Why Terracotta Cracks When the Frost Arrives

Terracotta is fired clay, and for all its charm it is a porous material. Leave a pot out through a wet British autumn and the walls themselves soak up water like a sponge. When the temperature drops below zero, that trapped water expands as it turns to ice, and the pressure it exerts is considerable. Fired clay copes well with being squeezed but very poorly with being pushed apart from the inside, so what begins as a hairline crack in January is often a split pot by February.

The compost inside is the second half of the problem. Wet compost freezes solid and expands, pressing outwards against the sides of the pot with surprising force. A half-barrel of saturated, frozen compost is essentially a slow-motion battering ram. Everything below is aimed at two simple goals: keeping the walls of the pot dry, and keeping the compost free-draining.

Move Smaller Pots Under Cover First

Start with anything you can physically lift, and give priority to pots under about 30cm in diameter. These have thin walls and a small volume of compost, so they freeze right through and thaw again far more quickly than a large container does. A cold greenhouse, an unheated porch, a garage, or even a shed with a window will do nicely. So will a sheltered spot against a south-facing house wall, which holds a little residual warmth and blocks the worst of the wind.

• Group pots together so they shelter one another and reduce wind chill.
• Push them close to the wall rather than leaving them exposed at the front of a patio.
• Stand them on a plank, a piece of polystyrene or a wooden pallet if the floor is bare concrete or stone, which draws heat out of the pot.
• Avoid bringing hardy evergreens into a heated room — they need a cool dormant period, and warm air will push them into soft growth that the spring frosts will shred.

Hardy evergreens such as bay, Skimmia, Sarcococca and Choisya are perfectly capable of sitting out a British winter, but they resent having their roots frozen solid for weeks on end. Shelter is about protecting the rootball, not the foliage.

Raise Every Pot Off the Ground

This is the single cheapest and most effective thing you can do. A pot standing flat on a patio forms a seal around its base, so water cannot escape and simply pools underneath. That standing water is the first thing to freeze, and the base is the first part of the pot to fail.

Pot feet are made for the job, but wooden battens, bricks, slate offcuts and chunks of leftover polystyrene all work just as well. Aim for two to three centimetres of clear air beneath the base, and set the pot on a very slight tilt if you can. Tipping one side up a fraction lets rainwater run off the surface of the compost rather than sitting on it, though don't do this with plants that genuinely like winter wet.

Wrapping Larger Containers

Big pots that you cannot sensibly move need insulation instead. Bubble wrap is the traditional standby: wrap two or three layers around the sides and secure it with twine rather than tape, which perishes in the cold. Leave the top open. The plant still needs light and air, and covering the surface traps damp and encourages mould.

Hessian looks better and breathes more freely, which suits evergreens that dislike a stagnant, humid microclimate. Old coffee sacks, hessian potato sacks and even a length of old carpet wrapped around the base will all do the job. For the very largest containers, pack straw around the sides and hold it in place with a hessian jacket tied at top and bottom.

• Wrap only when the pot walls are dry — wrapping a wet pot simply traps the moisture you are trying to keep out.
• Keep the drainage hole completely clear at all times.
• Mulch the surface with three to five centimetres of grit, bark or straw to insulate the compost and keep the crown of the plant dry.

Compost, Drainage and Winter Watering

Good drainage starts in autumn. Place a single broken crock or a piece of slate over the drainage hole rather than a thick layer of rubble, which can actually hinder flow. Stop feeding in late summer, because soft, sappy growth is the first thing frost damages.

Water sparingly through the winter. Evergreens continue to lose moisture through their leaves on mild, windy days, so a light watering on a mild morning will help — but only when the compost is nearly dry and no hard frost is forecast. Never let a saucer stand full of water, and never water in the evening when the temperature is about to plunge.

If a Pot Cracks Anyway

A hairline crack is not a disaster. Bind it tightly with a couple of turns of duct tape or a strap of wire and it will usually hold for another season or two. A structural split means repotting into something frost-proof in spring, once the risk of hard frost has passed.

Check your containers in late winter rather than midwinter. The thaw does as much damage as the freeze, because water seeps into the cracks and then expands again the following night. Unwrap and uncover once the worst of the frosts are past, usually towards the end of March, and give the surface of the compost a gentle fork over to let the air in. Your pots, and the evergreens in them, will thank you for the trouble.