Why containers struggle in a British winter

A plant in a border has the whole garden around its roots. Soil at spade depth stays a few degrees warmer than the air and freezes slowly, if at all. A patio pot has none of that comfort. Compost in a container is exposed on every side, so a single sharp night can freeze the entire rootball solid, and repeated freeze-thaw cycles are what actually kill plants rather than one cold snap. Wet is often the bigger enemy here: cold, sodden compost rots the roots of lavender, rosemary, olives, agapanthus and pelargoniums long before frost gets a chance. Wind adds a third problem, drying compost out even in December and stripping moisture from evergreens. So think of winter protection as three jobs at once — keeping roots warmer, keeping compost drier and keeping wind off.

Move pots to a sheltered spot

Before you buy anything, move things. A south or west-facing house wall absorbs heat during the day and releases it overnight, giving you a genuine microclimate. Then group pots together so the containers touch and the foliage interlaces. Grouping cuts wind exposure and shares what little warmth there is. Put the toughest plants on the outside of the cluster and the tenderest in the middle, tucked closest to the wall.

• Raise every pot onto feet, bricks or blocks so drainage holes are never sitting in water on paving. Even two centimetres helps.
• Empty saucers and trays, and turn them upside down or put them away for the winter.
• Keep pots clear of dripping gutters, downpipes and spots where snow slides off a roof.
• Move the most precious containers under a porch, into a cold greenhouse, unheated conservatory or a light shed.
• Check nothing is smothering the crown of the plant. Fallen leaves left to sit wet on top of compost cause rot.

Wrap the container, not the plant

Insulating the pot is one of the most effective things you can do, and it is cheap. A couple of wraps of bubble wrap around the outside of the container, secured with twine or a strip of tape, will buffer the rootball against frost and stop terracotta and glazed pots cracking. Leave the drainage hole clear, and take the wrap off in spring as growth restarts.

The key point is that you are wrapping the pot, not the foliage. Draping bubble wrap over a plant traps damp air around the leaves and invites botrytis. For really cold gardens, hessian, an old blanket, straw packed into a cardboard box with the pot stood inside, or a jacket of straw held in place with netting all work well. If you have a spare patch of soil, sink the pot up to its rim in the ground and mulch over the top — the earth is the best insulator there is, and roots will come through winter far better for it.

Cut back on watering and feeding

Overwatering is the single commonest way to lose container plants over winter. Growth has stopped, so roots use a fraction of the water they did in July. Water only when the compost is nearly dry at knuckle depth, and only on a mild morning when the temperature is above roughly 5°C. Never water onto frozen compost: it sits on the surface and freezes around the crown of the plant.

Stop feeding completely from October until March. Feeding now pushes soft, sappy growth that is the first thing frost destroys. If you have plants under cover, you control the moisture, which is a big advantage — a cold greenhouse or porch keeps the worst of the rain off and lets you water sparingly when it is genuinely needed.

Fleece covers for frosty nights

Horticultural fleece is your overnight armour. Go for a weight of 17gsm or more, and double it for anything tender or when a hard frost of -5°C or below is forecast. Better still, drape it over a simple frame of bamboo canes, hoops or a cloche so the fabric is not resting on the leaves. Wet fleece lying against foliage can freeze to it and do more damage than the cold alone.

• Secure covers with pegs, bricks or twine so wind cannot rip them loose.
• Remove covers in the morning, or at least on mild, still days, to let air circulate and prevent mould.
• Old net curtains, hessian and a double layer of bubble wrap over a frame all do the same job.
• For alpines and succulents, a sheet of glass or a clear cloche that keeps rain off the crown is more useful than insulation.

A plant left sealed under cover for weeks on end with no ventilation will rot faster than it would have suffered from frost.

Know your plants' limits

Some plants simply will not take a UK winter outdoors, whatever you do. Pelargoniums, tender fuchsias, begonias, citrus, olives and many aeoniums are among them. If you can, lift them into a bright, frost-free spot — a windowsill, conservatory or cool spare room. Pelargoniums can be kept almost dry and half-dormant in a light shed; citrus need light and will drop leaves in the dark. If moving them is impossible, treat them as annuals and take cuttings in late summer as insurance. As a rough guide, plants rated H1 and H1c need frost-free conditions, H2 will take 1–5°C, and H3 copes with about -5°C. Check the label, mulch the crowns with grit or bark, and give the whole patio one last tidy before the cold settles in.