How to Protect Pots and Containers from Freezing

Why Frost Cracks Pots

Terracotta and unglazed ceramic are porous materials, and that is exactly what makes them beautiful — and vulnerable. Over a season, the walls of a pot absorb water from the compost and from rainfall. When the temperature dips below freezing, that trapped moisture turns to ice and expands by roughly nine per cent. The pressure has nowhere to go but outwards, and the pot loses the argument. What starts as a hairline crack in November can be a split, crumbling container by February.

Glazed pots are not automatically safe. If a chip or an unglazed base allows water to creep behind the glaze, the same freeze-thaw cycle can cause the surface to flake or spall. Even pots sold as frost-proof are only frost-resistant to a degree — the phrase means they will survive most winters, not every one. Plastic, resin, fibreglass and wooden containers flex or shrug off the cold, which is why they are the low-risk choice if you garden somewhere exposed. But you do not need to give up on terracotta. The goal is simply to keep water out of the pot walls and stop it pooling around the roots.

Get Containers Off the Ground

The single most effective thing you can do is lift pots off the ground. Paving and soil freeze first and stay frozen longest, and a pot sitting flat on either will have its drainage hole sealed against a sheet of ice. Water has nowhere to escape, so it backs up through the compost and into the pot walls.

• Stand pots on purpose-made terracotta or metal pot feet, or on bricks, slate offcuts or wooden battens.
• Aim for at least 2–3cm of clearance so air can circulate underneath.
• Avoid single narrow feet on tall, heavy pots — three or four wide supports spread the load and stay stable in gales.
• Remove saucers and drip trays for the winter. If you need them for a treasured pot, tip them out regularly and line them with a shallow layer of gravel so the base is never sitting in water.

If your pots stand on a slightly uneven surface, so much the better — a gentle tilt sheds rainwater away from the pot rather than letting it collect beneath it.

Wrap Them for Warmth

Insulation works by trapping a layer of still air, and bubble wrap does this beautifully. Wrap it around the outside of the pot, not over the plant, and use two layers for a large container or a specimen you would hate to lose. Secure it with twine rather than tape — tape goes brittle in the cold and peels away just when you need it.

Alternatives that look rather smarter include hessian, jute sacks, old wool blankets and straw packed into a hessian collar. Whatever you use, leave the top of the compost and the drainage holes clear so the pot can still breathe and drain.

Two more tricks are worth knowing. First, group pots together — they shelter one another and reduce the surface area exposed to wind. Second, push them against a house wall, ideally one facing south or west, where brick and masonry release stored heat slowly through the night. A few centimetres closer to the wall can make a genuine difference on a hard frost night. In severe spells, drape fleece over the top growth, but lift it during the day so foliage does not sit damp and go mouldy.

Move Smaller Pots Under Cover

Anything you can lift without straining should be moved. An unheated greenhouse, a cold frame, a porch, a garage or a shed will all keep the worst of the frost off. Grouping pots inside a cold frame or against a sheltered wall multiplies the benefit.

Light matters as much as warmth. Evergreens, alpines and winter-flowering plants need good light to stay healthy, so a bright, unheated greenhouse or a sheltered porch is ideal. Plants that are fully dormant and leafless — deciduous shrubs and trees, for instance — will tolerate a darker garage or shed, but they will need far less water while they are there. Check them every fortnight or so for mould, slug damage and drying out.

Water Less, Drain Better

More container plants are killed by winter kindness than by winter cold. Growth has stopped, so the plant uses very little water, and soggy compost freezes into a solid block that damages roots and rots crowns. Ease right back on watering from late autumn onwards, and stop entirely during freezing spells.

That said, do not let pots under cover dry out completely. Evergreens continue to lose moisture through their leaves on mild, windy days, and dry roots combined with wind can cause browning and dieback. Water sparingly on a mild morning every few weeks if the compost feels dry at knuckle depth.

Make sure drainage holes are clear before winter sets in. A piece of broken pot or a square of mesh over the hole stops compost blocking it while still letting water through. Never let a pot stand in a puddle, and consider a windbreak — a length of hessian on canes — for exposed, windy sites, since wind chill strips moisture from foliage as surely as frost does.

Choose Frost-Proof Pots and Hardy Plants

If you are buying containers for a cold, exposed spot, look for frost-proof terracotta, which is fired at a higher temperature and is noticeably less porous than the cheap, thin sort. Stone, concrete, fibreglass and good-quality plastic are the most reliable choices. Glazed pots are fine if the glaze covers the inside as well, and if there are no chips. Hand-painted decorative pots are best treated as summer only.

Finally, plant with the pot in mind. Fully hardy shrubs, evergreens, hardy herbs such as rosemary, thyme and sage, hellebores, winter heathers and hardy cyclamen will all come through a British winter outdoors with sensible protection. After snowfall, gently knock the weight off evergreens before branches bend or snap — though a layer of snow on the compost itself is a fine insulator, so leave that in place.