How do I stop container plants dying of winter wet?

British winters are wet more reliably than they are cold. A pot in the open receives a season's rainfall on a small surface area with nowhere for the water to go once the compost is saturated, and roots that cannot get air die whether it freezes or not. When it does freeze, the saturated rootball expands and takes the pot with it. The two pots in the diagram are the same plant with two different outcomes.

Step by step

1. Take every saucer away in autumn.A saucer through winter holds the pot base permanently in standing water. This one change saves more plants than anything else on the list. Put them back in late spring.
2. Raise every pot on feet.Three pot feet, tile offcuts or slate pieces under each pot, so the drainage hole is clear of the paving and air can move underneath. Check the hole is not blocked with root or silt while you are there.
3. Move pots out of the open rain.Under the eaves, into a porch, against a house wall on the sheltered side, or under a bench. A pot that receives half the winter rain has half the problem. Remember to check those pots for dryness through the winter instead.
4. Group pots together against a wall.Grouping reduces wind chill on the pot sides, the pots shelter each other, and heat leaking through a house wall keeps the group several degrees warmer. Put the most vulnerable plants in the middle.
5. Insulate the pot, not the plant.Bubble insulation, hessian or fleece wrapped around the container keeps the rootball from freezing solid. Wrapping the foliage traps moisture against the leaves and causes rot. Leave the top open so rain can still drain through.
6. Tilt the pot very slightly, or mulch the surface.A slight tilt sheds surface water off the top rather than letting it pond. A gravel or slate mulch does the same job for a level pot and keeps the crown of the plant dry.
7. Stop feeding in late summer.Nitrogen after about August produces soft growth that has not hardened off by the first frosts. Stop feeding well before autumn so the plant goes into winter woody and tough.
8. Reduce watering to almost nothing for pots in the open.They are already receiving all the water they need. Only pots in a rain shadow need any attention, and then only enough to keep the compost just moist.

Tips

• Anything grown in a Mediterranean or gritty mix — lavender, rosemary, thyme, agaves, alpines — is more at risk from winter wet than from winter cold. Get these under cover or under eaves as a priority.
• Terracotta pots crack because the water in the clay freezes, so keeping them draining and out of standing water protects the pot as well as the plant.
• A sheet of glass or clear acrylic propped over an alpine trough keeps the rain off while leaving all four sides open. That is the traditional and still the best solution for a pot that must stay outside.

Common mistakes

• Wrapping the whole plant in plastic — It traps moisture, excludes air and cooks the plant on a sunny day. Fleece or hessian, and around the pot rather than over the leaves.
• Bringing hardy plants into a heated house — Warmth and low light break dormancy, produce weak growth, and the plant is then killed when it goes back out. An unheated porch, shed or greenhouse is the right place.
• Putting pots into the shelter of a fence in the lowest corner of the garden — Cold air flows downhill and pools against a solid barrier. That corner is the frost pocket. Against a house wall, higher up, is far better.

Checklist

• Saucers removed and stored
• Every pot on feet with the drainage hole clear
• Vulnerable pots moved under eaves or into a porch
• Pots grouped against a sheltered wall
• Containers wrapped, foliage left open
• Feeding stopped since late summer
• Rain-shadowed pots on a fortnightly moisture check

Questions people ask