. Without clouds, the warmth stored in the ground escapes into the sky. Cloudy or windy nights rarely bring frost, even when they’re cold.

Check the forecast in the afternoon. If the overnight low is 3 °C (37 °F) or below, cover tender plants. Frost forms at ground level before the air at thermometer height reaches zero.

Some spots get frost more than others. Cold air flows downhill like water and settles in hollows, at the bottom of slopes and against solid fences. Those frost pockets can stay frosty a week or two later in spring.

Ways to cover plants

• Horticultural fleece. Light enough to lay straight over plants, and lets light and water through. One layer gives a couple of degrees of protection; two layers give more. Peg or weigh down the edges.
• Cloches. Glass, plastic or a cut-off drinks bottle over single plants. Open them on sunny days so plants don’t overheat.
• Cold frames. A box with a clear lid. Close it at night and on cold days, and cover with an old blanket or sacking on very cold nights.
• Upturned pots, buckets and boxes. Quick cover for single plants on one cold night. Remove them in the morning, as plants need light.
• Old sheets and blankets. Drape over larger plants or rows, ideally over stakes so they don’t crush the leaves.

Put covers on in the late afternoon to trap the day’s warmth, and take them off in the morning once the frost has lifted.

Other ways to help

• Water during the day. Moist soil holds and releases more heat than dry soil.
• Mulch. A layer of straw or compost around the base protects roots and crowns.
• Move pots. Shift containers against a house wall, into a porch or a greenhouse.
• Don’t feed late in the year. Soft new growth is the most easily damaged.

Which plants need it

Tender crops are killed or badly set back by any frost: tomatoes, peppers, chillies, aubergines, courgettes, squash, cucumbers, sweetcorn, beans and basil. Keep them indoors or under cover until after your last frost, and protect early plantings.

Half-hardy plants, such as early potatoes, survive a light frost but their leaves get damaged. Cover them, or draw soil over young potato shoots.

Hardy crops like kale, leeks, Brussels sprouts, parsnips and garlic cope with frost, and some taste sweeter for it. They only need protection in very hard frosts.

Spring and autumn frosts

In spring, the risk is a late frost after you’ve planted out. Waiting a week or two past your average last frost date is the simplest protection of all.

In autumn, the first frost ends tender crops. If one is forecast, cover the plants for a few more weeks of picking, or harvest everything that’s ready beforehand.

After a frost

Don’t rush to cut off damaged growth. The blackened leaves protect the growth below if there’s another cold night. Wait a week or so, then trim back to healthy growth.

If plants were caught uncovered, mist them with water before the sun reaches them. Thawing slowly does less damage than thawing fast.

Your own last frost date is the key number: see the last frost in the UK, and hardening off seedlings for getting plants ready to go out.