Why pots are the toughest place to spend a British winter

A plant in the ground has the whole mass of the soil around it, and soil holds on to a surprising amount of warmth. A plant in a pot has a few litres of compost exposed to cold air on every side and to frost from below. That is why the same plant that sails through winter in a border can be killed outright in a container, and why a small pot is the most vulnerable thing in the garden — a two-litre pot can freeze solid in a single hard night.

Terracotta makes matters worse. It is porous, so it soaks up water, expands as that water freezes and can split cleanly down the side. Sometimes the pot goes and the plant survives; more often it is the other way round. As a rough rule, treat a container plant as one or two notches less hardy than the same plant in the ground, and plan accordingly.

Move the tender ones under cover, and move them in order

Start with the plants that would be genuinely painful to replace: citrus, olives, aeoniums, pelargoniums, tender fuchsias, agapanthus, small bays, cannas, hedychium and anything you bought as a summer treat. Get them in before the first proper frost rather than the morning after it.

An unheated greenhouse, a conservatory, a cold frame, a cool porch or a garage with a window are all ideal. The aim is cool and bright, not warm and cosy — a temperature of around 5–10°C suits most of these plants perfectly. Heat them up and you will get pale, leggy growth, an early flush of aphids and a miserable plant by March.

• Tender evergreens first, because their leaves keep losing moisture all winter and scorch quickly in cold wind.
• Deciduous tender plants next — cannas, hedychiums and bananas can be lifted, cut back and stored in barely damp compost.
• Ventilate on mild, still days to keep the air moving and mould at bay.
• A dark shed is a last resort. If that is all you have, keep those plants almost dry and check them fortnightly for rot.

Insulate the container, not just the foliage

For pots that are staying outside, the trick is to protect the rootball from all directions, not just the top. A few minutes with a roll of bubble wrap and some string pays for itself many times over.

• Wrap two layers of bubble wrap around the pot, tied firmly but not tightly, leaving the top open so the compost can breathe and rain can drain.
• Raise every pot onto feet, bricks or a couple of battens so water never sits against the drainage holes.
• Group pots together, shoulder to shoulder. They shelter one another, and the mass of compost freezes far more slowly than a lone container.
• Stand them against a house wall, ideally south- or west-facing, where stored heat leaks out overnight.
• Mulch the compost surface with a couple of centimetres of bark or straw, keeping it clear of the stems.

Wrap the pot, not the plant, in plastic. Anything that seals in moisture will encourage mould and rot, which kills more overwintering plants than cold ever does.

Wrap the tops of borderline plants

Horticultural fleece is the gardener's best friend for foliage, but it works far better stretched over a simple frame of canes than pressed against wet leaves. For tree ferns, stuff the crown with dry straw and hold it in place with a little fleece or an old net curtain; the crown is the growing point and the only part that truly matters. Tie cordyline leaves upright in a bundle so water runs off rather than pooling and rotting the crown.

For bananas, cut the leaves back and cage the trunk with wire netting packed with straw. Palms and tender shrubs appreciate a fleece jacket, removed on mild days so the plant can breathe. Never use bin bags or plastic sheeting directly on foliage — you will trap condensation and lose the plant to rot.

Cut back the watering — that is half the battle

Growth has stopped, light levels are low and compost in a pot stays wet for weeks. More container plants drown in winter than freeze, so ease right off.

• Under cover, water sparingly — perhaps once a fortnight — and only when the top five centimetres of compost feel dry.
• Outside, let the rain do the work, but empty saucers and trays so nothing stands in water.
• Never water when frost is forecast. Wet compost freezes harder and does far more damage than dry compost.
• Stop feeding altogether until spring. Fertiliser encourages soft growth that frost will shred.

Keep checking, and be ready to act on a cold night

Once a week, walk the garden with a torch and look for trouble: grey mould on leaves, soft brown patches at the base of stems, slugs and snails tucked into pot rims. Clear fallen leaves, which shelter pests and hold damp against stems, and open greenhouse vents whenever the weather allows.

Keep fleece, bubble wrap and string within easy reach, and watch the forecast. On the afternoon before a clear, still, freezing night, that is the moment to act — an extra layer put on before dusk does far more good than a rescue attempt the next morning. A little foresight now, and your pots will come through to spring ready to grow away strongly.