What usually kills plants isn't the frost itself but the combination of frozen ground and sunshine: the top growth transpires water the roots can't replace from frozen soil, and the plant dries out. That's why for most perennials and woody plants it matters more to protect the roots and the base than the leaves.
What needs protection and what doesn't
| Group | Protection | How |
|---|---|---|
| Hardy vegetables (kale, lamb's lettuce, spinach) | not needed | light fleece only to extend picking |
| Last summer crops (tomatoes, peppers) | short-term only | fleece overnight, buys 2–3 more weeks |
| Newly planted trees and shrubs | yes, first 2 winters | mulch over the roots, wrap the stem |
| Roses | yes | earth up 20 cm over the graft union |
| Tender perennials (fuchsia, hydrangea) | yes | mulch plus fleece over the top growth |
| Fig, pomegranate | yes, thoroughly | wrap in fleece, mulch the roots heavily |
| Plants in containers | always | insulate the pot, not just the plant |
Fleece yes, plastic no
This is the most common and most expensive mistake. Plastic doesn't breathe — moisture condenses underneath, the plant cooks on a mild day, and in frost it ends up sheathed in ice. Horticultural fleece lets air and water vapour through while still holding several degrees. Alternatives: hessian, reed matting, conifer branches.
Conifer branches have the added advantage of not trapping moisture and being easy to pull off in spring — and the plant underneath won't rot even if you forget to remove them a week early.
Container plants are the most vulnerable
In the ground, surrounding soil insulates the roots; in a pot they're exposed to frost from every side and freeze many degrees harder. So:
- Wrap the pot in bubble wrap, hessian or an old blanket — here plastic on the outside matters least, because it insulates the root ball rather than the plant.
- Stand it on polystyrene or wood, never straight on paving — cold from below is the worst of it.
- Move pots against the house wall, ideally on the south or east side where they're sheltered.
- Water occasionally through winter — drying out in a pot kills more container plants than frost does.
When to put protection on and when to take it off
Don't rush to put it on. Plants need gradual cooling to harden off at all; wrap in mid-October and you get a soft plant that suffers more in December than an unprotected one would. The right time is after a few light frosts, usually in November.
Do rush to take it off. In spring, as soon as sustained frosts are over, remove the wrapping — it warms up fast underneath, the plant starts into growth, and then the first spring frost catches it at its most vulnerable. Pull the mulch back from the crown so the base can dry out.
Frequently asked questions
When does the first frost arrive?
In most inland areas between mid-September and mid-October, and earlier in upland areas and frost pockets. What matters isn't the average temperature but a clear, still night — that's when ground-level temperature falls fastest and furthest.
Can I use plastic sheeting to cover plants?
Overnight, as an emergency measure against a first frost, yes; as winter protection, no. Plastic doesn't breathe, moisture condenses under it, and the plant either cooks or ends up encased in ice. For winter use horticultural fleece, hessian or conifer branches.
Do hardy vegetables need winter protection too?
Kale, lamb's lettuce, spinach and leeks don't need it — they take frost, and some actually sweeten after it. Light fleece is only worth it if you want to keep picking through harder frosts, since it keeps the leaves accessible and unfrozen.