Techniques
Cold frame
A low, unheated box with a clear, sloping lid that traps warmth — used to harden off seedlings, raise early crops and overwinter tender plants.
A cold frame is a simple, unheated growing box with low sides and a clear, sloping lid (the "light"). It is one of the most useful pieces of kit a UK beginner can own, because it gently bridges the gap between a warm windowsill or greenhouse and the cold, exposed open ground.
How it works
A cold frame works by passive solar gain. Sunlight passes through the glass or clear plastic lid, warms the soil and air inside, and the closed frame traps that heat — so the temperature inside stays several degrees above the outside, and frost is kept off tender growth. There is no heater involved; the sun does all the work. The sloping lid faces the light to catch as much sun as possible and to shed rain.
The single most important skill is venting. On a bright day, even in March, a closed frame can quickly overheat and cook your plants or trigger fungal problems. Prop the lid open a few inches on mild, sunny days and close it again before evening. On warm summer days you can remove the lid entirely.
What you'll use it for
- Hardening off: the classic job. Use it to harden off seedlings raised indoors, gradually acclimatising them to outdoor conditions before they go into the ground.
- Early sowings: sow hardy crops such as broad beans, salad leaves, peas and beetroot a few weeks earlier than you could outdoors.
- Overwintering: protect tender plants like pelargoniums, or keep autumn-sown sweet peas and biennials going through the colder months.
- Winter salads: raise lettuce, rocket, mizuna and other cut-and-come-again leaves for picking through the dark months.
Where to put it
Site your cold frame in a sunny, sheltered spot — ideally south-facing against a wall or fence that blocks cold winds and stores a little background warmth. Stand it on a firm, level base so the lid seals well, and keep it close to the house so checking and venting it is no trouble.
A cold frame is essentially a more permanent, walk-up version of a cloche: where a cloche is moved around the beds to cover plants in place, a cold frame is a fixed home base for trays and pots. Many gardeners use both. You can buy aluminium-and-glass frames from the usual garden centres, but a perfectly good frame can be knocked together from scrap timber and an old window — which is how most allotments have always done it.
In a UK garden
In the UK's changeable spring, a cold frame buys you two to four weeks on either end of the season — letting you sow earlier and toughen plants before the last frosts pass in May.
Example
Move a tray of broad bean seedlings from the windowsill into the cold frame in March, propping the lid open on mild days, so they are ready to plant out by April.