What is the difference between frost-proof and frost-resistant pots?

Terracotta does not break because it gets cold. It breaks because it is porous, so it soaks up water; when that water freezes it expands by about a tenth and forces the clay apart from inside. Everything about protecting a pot follows from that: keep the water out and the frost is harmless.

Step by step

1. Understand what firing temperature changes.Low-fired terracotta stays open and porous and can absorb several per cent of its own weight in water. High-fired clay, taken close to vitrification, absorbs very little. That absorption figure is the whole difference between the two labels.
2. Read 'frost-proof' as a manufacturer's guarantee and 'frost-resistant' as a hope.Frost-proof pots from reputable makers often carry a stated guarantee of some years against frost damage. Frost-resistant carries no such promise and describes ordinary garden-centre terracotta that will usually be fine and sometimes will not.
3. Test an unlabelled pot by weight and sound.Tap the rim with a knuckle. A high-fired pot rings; a soft low-fired one gives a dull thud. A high-fired pot is also noticeably heavier and denser for its size, and water beads on the surface rather than darkening it instantly.
4. Take the pot off the ground whichever type it is.A pot standing flat on paving wicks water up through its base and cannot drain. Pot feet or three tile offcuts under the rim solve most winter pot breakage on their own.
5. Empty the saucer, or take it away entirely.A saucer left under a pot through winter keeps the base permanently soaked. Saucers are a summer tool. Remove them in autumn and put them back in late spring.
6. Move genuinely vulnerable pots under cover.Glazed pots are the most at risk of all, because the glaze traps water inside the clay body with nowhere to escape. Anything decorative, old or expensive is better spending winter in a dry porch or under the eaves, planted or not.

Tips

• An empty pot left out over winter still breaks. Store empties upside down or on their side so they cannot fill with rain.
• Concrete, fibreglass, resin and thick plastic are all effectively frost-proof. Thin plastic goes brittle in cold and cracks when knocked rather than when frozen.
• Flaking or spalling on the outside face of a terracotta pot is early frost damage. It will get worse each winter and there is no repair — move that pot under cover.

Common mistakes

• Wrapping the plant and leaving the pot standing in a puddle — The pot breaks and the roots drown. Winter container protection is about drainage and shelter from rain far more than about warmth.
• Assuming an expensive glazed pot must be frost-proof — Glaze is a finish, not a firing spec. Many glazed pots are low-fired underneath and the sealed surface makes frost damage more likely, not less.

Alternatives

• Plastic liner inside a decorative pot: Plant into a plain plastic pot that sits inside the terracotta one. The decorative pot never holds wet compost, and you can lift the liner out to move or protect the plant.
• Fibreclay or polyresin: Looks close to terracotta at a distance, weighs a fraction, and is genuinely frost-proof. It fades in strong sun over several years and is not repairable when cracked.

Questions people ask