Frost is the winter gardener's main enemy, and frost protection is the skill of keeping plants alive through it. A frost damages plants in two ways: ice forms inside the plant's cells and bursts them, and the cold dries the plant out, because frozen soil means the roots cannot take up water to replace what the leaves lose. Tender plants can be killed by a single light frost. Hardy plants can be damaged by a late frost after they have started growing. And even hardy plants can be damaged by frost heaving, where repeated freezing and thawing lifts them out of the soil. This guide explains how frost works, which plants need protection, and the practical methods that actually work.
The first thing to know is that frost is not the same everywhere in a garden. Cold air is heavy, so it flows downhill and pools in hollows and at the bottom of slopes — these are frost pockets, and they can be several degrees colder than a spot a few metres away on a slope or near a wall. A wall that faces the sun absorbs heat during the day and releases it at night, so plants against it are protected. A hedge or fence can shelter plants from cold wind, which makes frost damage worse by drying the plant out. Choosing where to plant is the first and cheapest form of frost protection, and it is worth thinking about before you buy a tender plant.
Which plants need frost protection
Tender plants need protection from the first frost: tomatoes, courgettes, beans, basil, pelargoniums, citrus in pots, and anything from a warm climate. Half-hardy plants — dahlias, fuchsias, some salvias — will survive a light frost but not a hard one, and they need protection or lifting in cold zones. Hardy plants — most trees, shrubs, bulbs and hardy perennials — need no protection in their zone, but they can be damaged by a late frost after growth has started, or by frost heaving if they are newly planted or shallow-rooted. Our growing tomatoes and growing basil guides cover two of the most frost-tender crops, and the zone 5 winter gardening and zone 8 winter gardening guides show how frost risk differs between a cold zone and a mild one.
In zones 11 to 13, which are frost-free, frost protection barely applies — there is no frost to protect against. In zones 9 and 10, frosts are light and infrequent, and a fleece on the coldest nights is usually enough. In zones 5 to 8, frosts are regular and protection is a routine part of winter gardening. In zones 1 to 4, frosts are severe and long, and the honest plan for tender plants is to grow them under cover or bring them indoors — no fleece will keep a tomato alive through a zone 3 winter. Frost dates vary even within a zone, so treat the ranges in this guide as averages and check your local extension service for your area. The frost date calculator gives average first and last frost dates by zone.
Fleece, cloches and covers
Horticultural fleece is the most useful frost protection material for most gardeners. It is a lightweight, breathable fabric that you drape over plants on a frosty night and remove in the morning. It protects by a few degrees — enough to save a plant from a light frost, not enough to save it from a hard freeze. Use it double-layered for extra protection, hold it clear of the leaves with hoops or canes if you can, because fleece that touches wet leaves can freeze to them and damage them, and remove it during the day so the plants get light and air. Do not use plastic sheeting directly on plants: plastic does not breathe, it traps moisture, and it can freeze to leaves and do more harm than good. If you must use plastic, hold it clear of the plant on a frame, and remove it in the morning.
Cloches — bell-shaped or tent-shaped covers placed over individual plants — protect salads, herbs and young plants from frost and wind, and they warm the soil during the day. Cold frames, which are low boxes with a glass or plastic lid, do the same job for a larger area, and they are the best protection for winter salads and early sowings. Keep the lid open on mild days so the plants do not overheat or get diseased in still, damp air, and close it before night. Our winter mulching guide covers the mulch that protects roots, and the winter vegetable care guide covers frost protection for food crops.
Mulch and soil protection
Mulch protects roots, which is where frost does much of its damage. A layer of straw, leaves or bark 7 to 10 cm deep steadies soil temperature, so the ground freezes once and stays frozen rather than freezing and thawing repeatedly, and it stops frost heaving, which lifts shallow-rooted plants out of the soil. Mulch around the base of roses, shrubs and newly planted trees, and over beds of garlic and overwintering vegetables. Keep mulch away from stems and trunks, because mulch piled against a stem holds moisture and invites rot. The mulch calculator works out how much material you need. Water the soil before a frost if it is dry, because moist soil holds heat better than dry soil, and a well-watered plant is less likely to be damaged than a dry one — but do not water when the ground is already frozen.
