Protecting Pots and Containers from Frost (UK)

The first frost hits plants in pots harder than the same plants in the ground. It's not that pots attract frost; it's that a container leaves the roots out in the open air, with nothing around them but a few centimetres of plastic or clay. Soil in a bed holds onto warmth and shields roots from the worst of a cold night. A pot doesn't.

The fixes are simple and mostly free: put pots where it's warmer, let them drain, wrap the container rather than just the plant, and bring the tender ones in. This guide covers each step. It's part of our cluster on protecting plants from frost.

Why pots are more at risk than the ground

Think of a plant in a border. Its roots are deep in a big body of soil that warmed up all summer and gives that heat back slowly. Frost has to work down from the surface, and on most UK winter nights it doesn't get far.

Now think of the same plant in a pot on the patio. Cold air touches the pot's sides and base as well as its top. On a hard night the whole rootball can freeze. Roots are generally less hardy than the stems and leaves above them, because in the ground they never have to face that cold. So a plant that would be perfectly happy in a border can be killed in a pot by the same night.

Two things make it worse:

• Small pots. Less compost means less stored warmth and a rootball that freezes through faster. A big container is much safer than a small one; our guide to choosing pots and sizes explains why bigger is usually better for other reasons too.
• Wet compost. Compost that's waterlogged freezes into a solid block. Roots sitting in cold, saturated compost can rot even if they don't freeze. Mediterranean herbs, as our guide to overwintering Mediterranean herbs explains, are more often killed by winter wet than by cold.

Step 1: Group them in a sheltered spot

The quickest win is to move pots to the warmest part of the garden. In most gardens that's against the house wall, ideally one that gets some sun. Walls give off a little warmth at night and shelter pots from wind, and the eaves keep off some of the rain.

Group the pots together. Pots huddled side by side shield each other: only the outer ones have their sides exposed. Put the hardiest, biggest pots on the outside and the most delicate in the middle.

Avoid the bottom of a slope or a dip in the garden. Cold air sinks and collects there on still nights, making a frost pocket that can be noticeably colder than the rest of the garden.

Step 2: Lift them so they drain

A pot standing flat on a patio slab often sits in its own drainage water for weeks in a wet winter. Lift it onto pot feet, a couple of bricks or a few offcuts of wood, so water can run out of the drainage holes freely.

This does two jobs. The compost stays moister rather than soaked, so it's less likely to freeze solid. And the pot isn't sitting in a puddle that freezes around its base. Check the drainage holes aren't blocked by roots or debris while you're at it. If you're potting something up for winter, use a free-draining compost; our guide to growing food in containers covers the mixes.

Step 3: Wrap the pot, not just the plant

When people protect potted plants they tend to throw fleece over the top and leave the pot bare. But in a container it's the roots that need protecting most. Wrap the pot itself.

• Hessian is cheap, breathes, and looks fine on a patio. Wrap a couple of layers and tie with string.
• Bubble wrap traps air and keeps the cold off well. Greenhouse bubble insulation is tougher than packaging bubble wrap if you want it to last several winters.
• Fleece works too, wrapped in several layers around the pot.

Leave the top of the compost and the drainage holes clear, so rain isn't funnelled in and water can still get out. For a plant whose top growth is also tender, add a layer of fleece over the plant on the coldest nights, and take it off again on mild days so it gets light and air.

Step 4: Bring tender plants under cover

Wrapping and grouping help hardy and borderline plants through a cold snap. They won't save a truly tender plant from a hard frost. Pelargoniums, fuchsias you want to keep, citrus, chillies and other tender plants in pots need to go somewhere frost-free before the first frost: a porch, a cool bright room, a greenhouse kept just above freezing, or a frost-free shed for dormant plants.

Our guide to overwintering tender plants goes through where each one is happiest, and overwintering chillies covers keeping chilli plants going for a second year. If you're not sure which of your plants count as tender, see hardy or tender?.

Timing matters. Use the frost date checker to see when the first frost typically arrives in your region, and aim to have tender pots moved before then. Those are typical dates, not a forecast: on a cold, clear, still evening, check the Met Office forecast and move or cover pots anyway.

Step 5: Choose and care for pots that last

Terracotta that isn't frost-proof can crack. Unglazed clay is porous, as our guide to choosing pots explains, so it soaks up water. When that water freezes inside the clay it expands and can flake the surface or split the pot. Pots sold as frost-proof are fired to resist this. If you have pots that aren't, empty them and store them dry under cover for the winter, or at least lift them off wet ground so they drain.

Plastic doesn't crack in frost, but it gives the roots the least insulation of any pot, so wrapping matters more. Glazed ceramic varies; if in doubt, treat it like unglazed terracotta.

Big pots of fruit and shrubs are too heavy to move easily, so give them their winter spot in autumn while you can still shift them. Fruit trees, blueberries and other potted fruit are hardy in themselves, but their roots are still exposed in a container; our guides to fruit trees in pots and blueberries in pots cover them in more detail.

Winter watering

Most pots outside get plenty of water from the rain over winter. Check them now and then, especially those against a house wall under the eaves, where rain may not reach. Water only when the compost is drying out, and do it on a mild morning so the surface isn't wet going into a cold night.

Pots under cover, in a greenhouse, porch or shed, need the most attention: it's easy to forget them. Keep the compost just moist for evergreens and barely damp for dormant plants. Overwatering in cold weather is a common way to lose a plant that would otherwise have come through.

A quick winter checklist for pots

1. Move pots against a sheltered wall and group them.
2. Lift them onto pot feet or bricks.
3. Wrap the pots of anything borderline in hessian, bubble wrap or fleece.
4. Move tender plants somewhere frost-free before the first frost.
5. Empty and store pots that aren't frost-proof.
6. Water sparingly, on mild mornings.

For the rest of the garden, see what to do before the first frost and the full guide to protecting plants from frost.