Cold Frame Greenhouse | Extend Your Growing Season

A cold frame greenhouse is a bottomless, sloped-lid enclosure that traps solar heat to protect plants from frost and extend the growing season by weeks in each direction.

One sunny winter afternoon can push a cold frame’s interior to 70°F while the outside air hovers near freezing. That captured warmth creates a microclimate that keeps greens alive through light frosts, hardens off spring seedlings, and lets you start cold-weather crops earlier than any calendar says you should. Whether you build from scrap lumber or repurpose an old window sash, the design principles stay the same: a sloped box with a transparent lid, placed to catch the low winter sun.

What Makes a Cold Frame Work

A cold frame is a passive heat trap. The clear lid lets sunlight in, and the enclosed box keeps that warmth from escaping. The bottom is open to the ground, which means the soil itself stores heat and releases it at night. The sloped lid is essential — a flat lid can’t shed snow or water, and it won’t catch the angled winter sun the way a tilted surface will.

Typical dimensions from build guides include a 4-foot-by-4-foot frame with a 7-inch front and 12-inch back, or a 3-foot-by-6-foot frame with a 14-inch front and 18-inch back. The back should always be taller than the front so the lid angles toward the sun. Place the frame facing south if possible; west is the second-best option, then east, then north. Tucking the cold frame against a house wall or shed adds wind protection and some reflected warmth.

How to Build Your Own Cold Frame

The simplest build starts with untreated lumber — cedar is ideal because it resists rot without chemicals. Build a rectangular box with the back wall 4 to 6 inches taller than the front. Cut the side pieces with a sloped top edge to match that height difference. The lid can be framed from 2×2 lumber and covered with clear polycarbonate, acrylic, or heavy-duty polyethylene sheeting stapled in place and trimmed. Hinge the lid to the taller back wall so it lifts upward.

• Cut back wall to the desired height (e.g., 12 inches for a 4-foot-wide frame).
• Cut front wall shorter (e.g., 7 inches for the same frame).
• Cut side panels with angled tops to match the height difference.
• Assemble the box with screws or nails.
• Build the lid frame, attach the transparent cover, and hinge it to the back wall.
• Add weatherstripping around the lid’s contact edge to seal gaps.

For readers ready to buy rather than build, our tested roundup of greenhouse-with-cold-frame combos covers the best pre-built options that combine full greenhouse space with cold-frame utility.

Using Your Cold Frame — Ventilation and Winter Care

A cold frame overheats fast on a sunny day, even in winter. Open the lid a crack when the interior temperature rises above the mid-40s°F — one source recommends venting above 50°F (10°C). A stick, a hinged prop, or a ball chain can hold the lid open at the right angle. Close it by late afternoon to trap the day’s warmth. If a hard freeze is predicted, throw an old blanket or heavy burlap over the frame overnight for extra insulation.

FAQs

Can I use an old window for the lid?

Yes. An old window sash works perfectly as a cold frame lid. If the glass is missing, replace it with fiberglass or polycarbonate rather than installing new glass — it’s safer and handles temperature swings better.

How much warmer is a cold frame than outside?

On a sunny day, the interior can be 30°F to 40°F warmer than the outside air. Overnight, the difference shrinks to 5°F to 10°F, which is why adding an insulating blanket during severe cold helps.

Do I need to water plants inside a cold frame?

Less often than you’d expect. The enclosed environment holds moisture well, and the open bottom lets ground moisture rise. Check soil weekly and water sparingly — overwatering in a cold frame invites mold and rot.