Simple Ways to Protect Tender Plants from Frost

Understanding what frost actually does

Frost is deceptively quiet. You go to bed with a mild evening and wake to a garden that looks as though someone has passed a flame over the softest leaves. What has happened is simple enough: on a clear, still night, heat radiates away from the ground and from plant surfaces, and the temperature of leaves can drop several degrees below the air temperature around them. Water inside the plant cells freezes, ice crystals expand, and the cell walls rupture. By morning the damage is already done.

It helps to know the difference between an air frost, when the temperature measured in a standard screen at around 1.5 metres falls below freezing, and a ground frost, when the surface of grass or soil dips below zero while the air above stays just above it. Ground frost is far more common in the UK, especially in spring and autumn, and it is quite capable of blackening young potato shoots or scorching the tips of a tender shrub. Low-lying spots are worst: cold air drains downhill like water and pools in hollows, behind walls and at the bottom of slopes. If your plot sits in one of those frost pockets, you will need to be a little more vigilant than your neighbour up the hill.

Start with the soil and the roots

Before you worry about leaves, look after what is underground. Roots are hardier than top growth, but they still suffer when the soil freezes solid around them, and a plant that loses its roots cannot recover no matter how well its foliage survived.

A generous mulch is the simplest insulation you can give them. Spread a layer of well-rotted compost, bark chippings, leafmould or straw roughly 5–7cm deep over the soil surface around your plants. This traps air, slows heat loss from the ground, and keeps the soil temperature steadier through the night. Do keep the mulch clear of stems and crowns — piling it against the base of a plant can trap damp and encourage rot, which is a worse fate than a light nip of frost.

It is also worth knowing that damp soil holds heat better than dry soil. If a cold night is forecast and the ground is dry, water thoroughly during the morning, not the evening. Wet foliage at dusk is a gift to frost, but moist soil below ground will release warmth slowly overnight.

Fleece, cloches and makeshift covers

For anything above ground, a physical barrier is the answer. Horticultural fleece is the workhorse of the frost-protection toolkit. A single layer of lightweight fleece will buy you roughly two to three degrees of protection; doubling it, or choosing a heavier grade, adds a little more. Drape it loosely over plants and weight the edges with stones or pegs so it does not blow about, but avoid pulling it tight against the leaves — the trapped air layer is what insulates.

• Cloches are excellent for individual plants, especially young seedlings and salads. A traditional glass cloche is beautiful, but a plastic bell cloche or even a cut-down clear drinks bottle does the job just as well.
• Fleece tunnels on hoops suit rows of vegetables and low-growing flowers, and are far quicker to move than individual covers.
• Bubble wrap wrapped around a frame makes a serviceable temporary cold frame for a few precious plants.
• Hessian or old bedsheets will do at a push, though they are heavy and let less light through, so remove them promptly in the morning.

Whatever you use, take covers off during the day. Plants need light and air, and a closed cloche left in place through a sunny morning can cook tender growth just as effectively as frost.

Pots, containers and smaller plants

Plants in containers are far more vulnerable than those in the ground because their roots have only a thin wall of compost and a pot between them and the freezing air. Three quick moves make a real difference. First, raise pots onto feet or bricks so they are not sitting on freezing paving. Second, wrap the pot in bubble wrap, hessian or an old blanket, tied gently with twine. Third, group pots together against a house wall or in a sheltered corner, where they will benefit from radiated warmth and protection from wind.

For borderline-hardy shrubs in the ground, a simple teepee of fleece or a wrap of hessian around the crown will often see them through. Tender tree ferns and bananas benefit from having their growing points packed with straw and the fronds tied up and wrapped.

Watering, timing and small habits that help

Frost protection is as much about routine as equipment. Make a habit of checking the forecast each afternoon, and if a cold night is coming, get your covers on before dusk — the aim is to trap the day's warmth, not to fight a battle you have already lost. Harden off plants gradually rather than flinging them outside, and never feed tender plants in cold weather; soft, sappy growth produced by a late feed is the most frost-sensitive tissue of all. Keep the ground around vulnerable plants clear of debris, since a clear soil surface warms more readily during the day and releases that warmth at night. Finally, resist the urge to cut back frosted growth straight away. Those damaged leaves and stems provide a little extra shelter, and you will be able to see clearly what is truly dead once new growth appears in spring.

If frost catches you out anyway

Sometimes it will, however careful you are. Do not panic, and do not rush out with the secateurs. Wait until the plant has thawed naturally and the sun is off it, then assess the damage. Blackened, mushy growth can be trimmed back to healthy tissue, but if you are unsure, leave it until spring. Many plants that look ruined in the days after a frost will surprise you by pushing up fresh shoots from the base. A little patience, a little fleece, and a good mulch will carry most gardens through the coldest nights the UK can offer.