How do I protect pots and containers from frost?

Why pots freeze when the ground does not, protecting roots rather than leaves, and saving the pot as well as the plant.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

Roots in pots freeze because there is no surrounding soil to buffer them. Group pots together against a wall, wrap them in fleece or bubble wrap, raise them off cold paving, and move the vulnerable ones under cover.

Plants that are perfectly hardy in the ground are frequently killed in pots, because the root ball freezes solid in a way it never would in open soil.

What you'll need

Horticultural fleece or bubble polythene
Pot feet or bricks
Hessian or straw for larger pots

Step by step

Understand the difference.Soil in the ground holds heat and rarely freezes deeply. A pot is surrounded by cold air on all sides, so the root ball can freeze solid — and frozen roots cannot take up water.
Protect the roots, not the top.Wrapping the foliage is largely cosmetic. It is the pot that needs insulating, because root death is what actually kills the plant.
Group pots together.Against a wall, ideally house wall rather than a boundary wall. They shelter each other and benefit from the heat the building loses.
Raise pots off the ground.Pot feet or bricks. It stops the base freezing to cold paving and, just as importantly, lets water drain away.
Wrap the pot itself.Bubble polythene, hessian, or fleece around the container. Several layers on anything valuable, tied on so wind does not remove it.
Keep them on the dry side.Waterlogged compost freezes solid and expands, which both damages roots and cracks pots. Most winter container losses are from wet rather than cold.
Move the tender ones under cover.Unheated greenhouse, cold frame, porch or shed. Even an unheated space is several degrees warmer than the open garden on a frosty night.
Protect the pot as well as the plant.Terracotta absorbs water and cracks when it freezes. Frost-proof pots are marked as such; ordinary terracotta should be emptied and stored or wrapped.
Use fleece over evergreen foliage in hard frosts.A double layer for a few nights, removed when the weather eases. Left on permanently it causes soft growth and fungal problems.
Do not feed in late autumn.It encourages soft growth that is far more vulnerable to frost. Feeding should stop well before the first frosts.
Check for wind as well as cold.Cold drying wind kills evergreens by desiccation when the roots are frozen and cannot replace lost moisture. Shelter matters as much as insulation.

Tips

Insulate the pot, not the plant. Frozen roots are what kills container plants.
Raise pots off paving. Drainage matters more than most people realise in winter.
Ordinary terracotta cracks when wet compost freezes. Check whether yours is frost-proof.

Common mistakes

Wrapping the foliage and leaving the pot exposed — The roots are what freeze and die. Fleece over the top looks like protection but does very little if the container itself is unprotected.
Leaving pots standing in saucers over winter — Waterlogged compost freezes solid, expanding and damaging roots, and it cracks terracotta. Winter wet kills more container plants than cold does.

If it doesn't work

Terracotta pot cracked over winter

Cause: Wet compost freezing and expanding — Fix: Not repairable. Use frost-proof pots, or empty and store them over winter.

Hardy plant died in a pot

Cause: Root ball froze solid — Fix: Insulate the container and group pots together next winter.

Evergreen leaves browned and crisp

Cause: Wind desiccation while roots were frozen — Fix: Provide wind shelter as well as insulation.

Plant rotted at the base

Cause: Waterlogged compost — Fix: Raise pots on feet, check drainage holes, and move under cover in prolonged wet.

Questions people ask

Why do hardy plants die in pots?

Because the root ball freezes in a way it never would in the ground. Hardiness ratings assume roots are in soil, which buffers temperature — a pot has cold air on every side.

Should I bring pots indoors?

Into an unheated shelter such as a greenhouse, cold frame, porch or shed, yes for tender plants. A heated house is usually too warm and dry and causes its own problems.

Is bubble wrap or fleece better?

Bubble polythene insulates the pot better; fleece is more breathable and better over foliage. Using bubble wrap on the container and fleece on the plant is a sensible combination.