How to use a cold frame
Updated: October 2026
You have a low glazed box on the patio or the allotment, and the seedlings on the windowsill are ready to leave the house. How to use a cold frame is mostly timing: open by day, shut at night, and stop before a sunny lid cooks the tray. Peter F. followed Royal Horticultural Society guidance on hardening off, ventilation and frost, checked on 20 October 2026. This page does not rank kits. The [cold frame comparison](/hub/raised-beds-greenhouses-and-compost/cold-frame) does that. A qualifying purchase from the main table can earn a commission at no extra cost to you.
You have a low glazed box on the patio or the allotment, and the seedlings on the windowsill are ready to leave the house.
You have a low glazed box on the patio or the allotment, and the seedlings on the windowsill are ready to leave the house. How to use a cold frame is mostly timing: open by day, shut at night, and stop before a sunny lid cooks the tray. Peter F. followed Royal Horticultural Society guidance on hardening off, ventilation and frost, checked on 20 October 2026. This page does not rank kits. The cold frame comparison does that. A qualifying purchase from the main table can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Form | Height | Glazing | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Outsunny Outsunny fir cold frame | Low single lid | 40 cm | Polycarbonate | 8.6 | about £46.99 | View offer |
 Spear & Jackson Spear & Jackson cold frame | Low single lid | 46 cm | Plastic panes | 8.0 | about £68.94 | View offer | |
| Best value |  Outsunny Outsunny twin-lid cold frame | Low twin lid | 36 cm | Polycarbonate | 8.0 | about £47.59 | View offer |
On this page
Put the frame somewhere you can open it every day
A cold frame earns its place when you can reach the lid without moving pots first. The RHS treats soil, gravel and paving as normal bases. Paving keeps the timber off wet earth, because untreated wood at ground level rots faster. Level the ground so the lid actually meets the rim. A twist means a gap on cold nights and a lid that will not stay where you propped it. Peg a lightweight frame down. On slabs, where pegs will not bite, use weights on the frame, not on the glazing. If you will not open it on a weekday morning before work, the plants are safer on a porch with fleece than in a box you forget.
Harden off for about two weeks
The RHS method for tender plants is a cold frame with the lid slightly open by day and closed at night for about two weeks before you plant out. Start with plants that have already been growing in the light indoors, not with seedlings you have only just pricked out and then shocked twice. On the first mornings, crack the lid a small amount and close it before the temperature drops. Lengthen the open period through the fortnight. A cloche gives less frost protection than a cold frame. The RHS points to fleece and to later planting dates when a severe frost is likely. These boxes do not come with a degree rating.
Sunny days, watering and the mistakes that lose a tray
The failure described most often is a shut lid on a bright day. The same cover that holds warmth at night traps heat at noon. Open the lid, or prop it, as soon as the sun is on the glazing. Water in the morning so the compost is not sitting wet and cold overnight. A closed, damp frame grows mould on leaves and on the underside of the lid. The RHS notes that bubble plastic inside a frame cuts light and still needs airflow if you want to limit that mould. Use a sheet only in a cold spell, and take it out when you are venting hard. A typical RHS cold frame depth is about 45–60 cm. Many flat-pack lids are a little lower than that, so tall pots hit the cover and stop it shutting.
When you should stop and ask someone else
You can build and run a timber and polycarbonate kit yourself. Call in help when the job leaves that kit. Mains heating or any wiring in a greenhouse is electrical work, and these frames are not mains products with a UK plug to judge. Broken horticultural glass that needs proper disposal is also outside a plastic-panel kit. If the timber arrives warped, the lid will not sit, or a door panel is missing, use your purchase rights rather than forcing a crooked box into service. The Consumer Rights Act 2015 section 22 gives you 30 days to reject goods that are not of satisfactory quality. If you travel for a week in April, ask a neighbour to vent it, or move the trays back indoors. Lid hardware is on cold frame ventilation.
Common questions
How long should I harden plants off in a cold frame?
The RHS allowance is about two weeks, with the lid slightly open by day and closed at night, before planting out. Stretch the daytime opening through that fortnight rather than flinging the lid off on day one. A frost forecast still means you close up, and a severe frost may need fleece as well. The frame is a step between the house and the bed, not a heated greenhouse.
Should the lid stay shut on a sunny day?
No. A shut lid in sun overheats seedlings quickly, which is the usual way people lose a tray. Prop or open the cover once the sun is on it, then close it again before the evening chill. The RHS treats daytime ventilation as normal cold-frame practice. If you cannot get home to open it, do not leave tender plants inside on a clear spring day.
Will a plastic lid protect plants from frost?
It moderates the cold and, with the lid shut at night, shelters plants better than a cloche, which is the RHS comparison. It does not guarantee survival in a hard frost, and these panes do not come with a frost temperature. Add fleece inside the frame, or delay planting, when a severe frost is forecast. Bubble plastic can help in a cold spell and must come out when you need airflow.
Can I stand the frame on paving?
Yes. Paving, gravel and soil are all normal bases. Paving keeps the timber drier than bare soil, as long as the frame is level and the lid can close. Pegs often fail on slabs, so weight the frame on a windy patio. The frame will not warm the whole patio. It only covers the pots inside the rim.
When should I treat the timber?
Before the first wet winter, and again when the coating has worn, especially on fir or soft wood that sits near damp ground. UK buyers of flat-pack frames commonly report that a coat is needed even when the listing sounds as if the wood is ready. Do that on a dry day with the plants removed, and let the treatment cure before you put trays back.
Who should I call if I want heat or the glass breaks?
Anyone competent with mains electrics, if you are adding heat to a proper greenhouse. These cold frames are not plug-in appliances. Broken horticultural glass is a disposal and safety job, and it is a different material from the polycarbonate or plastic panes on ordinary kits. A crooked flat-pack is a returns question under the Consumer Rights Act 2015, not a glazier’s visit. Day-to-day venting stays with you.
Related pages
Sources
Public UK sources from the research digest, last checked on 20 October 2026. Amazon.co.uk listing lines are the price and review source for the six ASINs. No frost or assembly test was run.
- RHS — Coldframes and mini-greenhouses (retrieved 20.10.2026)
- RHS — Choosing greenhouses (retrieved 20.10.2026)
- RHS — Hardening off tender plants (retrieved 20.10.2026)
- RHS — Cloches (retrieved 20.10.2026)
- RHS — Frost damage prevention (retrieved 20.10.2026)
- Gardeners' World — Best cold frames 2025 (retrieved 20.10.2026)
- Consumer Rights Act 2015, section 22 (retrieved 20.10.2026)
- GOV.UK — Returns and refunds (retrieved 20.10.2026)