Cold frame
A cold frame is a low, glazed box that captures solar heat — somewhere between an open bed and a full greenhouse. It extends the season 4–8 weeks at each end, hardens off seedlings gently, and keeps salads growing through winter in mild zones. Much cheaper than a greenhouse, and it fits on a single SFG bed.
Why a cold frame?
Harvest 4–8 weeks longer each side of the season — start spring sowings in March, pick salads into December.
Hardens off seedlings from the greenhouse or windowsill — they adapt to wind and temperature swings gradually before planting out.
Cheaper, smaller and simpler than a greenhouse — no foundation, no permit, fits on any bed or patio corner.
Passive solar — needs no heating, no electricity, no plumbing. Just sun and a hinged lid.
Types
Four designs, from DIY-simple to semi-permanent.
Classic wooden box
Wooden frame higher at the back (~50 cm) than the front (~30 cm), with an old window as a hinged lid. Budget-friendly, well-insulated, easy to build in an afternoon.
Glass & aluminium kit
A ready-made cold frame, more durable and attractive but pricier. Look for double-walled polycarbonate — it insulates better than single glass.
Hot bed (Mistbeet)
A classic cold frame with a 30–50 cm layer of fresh horse manure or half-rotted compost below the soil. The decomposition generates 4–6 weeks of heat — you can sow as early as February.
Mini polytunnel
Arched PVC pipes over a bed with clear plastic film. Cheapest option, covers longer rows, easy to remove. Weigh the edges down against wind.
Building step by step
Pick a south- or south-east-facing spot, sheltered from north winds — against a wall or hedge is ideal.
Build the box sloping down toward the front (back ~50 cm, front ~30 cm) so the lid tilts toward the sun.
Use untreated wood — pressure-treated timber leaches chemicals into the soil. Cedar or larch last longest without treatment.
Fit an old window or a sheet of clear double-walled polycarbonate as a hinged lid. Add a wooden stay so you can prop it open at different heights.
Dig 15–20 cm into the ground inside the frame and fill with SFG soil or Mel's Mix. Level the surface below the frame.
Drop in a min-max thermometer — essential for knowing whether to ventilate. Bury a second one in the soil for sowing-temperature decisions.
What to grow
A cold frame is a salad and hardy-vegetable machine. Timing matters more than variety — sow earlier in spring, later in autumn than you would outdoors.
Spring (Feb–Apr)
Lettuce, rocket, spinach, lamb's lettuce, mustard — direct sow from mid-February.
Radishes, early carrots, kohlrabi — 2–4 weeks ahead of outdoor sowing windows.
Harden off greenhouse or windowsill seedlings here for 7–10 days before planting out.
Once outdoor sowing is possible, switch to summer crops or remove the lid entirely.
Autumn–Winter (Sep–Feb)
Winter salads — lamb's lettuce (Feldsalat), claytonia, winter purslane, mizuna, mustards.
Hardy herbs — chervil, parsley and chives survive under glass all winter.
Spring onions and spinach overwinter happily for an early April harvest.
When nights drop below −5 °C, pile straw or leaves on the glass for extra insulation.
Daily care
Ventilate on sunny days — even in February, a closed frame can hit 40 °C and cook your plants. Prop the lid open 5–15 cm whenever the outside temperature is above 10 °C.
Close the lid before sunset to trap the day's heat against the cold night.
Shade in summer if you use it year-round — whitewash the glass or drape a shade cloth over it from June to August.
Water sparingly and in the morning — soil under glass dries slowly, and wet leaves at nightfall invite mildew.
A reliable cold frame is often more useful than a full greenhouse: a smaller climate is easier to warm passively, less ventilation work, and it fits on a single SFG bed. Build the wooden box with an old window first — invest in something larger only once you know you'll use it.