Protecting Container Plants from Winter Frost

Why Pots Feel the Cold Far More Than Borders

A plant in open ground has a whole garden's worth of soil beneath it, acting as a slow-release heat store. A plant in a pot has a bucket of compost and nothing else. Frost can attack from the top, the sides and the base all at once, and a modest 15-litre container can freeze solid right through to the middle.

There are two risks, and people usually worry about only one. The first is root damage: once the compost around the roots drops below freezing, the water inside the root cells expands and ruptures them. The second, and often the one that finishes a plant off, is waterlogging. Cold, saturated compost in winter is slow to drain, and roots sitting in it rot quietly long before the spring.

Terracotta adds a third. It is porous, so it soaks up moisture, and when that water freezes it expands. That is how you get the tell-tale flaking and hairline cracks down the side. Solutions after the frost do not exist, but preventions are simple and take an afternoon.

Move Pots Into a Sheltered Microclimate

The first and cheapest job is to move containers to a kinder spot. A south- or west-facing house wall is the warmest place in most British gardens; the brickwork absorbs heat through the day and releases it through the evening. Try to get pots within about 60cm of the wall, where the benefit is strongest.

• Group pots together. Clustered containers shelter one another, and the mass of compost holds warmth far better than a single pot standing alone. Put the hardiest plants on the outside and the tender ones in the middle.
• Avoid the soggy spots. Step away from the bottom of a downpipe, the sloped gully by a gate, and any corner where water pools on paving after rain.
• Think about wind before temperature. A drying east wind in January does more harm to evergreens than a short cold snap. If you have a choice, keep bay, olive, rosemary and camellias where the prevailing wind is broken.
• Watch overhanging trees. A pot under a dripping canopy or a branch that dumps snow stays damper and colder than one in the open.

Raise Every Pot Off the Ground

If you do only one thing this winter, do this one. Standing pots directly on paving, decking or a path is the leading cause of winter losses, because water cannot escape and the cold conducts straight up from the frozen surface beneath.

Pot feet are ideal, but bricks, timber offcuts, broken slabs and even two short battens underneath will do. You are aiming for 2–3cm of clear air under the drainage holes. Water drains freely, air circulates, and the base of the pot is no longer glued to a slab of ice.

Remove saucers and drip trays for the winter — they fill with water and never drain — and check that the drainage holes are not blocked by a pan of old, compacted roots. If a hole is sealed, ease it open with a bamboo cane from underneath.

Wrap and Insulate Terracotta Containers

Terracotta, and the thin plastic pots that imitate it, need wrapping. Bubble wrap is the most effective and the least attractive; hessian or an old wool blanket tied with twine looks far better against a wall and insulates perfectly well. Two layers of either is plenty.

Wrap the sides and, if you can, the base of the pot. Do not cover the drainage holes, and do not wrap over the crown of the plant or bury the stems — damp trapped around the base of a plant is an invitation to rot. Tie the wrapping loosely so air still moves.

Small pots are best treated differently. Plunge them into a larger empty pot padded with scrunched newspaper, tuck them into a bag of multi-purpose compost, or sink the whole thing into a spare corner of the vegetable bed and mulch over the top. A 5cm layer of bark chippings or horticultural grit over the compost surface keeps the crown dry and insulates the roots below; leave a small clear ring around the stems. Dark glazed pots have a small advantage here, as they absorb more heat during the day.

The Potting Shed and Allotment Retreat as a Winter Refuge

An unheated potting shed, garage or the back of an allotment cabin will carry a surprising number of tender plants through to March. Frost still gets inside, but wind, rain and the freeze-thaw cycle do not, and that makes the difference between a plant that sulks and one that dies.

• Insulate the bench. A sheet of polystyrene or an old pallet under the pots stops cold seeping up from the floor.
• Cluster and layer. Push pots together and stuff crumpled newspaper or bubble wrap between them.
• Ventilate on mild days. Open the door for an hour when it is above freezing to let damp air out and stop mould settling on foliage.
• Mind the mice. Wrapped pots and hessian make excellent nests, and stored compost sacks are a larder. Check pots periodically and keep traps or a covered bait station in the corner.

Water very sparingly in a shed. Compost there dries slowly, and most plants would rather be a little dry than sitting wet in the cold.

Tending Pots Through the Frosty Months

Winter care is mostly about doing less, but doing it at the right moment.

Only water when the compost has genuinely dried out and the forecast is for a mild day. Water in the morning, never in the evening, so surplus drains away before the temperature drops. Do not feed — any soft growth you encourage now will be the first thing damaged. Check pots standing in the open after gales, and re-tie any wrapping that has come loose.

Snow is your friend on the compost surface: it insulates beautifully against the next hard frost. On the foliage it is another matter, so knock heavy wet snow off evergreens, conifers and clipped bay before the branches splay. Pick a dry, above-freezing day to do it, rather than bending frozen stems.

Come early spring, hold your nerve. Leave the wrapping and the mulch in place until the hard frosts have passed and you can see new growth starting, then unwrap, top-dress the compost and give everything a gentle feed. A little preparation now means your pots, your shed and your allotment corner will all come back into leaf without a single gap on the bench.