There is a persistent myth that a sheet of fleece or a row of cloches turns October into September. It does not. Covers generate no heat of their own — they simply slow the rate at which the ground gives back the warmth it absorbed during the day. On a still, frosty night that is worth two or three degrees, and at the shoulders of the season two or three degrees is the difference between a crop and a compost heap.
Used properly, cheap protection buys you three to five weeks at each end of the year and keeps salads, roots and hardy greens standing right through the winter. Used carelessly, it cooks seedlings in March and rots them in December. This guide covers what each type of cover actually gains you, the light rule that decides your whole autumn timetable, and how to set the lot up so the wind does not take it in November.
- What it gains 2–3 °C on a frosty night; 3–5 weeks at each end of the season
- Set up by end of September for autumn crops · February for a spring head start
- Daily job ventilate — a closed cold frame hits 35–40 °C in weak March sun
- The deadline crops must be near full size by early November, when growth stops
The rule nobody mentions: light ends the season, not frost
Somewhere around the start of November, day length at British and Dutch latitudes drops below roughly ten hours. Below that threshold plants effectively stop making new growth — not because they are cold, but because there is not enough light to build with. Growth does not restart properly until early February, when the ten-hour day returns.
That single fact reorganises everything. A cloche in December is not a growing device, it is a preserving device: it holds a crop you already grew, in good condition, until you want to eat it. Which means the real work happens now. Anything you intend to harvest in January has to reach close to its full size by early November, so lettuce, spinach, corn salad, winter radish and oriental greens all want sowing in late August and September — our September sowing list and the guide to autumn-sown hardy annuals both work to the same clock.
Fleece, cloche or cold frame?
Fleece & cloches
Light, cheap, movable — the workhorse for beds and rows.
- Around 2 °C from single fleece; 3–4 °C from two loose layers
- Fleece breathes and lets rain through; rigid cloches keep leaves dry but need ends left open
- Best for: covering a whole bed, warming soil before sowing, frost nights in May
Cold frame
A permanent box with a glazed lid — warmer, drier, more work.
- Holds 4–5 °C above outside and keeps winter rain off entirely
- Needs opening on bright days and closing by mid-afternoon
- Best for: overwintering young plants, forcing early salads, hardening off in spring
Between the two sits the low polytunnel — hoops plus clear polythene — which gives cold-frame warmth over a whole bed for cloche money, at the cost of needing ventilation just as urgently. Whichever you choose, remember that a thicker single layer is worse than two thin ones: it is the trapped air between layers that insulates, not the fabric.
Setting up a covered bed, step by step
- Sow or plant first, cover second Get the crop in by mid-September so it has real size before the light runs out. A cover over an empty bed in November achieves nothing.
- Weed and tidy before you close it in Every yellowing leaf you leave under a cover becomes a botrytis starter culture. Clear debris and thin crowded seedlings now, not in January.
- Hoop it so nothing touches the leaves Hoops every 90 cm, cover clear of the foliage. Fleece resting on wet leaves freezes onto them and scorches; it also lets cabbage whites lay straight through.
- Weight every edge Wind destroys far more covers than frost does. Bury the edges in a shallow trench, or weigh them with planks, bricks or bags of sand — pegs alone rarely survive a gale.
- Ventilate on every bright day Open frames and cloche ends in the morning whenever it is dry and above freezing; close by mid-afternoon so the bed traps the day's warmth before dusk.
Damp, not cold, is what kills things under cover
Most winter losses under protection are fungal, not frost damage. Still, humid air around crowded plants at 4 °C is ideal for grey mould, and a closed cover creates exactly that. Air movement is the antidote: open up whenever the weather allows, space plants more generously than you would in the open, and water in the morning so foliage dries before dark.
Watering is the other surprise. A rigid cloche or a cold-frame lid sheds rain completely, so the soil underneath can be bone dry in February while the rest of the garden is waterlogged. Check with a finger once a fortnight and give a modest drink at the base of the plants — our notes on watering and on damping off and seedling mould both apply under cover, only more so. A mulch around the plants insulates roots and keeps soil splash off the leaves.
The other end: stealing three weeks in spring
Season extension is not only an autumn trick. Soil temperature, not air temperature, decides when seed will germinate, and bare ground in February is both cold and sodden. Cover a bed with a cloche or a sheet of clear polythene two to three weeks before you intend to sow and you will find it dry enough to rake and several degrees warmer — which typically brings a March direct sowing forward by two to four weeks.
A cold frame earns its keep in the same weeks as the halfway house between windowsill and garden: plants started indoors move into the frame for a fortnight, lid opened progressively wider, before they face the open ground. It is also where module-raised brassicas such as spring cabbage are best grown on, and where a raised bed or a run of containers can be sheltered without moving anything indoors.
Common mistakes
- Covering too late. Protection preserves growth; it does not create it. Everything for winter eating needs its size by early November.
- Forgetting to ventilate. A shut cold frame in weak March sun reaches 35–40 °C and cooks seedlings in an afternoon.
- Fleece lying on the foliage. It freezes to wet leaves, scorches them, and gives pests a laying platform. Always hoop it.
- Trusting pegs in a gale. Bury or weight the edges properly, or you will be retrieving fleece from a hedge.
- Never watering. Rigid covers shed every drop of rain. Dry soil under a cloche in February is completely normal — and completely avoidable.
Frequently asked questions
How much frost protection does horticultural fleece really give?
A single standard layer is worth roughly 2 °C on a still night; two loose layers with air between them get you to 3–4 °C. That is enough for a light frost, not for a hard freeze — for anything genuinely tender, lifting and storing is the safer route, as in our guide to overwintering tender plants.
Can I use old windows or bubble wrap?
Yes — a cold frame made from an old window on a timber box works exactly as well as a bought one, and bubble wrap makes decent lining for the inside of a frame. Keep the glazing clean, though: winter light is scarce enough without a layer of grime on top.
What is the difference between a cold frame and a greenhouse?
Only scale and heating. Neither is heated by default; both work by trapping solar gain. A cold frame is simply a very small, very well-sealed greenhouse you cannot walk into — which makes it warmer per litre and far more prone to overheating.
Which crops are actually worth covering?
Winter lettuce, spinach, corn salad, mizuna and other oriental leaves, winter radish, chard, spring onions and overwintering salads repay it most. Truly hardy things — leeks, parsnips, kale, savoy cabbage — do not need a cover to survive, only to stay clean and pickable.
A few metres of fleece, a handful of hoops and a box with a lid are the cheapest yield increase in the garden — provided the sowing happens first. Get spinach, corn salad, winter lettuce, mizuna and winter radish in the ground this month, cover them in October, and you will be cutting fresh leaves while the rest of the plot sleeps. Browse the full vegetable seed collection or dig deeper in our growing guides library.
