Why Containers Need a Different Approach

A plant in the border has the whole mass of soil around its roots, and that soil rarely drops below freezing more than a few centimetres down. A container plant has none of that buffer. Its rootball is exposed on every side, freezing from the outside in, and in a wet British winter it is far more likely to be sitting in cold, sodden compost than frozen solid. Waterlogged roots at three or four degrees rot quietly, and by spring the plant is hollow and dead even though it never saw a hard frost.

The aim of winter protection is simple: keep the rootball insulated, keep the compost just damp rather than soaked, and keep the worst of the wet and wind off the top growth. It is far easier than it sounds.

Wrap the Pot Itself

Bubble wrap is the amateur gardener's best friend from November to March. It is cheap, it traps air, and it insulates the part that really matters: the rootball and the pot wall around it.

• Wait until the first proper cold snap is forecast, usually late November, then wrap pots of tender plants such as olives, citrus, bays, pelargoniums and agapanthus.
• Use two or three layers around the pot, with the bubble side facing inwards against the pot wall, so the air pockets sit where the cold will hit first.
• Keep the wrap level with the rim or just below it, and make sure it never covers the drainage holes or blocks the surface of the compost. Trapped water under plastic is worse than no protection at all.
• Tie it in place with twine rather than tape, so you can loosen it quickly if a mild spell arrives and the compost needs to breathe.

A terracotta pot needs this more than a plastic one, because terracotta is porous: water soaks into the wall, freezes, expands and splits the pot. If you have a treasured pot, wrapping it is cheap insurance.

Finally, top-dress the compost with a couple of centimetres of grit or bark chippings. It keeps the crown of the plant dry and stops heavy rain compacting the surface.

Lift Pots Off the Ground

This is the single most useful thing you can do, and the one most often skipped. A pot standing flat on paving, gravel or soil has its drainage holes sealed against a wet surface. Rain runs down the sides, pools underneath, and the bottom few centimetres of compost stay saturated for weeks.

• Stand every container on purpose-made pot feet, or on bricks, slate offcuts, wooden battens or even an upturned saucer.
• Aim for a gap of at least two to three centimetres under the base so air can move and water can run away freely.
• Remove drip trays and saucers for the winter. They collect exactly the water you are trying to get rid of.
• Group pots together in a sheltered corner, close but not touching, so they shelter each other and you can cover the whole cluster with a single sheet of fleece in a cold snap.

Lifting also helps with slugs and snails, which will happily use a pot base as a winter hotel.

Water Less — But Not Never

This is where most container plants are lost. Growth has stopped, days are short, and the compost dries out very slowly, so watering should drop to a trickle. But dormant is not dead, and evergreens such as bay, olive and rosemary continue to lose moisture through their leaves all winter.

• Check before you water by pushing a finger in to the second knuckle, or by the weight of the pot. A light pot needs a drink; a heavy one does not.
• Water sparingly, mid-morning on a mild day, so any surplus has time to drain before the night's cold arrives.
• Never let pots stand in water, and never water when the compost is already cold and wet.
• Plants under the shelter of a house wall or in a greenhouse may need the opposite treatment: check them fortnightly, as rain never reaches them.

The target is compost that feels like a wrung-out sponge — damp, never dripping.

Extra Care for the Tenderest

Some plants want more than insulation around the roots. Tree ferns, citrus and tender fuchsias appreciate a collar of straw or fleece packed around the crown, and tree ferns benefit from their fronds being tied up and the crown stuffed with dry straw. Move pots of the most tender subjects into a greenhouse, conservatory or cold frame before the first frost rather than after it.

If a hard frost is forecast, throw a double layer of horticultural fleece over the top growth and anchor it with pegs or bricks. Remove it during the day once the worst has passed, since trapped damp and stale air encourage mould. After every winter storm, do a quick round with a broom to clear snow and fallen leaves off the tops of pots and fleece, because a heavy, wet blanket is as damaging as the cold itself.