Protecting Young Plants from Late Frosts

Why late frosts are a moving target

In the UK, the last frost of spring can arrive from late March in sheltered Cornish valleys to the end of May in parts of Scotland, northern England and upland Wales. A late frost often strikes after a mild spell, when young plants have produced soft, sappy growth. That tender foliage freezes quickly because it contains plenty of water, and a single night at -2°C can blacken potato shoots, nip broad bean tips or check newly planted sweetcorn.

The key is not to trust a single date. Watch the forecast for clear, still nights, when heat escapes from the soil and cold air settles. Cloud and wind keep frost at bay, but a sudden calm, starry evening after a sunny day is the classic warning sign. If you have tender plants in the ground or waiting to go out, be ready to act before dusk.

Know your garden's frost pockets and warm spots

Cold air behaves like water: it flows downhill and pools in hollows. Walk your garden on a frosty morning and you will soon see where it lingers. Those are the places to avoid for early plantings of courgettes, runner beans, tomatoes, dahlias and sweetcorn. Save them for hardier crops such as brassicas, lettuce, onions and carrots.

• Frost pockets: low ground, the bottom of a slope, behind a solid fence or hedge, and anywhere the soil is heavy clay that warms slowly.
• Warmer spots: south or west-facing walls, patios with stored heat, raised beds near a house, and gentle slopes where cold air drains away.
• Containers: pots are more vulnerable than open ground because roots cannot draw on deep soil warmth. Raise them onto pot feet or bricks and group them together for mutual shelter.

Cloches, fleece and mesh: choose the right cover

A cover traps warmth from the soil and stops the coldest air from touching leaves. The best choice depends on how much protection you need and how long the cover will stay on.

• Glass or rigid plastic cloches: excellent for individual seedlings or short rows. A bell cloche or tunnel cloche can gain 3-5°C on a still night. Vent them during sunny days or plants will cook and become leggy.
• Horticultural fleece: lightweight, breathable and easy to drape over rows or hoop frames. A 17gsm fleece gives roughly 2-3°C of protection; a heavier 30gsm fleece or a double layer gives more. It lets rain and air through, so you can leave it on for several days if needed.
• Horticultural mesh: useful for shielding young plants from wind, birds and insects, but it offers little frost protection on its own. Use it over fleece, or choose fleece when frost is the main threat.
• Bubble wrap and newspaper: good emergency options for pots, cold frames and greenhouse glazing. Wrap containers, not foliage, and lay newspaper over low rows, weighing the edges down with stones or canes.

Secure covers well; flapping fleece rubs leaves and lets cold air underneath. Remove covers in the morning once the temperature rises.

Harden off tender seedlings gradually

Plants raised indoors or in a heated greenhouse have soft leaves and thin cuticles. Moving them straight into open ground is a shock even without frost. Harden them off over 7-14 days, depending on the species and the weather.

• Start with 2-3 hours outdoors in a sheltered, shaded spot on a mild day. Bring them back in at night.
• Increase outdoor time by an hour or two each day, gradually moving them into more sun and wind.
• Reduce watering slightly and stop feeding a week before planting. This slows soft growth and toughens leaves.
• Use a cold frame if you have one: open the lid a little wider each day, then leave it open overnight once nights are reliably above 7-10°C for tender plants.
• Plant out on a dull, calm day and water well. A cloudy day reduces transplant shock; a sunny, windy one can dry seedlings out before roots establish.

Even after hardening off, keep fleece or cloches nearby until the last frost has genuinely passed.

Night-time tactics when frost is forecast

Preparation during the day makes the biggest difference. Water the soil, not the leaves, in the morning if it is dry; damp soil holds more heat than dry soil. Mulch around established plants with compost or bark, but keep it clear of stems. Move pots under cover, or wrap them in bubble wrap and cluster them against a house wall.

• Cover tender seedlings with fleece, cloches or a double layer of newspaper before dusk. Do not wait until midnight.
• Drape fleece over hoop frames so it does not rest directly on wet foliage. If you must lay it on plants, use a light grade and remove it promptly in the morning.
• Place a sealed hot water bottle wrapped in a towel inside a small cold frame or cloche to lift the temperature slightly. Never use open flames or unattended heaters.
• Check plants at dawn. If fleece is frozen to leaves, do not pull it off; wait for it to thaw.

Frost can form even when the air temperature is forecast to be 2-3°C, especially in low, still spots. Trust your garden's feel and past experience.