Why Containers Are Always the First to Suffer

Plants in the ground have the whole earth to insulate them. The soil below the surface stays surprisingly mild, and roots can retreat downwards when a cold snap arrives. A plant in a pot has none of that luxury. Its root ball sits above ground, exposed to frost on every side, and it is usually surrounded by far less compost than a border would offer. The result is that a plant rated as hardy in open ground can be killed outright in a container during the same winter.

Add wind, which strips moisture from evergreens that cannot replace it from frozen compost, and heavy winter rain, which turns free-draining mixes into a sodden, airless mass, and you have the three main threats: freezing, drying and rotting. The good news is that all three are easy to manage with a little forward planning. Most winter losses in pots are entirely preventable.

Work Out What Actually Needs Protecting

Before you start wrapping things up, sort your containers into groups. It saves time, materials and space.

• Genuinely hardy shrubs, trees and perennials, such as most acers, viburnums and hardy herbs, need little more than a raised pot and sensible watering.
• Borderline hardy plants, including bay, olive, rosemary, agapanthus, hardy fuchsias and many grasses, usually survive outdoors but benefit from a sheltered spot and an insulated pot.
• Tender subjects such as pelargoniums, citrus, aeoniums, tree ferns, cannas and coleus need a frost-free or nearly frost-free place, or a serious wrapping regime.
• Terracotta and glazed pots deserve protection in their own right. Wet terracotta expands as it freezes and splits, so move empty or vulnerable pots under cover.

Be honest about your microclimate. A cold, exposed garden in the north of Scotland is a very different proposition from a sheltered courtyard in Cornwall, and a plant that sails through one may struggle in the other.

Move Pots to Shelter Before the Cold Bites

Position is the cheapest protection you will ever buy. Move containers close to the house, ideally against a south or west-facing wall, where the masonry releases stored warmth overnight. Grouping pots tightly together helps too: the plants shield one another from wind, and a cluster of root balls stays warmer than a row of isolated ones.

If you have an unheated greenhouse, cold frame, porch or garage with a window, use it. Even a shed will do for fully dormant plants, provided it stays dry. Evergreens need light, so reserve the darkest corners for things that have died back completely. Bring tender pots indoors early, before the first hard frost rather than after it, and check them for slugs, vine weevil grubs and fallen leaves as you go.

Insulate and Wrap the Root Ball

The roots are what you are protecting, not the foliage, so focus your efforts low down. Wrap the pot, not the plant.

• Bubble wrap is the workhorse. Wrap two or three layers around the outside of the pot, shiny side inwards, and secure it with string or tape. Leave the compost surface open to the rain and air.
• Hessian or old blankets work well for larger containers, tied around the pot and stood on a layer of the same material.
• Straw or leaf mould packed into a larger outer pot, or heaped around the base of a pot plunged into the ground, gives excellent insulation.
• Fleece or a cloche over the top is useful for evergreen foliage that risks wind scorch, but remove it on mild, still days to let air circulate.

Raise every outdoor pot onto pot feet, bricks or wooden battens. Standing a container directly on paving traps water underneath, and that pooled water is what freezes and cracks pots or rots roots. A gap of a few centimetres makes an enormous difference.

Ease Off the Watering

Overwatering kills more container plants in winter than cold does. Growth has slowed or stopped, so the plant is using very little, and compost that stays wet for weeks suffocates roots and invites rot. Water sparingly, only when the top few centimetres feel dry, and never when the compost is already cold and damp.

Timing matters. Water in the morning on a mild day, so surplus moisture can drain and evaporate before nightfall. Remove saucers and drip trays from outdoor pots so they cannot fill and freeze. Empty any water that collects in the base of decorative outer pots. If a container does freeze solid, do not water it at all until it has thawed completely.

There are exceptions. Evergreens, camellias and anything under the shelter of a house wall can dry out badly during a mild, windy spell, particularly if they are in the rain shadow of the eaves. Check them monthly and give a little water if the compost has gone dusty, but keep it modest.

Check Regularly Through to Spring

Winter protection is not a case of setting everything up in November and forgetting it until March. Walk the garden after storms and hard frosts. Straighten pots that have toppled, replace wrapping that has come loose, and firm back any plant that has been lifted by freeze-thaw cycles.

On mild, bright days, open greenhouses and cold frames for an hour or two to keep the air moving and deter mould. Watch for mould on compost surfaces and grey fuzzy growth on stems, and remove affected material promptly with clean secateurs. As February turns to March, resist the urge to unwrap too early. A single sharp frost after a warm spell does more damage than a whole January of steady cold. Keep the insulation in place until the risk of hard frost has genuinely passed, then remove it gradually, watering and feeding a little more as new growth appears. Your patience will be repaid with plants that start the season strong.