Why should I not cover plants with plastic sheeting in a frost?
The three separate ways polythene laid on a plant does more damage than no cover at all, and the one arrangement in which plastic genuinely helps.
- Difficulty
- beginner
- Time
- 7 min
- Read
- 3 min
Short answer
Where plastic touches a leaf it conducts heat straight out of it, so those leaves are frosted worse than uncovered ones. It traps condensation which drips back and refreezes on the plant. And it heats up fast in morning sun, so a cover left on cooks whatever is underneath. Plastic works only as a rain shield held clear on a frame with the sides open, and it has to come off in the morning.
Reaching for a sheet of polythene is the instinctive response to a frost warning, and it is one of the few interventions in gardening that reliably makes the outcome worse than doing nothing. The reasons are worth knowing because they also explain why fleece works.
Step by step
- Understand what a cover is meant to do.It intercepts the heat radiating from the plant and the soil to the open sky, and it holds a pocket of air that the soil's own warmth keeps a little above the outside temperature. Both jobs need an air gap and neither needs the material to be waterproof.
- Recognise where plastic fails at both.In contact it conducts rather than insulates, and being impermeable it converts the plant's own moisture into a liability instead of letting it escape.
- Use the one arrangement that works.Where the threat is winter wet rather than cold — a silver-leaved or alpine plant, a crown that must stay dry — a sheet of clear polythene or a pane of glass propped on a frame above the plant with all four sides open sheds rain without trapping air. That is a rain shield, not insulation.
- Substitute a breathable material for everything else.Fleece, hessian, mesh, several layers of newspaper for one night, or an upturned pot or bucket for a small plant. All of them beat polythene laid on foliage.
Tips
- If polythene is genuinely all you have on the night, build a frame from canes so nothing touches the plant, weight the edges, and set an alarm to take it off at first light.
- Bubble wrap has the same problems as flat polythene when it touches foliage, and its air cells make no difference to that. It belongs round the outside of pots.
- Cloches made of rigid clear plastic are a different case — they are a structure with an air gap built in, and they are ventilated. The failure mode described here is film laid onto a plant.
If it doesn't work
Leaves blackened in patches that exactly match where a plastic cover touched them, while uncovered leaves nearby are fine
Cause: Plastic is a good conductor compared with still air. A leaf in contact with cold plastic loses heat through it far faster than a leaf surrounded by air — Fix: Remove damaged tissue only once new growth shows what survived. Next time, hold any cover clear of the foliage on hoops or canes.
Water beaded and frozen on the plant under a cover, with worse damage than outside it
Cause: Plants and soil release moisture that cannot escape through plastic. It condenses on the inside, drips back and freezes on the leaves — Fix: Use a breathable material — fleece, hessian or mesh — for anything left on for more than a few hours.
Plants collapsed or scorched by the middle of the morning after a cold night
Cause: A sealed plastic cover in sunshine reaches high temperatures very quickly, and plants that were cold-shocked overnight cannot cope with it — Fix: Take plastic covers off first thing, without exception. If you cannot commit to that, do not use plastic.
The cover blew away and took plant growth with it
Cause: Polythene is a sail and has no give. Fleece tears at a weak point; polythene lifts the whole assembly — Fix: Weight or fix the edge continuously, and prefer a permeable material on any exposed site.
Questions people ask
Why does plastic work fine as a cloche then?
Because a cloche holds the material well clear of the plant, has a substantial air volume inside it and is ventilated. The problem is contact and sealing, not the material in the abstract.
Is black plastic different from clear?
For frost protection, no — both conduct and both trap moisture. Black plastic is useful for warming and clearing ground before planting, which is a different job entirely.
What about the plastic sheeting builders sell?
Same physics, more weight. It is a good rain shield on a frame and a bad plant cover, exactly like any other polythene.