Act before the first hard frost

In most of the UK, the first proper frost of winter arrives somewhere between late October and early December. Gardens in the north, in frost hollows and on heavy clay will feel it first; coastal Cornwall, west Wales and central London often hold out a fortnight or more longer. But the exact date matters less than the principle: tender plants are already suffering when the thermometer drops to 5°C, long before the ground turns white.

So pick a dry, mild afternoon in mid-autumn and do the work then, rather than dashing out in a panic at dusk with a torch. Compost that freezes solid in a pot is very hard to bring back, and damage to roots and growing tips is usually irreversible. Everything below is easier, kinder and more effective if done while plants are still actively growing and the soil is still warm.

Move pots under cover

Container-grown tender plants have the great advantage of being portable. A greenhouse, cold frame, unheated conservatory, porch, shed or garage will all do, but match the plant to the space.

• Dormant plants such as dahlias, cannas and agapanthus are happy in a dark but frost-free shed or garage.
• Evergreens such as citrus, olives, bay and pelargoniums need light, so a greenhouse, porch or conservatory suits them far better.
• Clean the glass first. Every bit of dirt costs you light, and light is what carries plants through winter.
• Group pots closely together to share warmth and shelter each other from wind, but leave enough room for air to move between them. Packed, stagnant air invites grey mould and botrytis.
• Raise pots onto feet, bricks or slats. This keeps drainage holes clear, deters slugs and stops worms blocking the base.

If a plant is too heavy to move, drag it to the shelter of a south-facing wall or under the eaves of the house. A little reflected warmth and protection from driving rain makes a real difference.

Wrap and insulate containers

Roots in a pot are far more vulnerable than roots in the ground, because a container freezes from all sides. Insulating the pot is one of the simplest and most rewarding jobs of the winter.

• Wrap the outside of the pot in bubble wrap or hessian, secured with twine or tape. Insulate the sides, not the top, and never seal the drainage hole.
• Stand pots on a piece of polystyrene or a thick plank to lift them off freezing paving.
• Mulch the surface with 5-8cm of bark, leaf mould or compost to protect the crown of the plant.
• For tree ferns and bananas, gather the fronds upwards, tie them, and pack the crown with straw or fleece before wrapping the whole head.
• Use horticultural fleece over foliage on evergreens and tender shrubs, held in place with clothes pegs. A double layer is surprisingly effective, and it lets light and air through.

Avoid plastic sheeting directly over leaves; it traps condensation, which does more harm than the cold itself. Terracotta and glazed pots can crack in a hard freeze, so if you cannot wrap them, at least move them out of the wind and off the ground.

Ease off the watering can

Overwatering is the single biggest killer of overwintering plants. Growth has slowed to a crawl, so roots use very little water, and wet compost in a cold greenhouse turns to a sour, rotting sludge.

• Water sparingly, perhaps once every two to three weeks, and only when the compost feels dry a few centimetres down.
• Remove saucers and trays so pots never stand in water.
• Stop feeding entirely from late autumn. Feeding encourages soft new growth that a frost will destroy.
• Water in the morning, never in the evening or on a frosty day, so any excess can drain away before nightfall.
• Keep cacti and succulents almost bone dry until spring.

Ventilate the greenhouse on mild, sunny days, opening a window or door for an hour or two. Moving air is your best defence against mould, and it costs nothing.

Look after plants in the ground

Some tender plants are staying put, and they need their own kind of care. Spread a generous mulch of leaf mould or compost over the root zone of anything borderline hardy, keeping it clear of stems so it does not smother the crown. Erect a windbreak of netting or fleece on the exposed side, because cold wind damages far more plants than cold still air.

Wrap the trunks of young or tender trees with hessian, and protect low-growing plants with a cloche. Try not to walk on frozen beds, as this can damage roots and crowns beneath your feet. If snow settles on evergreens, knock it off gently before the weight snaps branches.

After the cold snap

When the worst has passed, resist the urge to tidy everything at once. Check plants for grey, fuzzy mould and remove affected leaves and stems promptly. Do not prune frost-damaged growth straight away; the dead tissue shields what lies beneath, so wait until spring to cut back to healthy wood.

Keep watering minimal and ventilation generous. As light levels rise in February and March, plants will begin to stir again. That is the moment to resume gentle watering and feeding, and to start hardening off anything you plan to move back outdoors. Patience now means strong, healthy plants and a garden ready to romp away when the weather finally turns.