How to Build a Cold Frame for Winter Gardening

A cold frame is one of the most effective season-extension tools a homestead gardener can build, and it is one of the simplest structures in homestead woodworking. At its most basic, a cold frame is a box without a bottom with a transparent or translucent lid that admits sunlight while trapping heat. It creates a microclimate inside that runs 10 to 20 degrees Fahrenheit warmer than the outdoor air, which translates to 4 to 6 additional growing weeks on each end of the season.

With a cold frame, a gardener in Zone 6 (last frost mid-April, first frost mid-October) effectively gains the growing conditions of Zone 7, allowing fall planting of spinach, kale, mache, and overwintering root vegetables through November and December, and spring planting of cold-tolerant seedlings starting in late February.

This guide builds a 4×4-foot cold frame sized to fit a standard single-pane window, which is the free or cheap lid material that makes cold frames economically compelling.

Finding Your Lid First

Build the cold frame box to fit the lid, not the other way around. Lids for cold frames are ideally old single-pane glass windows, which can be found at architectural salvage shops, Habitat for Humanity ReStore locations, and on Craigslist or Facebook Marketplace for $5 to $20 or sometimes free.

Old storm windows are excellent cold frame lids. So are sliding glass patio door panels. A piece of twin-wall polycarbonate greenhouse panel cut to size is the best performance option if you are buying rather than finding: it insulates better than single-pane glass, weighs less, and does not shatter.

Avoid double-pane sealed windows as cold frame lids. They are heavy, difficult to hinge, and the seal integrity matters less than the weight.

Measure your window before cutting any lumber. The cold frame box dimensions come from the window dimensions, not from any standard size.

For this guide, assume a standard 4×4-foot single-pane window, which is a common size for old wooden storm windows and produces a practical cold frame footprint.

Materials List for a 4×4 Cold Frame

• (4) 2x10x4-foot boards for the walls (cedar or PT lumber)
• (1) 2x10x4-foot board ripped to a tapered height (for the sloped back wall, optional)
• (1) pair of exterior hinges for the lid
• (1) lid support chain or prop stick to hold lid open
• (4) corner brackets or 4-inch angle brackets for corners
• (16) 2.5-inch exterior deck screws
• (1) old window or polycarbonate panel for the lid

Note on sloped versus level walls: A sloped cold frame has a taller back wall and shorter front wall, angling the lid toward the sun. In the northern hemisphere, this south-facing slope maximizes solar gain by presenting a larger surface area to the low winter sun. A level-walled cold frame (all four walls the same height) is simpler to build and performs adequately in most situations.

Building a Simple Level Cold Frame

Step 1: Cut the four wall boards to length. Two boards at 48 inches for the sides, two boards at 44 inches for the front and back. The 44-inch boards will fit inside the 48-inch boards to produce a 4×4 outside dimension.

Step 2: Assemble the box. Stand the two side boards on edge. Position the front and back boards between them at each end. Drive two 2.5-inch exterior screws through each side board into the end of each front/back board. Check for square by measuring diagonally (equal measurements = square).

Alternatively, use metal corner brackets at each interior corner. This is faster, does not require perfect alignment, and produces a very strong joint.

Step 3: Set the assembled frame in its location. A south or southeast-facing location that receives full sun is ideal. The frame does not need to be anchored; its weight and the soil contact hold it in place.

Step 4: Hinge the lid. Attach two exterior hinges to the back wall of the frame and to the back edge of the window. Position the hinges so the window opens from the front, lifting upward from the front edge. This allows you to open the frame for ventilation and watering from the front.

Step 5: Attach a lid prop. A stick, a chain, or a dedicated lid prop keeps the lid open at various heights for ventilation without requiring you to hold it. An adjustable chain attached from the side of the lid to a hook on the box side allows several preset ventilation positions.

Building a Sloped Cold Frame for Better Solar Gain

A sloped cold frame tilts the lid toward the south, presenting a larger surface to the low winter sun and shedding rain and snow better than a horizontal lid.

The back wall is taller than the front wall by 4 to 6 inches for a 4-foot-deep cold frame. A 4-inch slope over 4 feet is a mild slope adequate for most situations.

Cut the side walls with a taper: Each side board must be cut from 14 inches tall at the back to 10 inches tall at the front (a 4-inch rise over 4 feet). Mark the taper on each side board and cut with a circular saw guided along a straightedge.

Set the back wall at 14 inches, the front wall at 10 inches, and the tapered side walls connect them. The window lid rests on the sloped top of the four walls.

Siting the Cold Frame

Location: Full sun from at least 10am to 3pm is minimum. South or southeast-facing is ideal in the northern hemisphere. Avoid locations where shade from a fence, building, or large shrub reduces available winter sun.

Soil preparation inside the frame: The cold frame sits directly on the soil. Before placing it, loosen the top 6 to 8 inches of soil inside the frame footprint and mix in compost. This gives your cold frame plantings the best possible growing medium.

Thermal mass inside the frame: Adding thermal mass inside the frame increases its temperature stability. A black-painted gallon jug of water, a black plastic container of water, or dark rocks along the inside walls absorb heat during the day and release it slowly at night, keeping temperatures several degrees warmer than the outdoor ambient temperature. This is particularly valuable for protecting crops on nights when outdoor temperatures drop below 20 degrees Fahrenheit.

What to Grow in a Cold Frame

Fall and early winter: Plant cold-tolerant greens 6 to 8 weeks before your average first hard frost. Spinach, kale, arugula, mache (corn salad), tatsoi, claytonia, and Asian mustard greens all tolerate cold frame conditions through temperatures that would kill them outdoors. These plants do not grow quickly in cold conditions but they hold and can be harvested intermittently through November and December in most temperate climates.

Overwintering: Carrots and beets left in the ground and protected by a cold frame can often be harvested through winter in Zones 6 and warmer. The cold frame prevents the ground from freezing solid, keeping root vegetables accessible for harvest.

Early spring: Start cold-tolerant seedlings inside the cold frame 4 to 6 weeks before you would transplant them outdoors. Onions, leeks, celery, and early brassicas (cabbage, broccoli) can be started in the cold frame in late winter and transplanted after the last frost.

Hardening off seedlings: A cold frame is ideal for hardening off seedlings started indoors. Move them to the cold frame for a week before outdoor transplanting, acclimating them to outdoor temperature variation without risk of frost damage.

Ventilation: The Most Important Cold Frame Management Task

The most common cold frame management mistake is leaving the lid closed on sunny days when outdoor temperatures are cool. Solar gain through the glass can raise cold frame interior temperatures to 80 or 90 degrees Fahrenheit on a clear 40-degree day, which stresses or kills cold-tolerant plants adapted to cooler conditions.

Prop the lid open on any day when outdoor temperatures will be above 40 degrees Fahrenheit and the sun is shining. Close it before temperatures drop in late afternoon.

A simple automatic vent opener, the same device used in greenhouse ridge vents, uses a wax-filled cylinder that expands when warm and contracts when cool to open and close the lid automatically. This eliminates the need to check and adjust the cold frame daily. They cost $30 to $50 and pay for themselves in the first season of use by preventing heat-related plant loss.

Automatic cold frame vent opener

Cold Frame Materials and Tools

Cedar 2×10 boards for cold frame walls

Twin-wall polycarbonate panel for lid if you cannot find an old window.

Exterior hinges for cold frame lid

Interior corner bracket set for box assembly

Min-max thermometer for monitoring cold frame temperature records overnight lows inside the frame to guide planting decisions.

See our full homestead woodworking series: homestead woodworking beginners guide, how to build raised garden beds, and DIY compost bin plans.