Simple Cold Frame

A bottomless box with a sloped glass or polycarbonate lid that captures solar heat and protects tender plants from frost.

Estimated time

1.5 – 2 hours

Lifespan

5–8 years

Soil needed

Bottomless — sits on existing bed soil; no bagged fill needed

Weight empty

30–50 lbs

Weight full

150–250 lbs with plants, soil, and lid

Best for

Overwintering greens, hardening off seedlings, starting cool-season crops early

Placement

Face the sloped lid south for maximum sun. Nestle against a wall or hedge for wind protection. Prop the lid open on sunny days.

Close-up of lettuce and spinach growing inside a wooden cold frame with a prop stick holding the polycarbonate lid open — A propped lid keeps greens from overheating on sunny days

A simple wooden cold frame with its lid open in a garden bed beside a fence — Set the bottomless box directly on prepared soil or over a raised bed

Boards to Buy

Boards to Buy	Quantity	What You Use Them For
1×12 cedar or rot-resistant board, 8 ft	1	Back wall (48 in × ~12 in)
1×8 cedar or rot-resistant board, 8 ft	1	Front wall (48 in × ~8 in)
1×12 cedar or rot-resistant board, 4 ft	1	Two side walls, ripped to slope 12 in → 8 in
1×2 cedar strips	2	Lid frame
Polycarbonate sheet or old window, 4 ft × 2 ft	1	Lid glazing
Exterior hinges	2	Attach lid to box
2.5-inch exterior screws	1 lb box	Assembly

·Exterior wood sealer or paint (outside only)
·Optional: automatic vent opener
·Optional: stick or prop for manual venting

Steps

1Cut the back wall 48 inches long from the 1×12 board.
2Cut the front wall 48 inches long from the 1×8 board.
3Cut two side walls 24 inches deep from the 1×12 board, then rip each so it slopes from 12 inches tall in back to 8 inches tall in front.
4Screw the front, back, and two sides together to form a sloped rectangle.
5Build a matching 4 ft × 2 ft lid frame from 1×2 strips.
6Attach the polycarbonate sheet or old window to the lid frame with screws and washers.
7Hinge the lid along the back wall.
8Set the bottomless box on prepared soil or over a raised bed.
9Prop the lid open 2–6 inches on sunny days, or install an automatic vent opener.
10Plant cold-hardy greens in late summer for winter harvest, or use it to harden off seedlings in spring.

Tools needed

·Circular saw or miter saw
·Table saw or circular saw with rip guide
·Cordless drill / driver
·Tape measure
·Speed square
·Safety glasses

Common mistakes to avoid

·Building a flat lid — rain pools and it overheats faster
·No venting — a closed cold frame can hit 80–90°F inside on a sunny winter day
·Using an old window with cracked glass or in a windy spot where shatter is a risk
·Placing it in deep shade or where water pools
·Filling the box with bagged soil instead of setting it on living ground
·Sealing or painting the inside of the box — plants need food-safe, untreated wood

Maintenance tips

Prop the lid 2–6 inches on sunny days, or install an automatic vent opener. Paint or seal only the outside wood. Lift or support the lid before heavy snow if it is glass; polycarbonate can stay put. Empty and scrub the box at season's end.

Community builds

See how other gardeners built their simple cold frame — and share photos of yours.

Build photos: 0 / 2 used

Upgrade

Paid plans include more build photos.

My Simple Cold Frame notes

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