Why Pots Are Harder Than Borders in Winter

A plant that shrugs off a Cumbrian January in open ground can be killed outright by the same weather in a pot. It is not the plant's fault. A container holds a small volume of compost, and that compost is exposed on every side: to the air, to the wind, and to the cold slab of paving beneath it. In a border, roots sit deep in a mass of soil that rarely drops below about 4°C. In a pot, the roots are the outermost thing in the party.

So the job of winterising is really three jobs at once: keeping crowns out of standing water, keeping rootballs from freezing and thawing repeatedly, and keeping the pots themselves in one piece until spring.

Raise Every Pot onto Feet

The simplest and most effective thing you can do, and it takes an afternoon. A pot sitting flat on paving or a patio slab blocks its own drainage hole. Water cannot escape, so it backs up, and the bottom third of the compost becomes a cold, airless sump. Saturated compost freezes harder and stays frozen longer than damp compost, and roots in that zone rot quietly through the winter.

• Use terracotta pot feet, three per pot, or timber offcuts, slate pads, or a couple of bricks set on edge.
• Aim for 2–4 cm of clear air under the base, enough for water to run out and air to move.
• Group smaller pots together on a shallow gravel tray. They shelter one another and reduce wind-rock.
• Empty every saucer. A saucer left out is a reservoir, and a reservoir in December is lethal to ferns and hellebores alike.

Large pots that must stay put are worth tilting slightly on a wedge over the worst of the winter, so the drainage hole becomes the lowest point.

Wrap Terracotta and Other Porous Pots

Terracotta is beautiful and porous, which is exactly what makes it vulnerable. Water soaks into the clay, expands as it freezes, and flakes the surface — spalling — or splits the pot outright. Glazed pots can go the same way if water creeps into the glaze or the rim. Wrap them.

• Bubble wrap around the sides, two layers on large pots, tied with twine rather than tape so it comes off easily.
• Hessian or sacking for a more respectable look against a doorstep; it insulates less well but keeps frost off the clay.
• Keep the top of the pot open. A permanently covered compost surface stays wet and encourages rot and mould.
• Move lighter pots against a house wall, into a cold frame, or under the eave of a shed where rain rarely reaches.

Watering Less — But Not Never

This is where most container plants die in winter, and usually from kindness. Growth has stopped, so demand for water drops to almost nothing, and a plant sitting in cold wet compost has no way to use it. Water far less often than in summer — but do not abandon the watering can entirely.

• Check by weight. A light pot needs a drink; a heavy one does not.
• Water in the morning, never in the evening, so any surplus drains before the night frost.
• Evergreens, including most ferns, continue to lose moisture through their leaves in mild spells and drying winds, and their roots cannot replace it from frozen compost.
• Never water when the compost is already frozen solid. Wait for a thaw.

Ferns and Other Shade Plants in Containers

Ferns are the great decorative workers of a shady garden, and most are bone hardy — but in pots they need their crowns protected. Deciduous kinds such as lady fern and male fern die back to a knuckle of dormant buds at compost level, and that knuckle is the vulnerable part. After the fronds collapse, mulch over the crown with 5–8 cm of bark chip or leaf mould, leaving a small gap around the very centre so it does not smother.

Evergreen sorts — hart's tongue, polypody, the shield ferns — keep their fronds and so keep losing water. They scorch badly in a drying east wind, and a single bright frosty morning can burn the foliage of a fern that spent all winter in full sun. Put them where they would be in nature: against a north or east wall, under a deciduous shrub, in deep shade. A dry fern in a frozen pot is a fern you will be replacing in March.

Shade perennials in pots — epimedium, hellebore, tiarella, wood anemone — all benefit from the same treatment: raised, wrapped, grouped, and watered sparingly. Cut back dead top growth in late winter so slugs and vine weevil have nowhere to hide, but leave the old foliage on hellebores until February to protect the emerging flower buds.

Through to Spring

Check your pots after every hard spell. Look for shattered rims, lifted crowns, and compost that has shrunk away from the sides of the pot, which lets cold air straight down to the roots. Firm gently and top-dress with fresh compost in February. Start watering properly again in March, when new growth appears, and repot anything that has been in the same container for three winters.

Do that much and your pots — and the ferns in them — will come through a wet British winter ready to unfurl. Winterising is not fussing. It is simply giving a small, exposed rootball the same protection the open ground gives for free.