Cold frames, hotbeds and the glasshouse

Full sun all winter, shelter from the wind, water close by, and ground that drains. Face the glass toward the equator: south in the northern hemisphere, north in the southern.

Avoid frost pockets, the low ground where cold air collects on still nights, and anywhere a building, tree or wall throws a shadow at noon in midwinter, when the sun is lowest. A wall behind the frame or glasshouse helps: it breaks the wind and stores heat.

1.8 × 1.8 m is the common size, 0.9 × 1.8 m the smallest. Make lengths a multiple of 0.9 m so the lights fit. Use boards 25–50 mm thick of a rot-resistant wood such as cedar, and never wood treated with a preservative, which injures plants (University of Nebraska). Put a 5 × 10 cm post in each corner.

Slope the top toward the sun, 1 in 12: on a frame 1.8 m wide, 20 cm high at the front and 35 cm at the back. Sink the boards 5–15 cm into the ground, bank soil against the outside, and seal every gap: a tight frame keeps heat in and mice out. The cold frame and hotbed build has the drawings.

Two lights of 0.9 × 1.8 m cover a 1.8 m frame, meeting over a bar down the middle. Glass passes the most light and lasts, but breaks and cuts: keep children away from a glazed frame. Hinge the lights at the back or leave them loose, and keep a notched prop to hold each one open at a set gap.

A hotbed is a cold frame over a heat source. Dig the pit 60 cm deep, lay 10 cm of coarse gravel for drainage, and fill with 20–30 cm of fresh horse manure mixed with 5–10 % straw, trodden firm and wetted with lukewarm water. Cover it with 7.5–10 cm of good, weed-free soil (University of Nebraska).

The manure heats as it rots, far too hot for seed at first. Push a thermometer into the soil and sow only when it has fallen to 24 °C. The bed gives weeks of heat, then cools; dig the spent manure out as compost the next season.

Seeds germinate best at 21–24 °C. After that, keep the air at plant height at 16–18 °C for cool-season crops and 18–24 °C for warm-season ones (University of Nebraska).

Sun on a closed frame overheats it quickly: prop the lights open on every sunny day and close them in mid-afternoon to hold heat for the night. On frosty nights cover the lights with sacking or straw mats. Water less, and feed less nitrogen, than outdoor crops need: warm, wet, rich conditions under glass make weak, leggy seedlings and invite disease.

Most annuals need 30–40 days from seed to transplant, so count back from the last spring frost and do not sow earlier. For the last week or two, take the lights off for more of each day, then leave them off at night, so the plants meet wind, full sun and cold gradually.

A glasshouse built against a masonry wall that faces the sun: the wall shelters it, stores the day's heat and gives it back at night.

• Roof: glass sloping at 30° or steeper, up to the wall. The low winter sun is reflected off a flat roof; FAO recommends at least 25–30°.
• Vents: lights that open at the top against the wall and at the front, together 15–30 % of the floor area (FAO), so hot air leaves high and cool air enters low.
• Heat: a stove outside the house whose brick flue runs along the floor inside and out through a chimney at the far end. The flue must be sound: fumes from a leak damage the crop and can poison people working inside.
• Inside: a border bed along the front, staging for pots against the back wall.

Wash the glass each autumn: dirt takes light the crop needs in winter. Replace broken panes at once. Glass is a fragile roof: never stand, kneel or lean on it. Work from below, from a ladder that does not lean on the glass, or from a staging with a guard rail, and replace panes from inside where the glazing allows it; in Britain about 7 people a year die falling through fragile roofs and roof lights (HSE).