A cold frame is a bottomless box with a transparent lid set over garden soil, and it adds 4-8 weeks to each end of the growing season without any fuel. Build the back wall 40-50 cm tall and the front wall 20-30 cm tall, so the lid slopes toward the sun, and keep the box to 120 by 180 cm so every plant can be reached without stepping in.
A cold frame is the smallest season-extension structure: a low box that lets sunlight in through a lid and traps the warmed air and soil beneath it.
A Cold Frame Works Because Sunlight Gets In Faster Than Heat Gets Out
Sunlight passes through the lid, warms the soil and the air inside, and the box stops that warmth blowing away. The soil is the heat store: it keeps releasing warmth after the sun drops, so seedlings inside sit warmer than the open garden on a cold night. The effect is large enough to add 4-8 weeks to both ends of the season, which is the difference between planting lettuce in early spring and waiting for the last frost to pass.
Three decisions shape every cold frame: which way the lid faces, how steeply it slopes, and how the box is opened on a warm day. The first two are fixed when you build. The third is a daily habit, and it is the one that kills crops when it is forgotten.
What You Need
- Four walls — boards, logs, stacked stone or earthbags, tall at the back and low at the front (see Basic Box Construction)
- A transparent lid in its own frame — old window sashes, plastic sheeting on a frame, or similar (see Lid Materials & Hinges)
- A hinge and a prop stick so the lid opens upward from the back
- A south-facing, well-drained spot in the Northern Hemisphere; in the Southern Hemisphere face the lid north. Finding True South covers the compass work
Method: Lay Out A Standard Frame
Step 1: Pick the site. Choose level ground with full winter sun and a wall, fence or hedge on the north side to break the wind. Avoid hollows where cold air pools; Cold Air Drainage and Frost Pockets explains why.
Step 2: Mark 120 by 180 cm. The 120 cm dimension runs from front to back, so you can reach the back row from the front. The 180 cm dimension runs along the south face. Do not make the box deeper front to back than your arm can reach.
Step 3: Build the box with a back-to-front slope. Back wall 40-50 cm, front wall 20-30 cm, side walls tapered between them. The slope drains rain off the lid and tilts it toward the sun.
Step 4: Fit the lid and prop. Hinge the lid at the back so it lifts upward. A prop stick holds it open at different heights.
Step 5: Learn to ventilate. Open the lid when the temperature inside passes 25-30 °C and close it in late afternoon to trap heat for the night. On a clear winter day a closed frame can overheat even when the air outside is cold.
Worked Example
Wall heights set the lid slope. With a 120 cm front-to-back depth, a 45 cm back wall and a 25 cm front wall, the lid rises 45 − 25 = 20 cm over 120 cm. The angle is arctan(20 / 120) = arctan(0.167) = 9.5°. Across the full range of wall heights (a 10 cm to 30 cm rise) the slope runs from arctan(10 / 120) = 4.8° to arctan(30 / 120) = 14.0°.
That is much flatter than a tilt equal to your latitude, which would be 40° at 40° latitude. A steeper lid needs a taller back wall or a shallower box; the arithmetic is in Basic Box Construction.
Common Mistakes
- Forgetting to vent — a closed frame in sun overheats and cooks seedlings. Open it on every sunny day, even a cold one.
- Making the box too deep — a plant you cannot reach is a plant you walk on, and trampled soil compacts.
- Facing the lid the wrong way — a lid facing away from the sun gets a fraction of the light and none of the benefit.
Safety
Glass lids and wind
A glass sash that falls shut or is thrown open by wind can shatter. Prop it firmly, and latch it closed in a gale. Wear gloves when handling broken glass.
What’s Next
- Learn next: Basic Box Construction — the walls, the slope and the lid frame in detail
- Combine with: Hotbed Variation (Manure-Heated) — add bottom heat for starting seedlings 8-12 weeks before the last frost