Containers and tender plants indoors
Plants in pots freeze harder than plants in the ground, because a pot has no earth around it to buffer the cold. Group pots together against a sheltered wall, wrap them in bubble wrap, hessian or fleece, raise them on pot feet so water drains away, and mulch the surface. Move tender container plants under cover before the first frost — into a greenhouse, a cold frame, a porch or a bright indoor spot. Our winter houseplant care guide covers the plants that come indoors, and the winter flower care guide covers container flowers. Check pots after frosts: if a terracotta pot has cracked, it is usually because water in the compost froze and expanded, which is why drainage and pot feet matter.
Greenhouses and cold frames in frost
A greenhouse protects plants from frost, but an unheated greenhouse can still freeze on a cold night, so tender plants inside it still need fleece or a second layer of protection. Ventilate a greenhouse on mild days, even in winter, because still, damp air encourages disease, and close it up before night. A cold frame should be closed at night and opened on mild days. Do not heat a greenhouse with an open flame or a heater that is not designed for the job, and do not leave a heater running unattended without checking that it is safe and suitable. Our greenhouse gardening guide covers greenhouse care through the year.
After a frost: what to do
Do not be too quick to cut back frost-damaged plants. The damaged leaves and stems protect the plant from further frost, and cutting them back in midwinter exposes fresh cuts to the next frost. Wait until spring, when you can see what is alive and what is dead, and then prune back to healthy wood. Do not water a frost-damaged plant heavily, and do not feed it — it needs time, not food. If a plant has been lifted by frost heaving, firm it back gently when the ground thaws. Be patient: a plant that looks dead in February can regrow from the base in May, and many perennials and shrubs recover from frost damage that looks fatal in winter. The garden tasks in February guide covers the late-winter checks that follow a frost.
Common mistakes
The most common mistake is using plastic sheeting directly on plants, which freezes to leaves and does more harm than good. The second is leaving fleece on all day, so plants are starved of light and air and disease builds up. The third is forgetting containers, which freeze harder than beds. The fourth is mulching against stems, which causes rot. The fifth is watering when the ground is frozen, which achieves nothing. The sixth is cutting back frost-damaged plants in midwinter, which exposes them to the next frost. And the seventh is assuming a greenhouse never freezes — an unheated greenhouse can freeze, and tender plants inside it still need protection. If you are unsure about frost protection for a particular plant in your area, check your local extension service for advice.
Frequently asked questions
What is the best frost protection for plants?
Horticultural fleece, used on frosty nights and removed in the morning, is the most useful protection for most gardeners. Mulch protects roots, cloches and cold frames protect small plants, and siting plants away from frost pockets and near warm walls is the cheapest protection of all.
Can I use plastic to protect plants from frost?
Only if it is held clear of the plant on a frame, and removed in the morning. Plastic laid directly on leaves traps moisture, does not breathe, and can freeze to the leaves and damage them. Fleece is a better choice for direct covering.
Should I water plants before a frost?
Yes, if the soil is dry. Moist soil holds heat better than dry soil, and a well-watered plant copes better with frost. Do not water when the ground is already frozen, and water the soil, not the leaves.
When should I remove frost protection?
Remove fleece and covers in the morning, so plants get light and air. Remove winter mulch gradually in spring, when the risk of hard frost has passed. Do not remove everything at once on the first warm day — a late frost after removal can do more damage than no protection at all.
Will my plants recover from frost damage?
Often, yes. Do not cut back frost-damaged growth in midwinter — it protects the plant from further frost. Wait until spring, prune back to healthy wood, and be patient. Many perennials and shrubs regrow from the base even when the top growth looks dead.