How to Build a Cold Frame for Winter Greens and Seedlings
Learning how to build a cold frame comes down to five decisions: a bottomless box no deeper than you can reach across, a clear lid sloped toward the south, glazing that lets light in and holds some heat, a site with full winter sun and shelter from the north wind, and a way to prop the lid open on sunny days. Get those right and a weekend build gives you spinach in January, lettuce weeks before the garden opens, and a safe halfway house for seedlings in spring. Get the siting or venting wrong and the best built frame either freezes or cooks.
Plan the starts that will fill it. Our free seed starting calendar counts back from your frost date so the frame is built and ready when the first trays need it.
Updated September 2026
What a cold frame does, and what it does not
A cold frame is a low box with a transparent lid, set directly on the soil. Sunlight passes through the lid and warms the soil and air inside. The box walls block wind, and the closed lid slows the loss of that stored heat overnight. There is no heater, fan or power involved.
That simple structure does three jobs well:
- Holds cold hardy greens through winter. Spinach, mache, claytonia, kale, scallions and hardy lettuce planted in late summer or early fall stay alive and harvestable well into winter, then start growing again weeks ahead of the open garden in late winter.
- Starts and hardens off seedlings in spring. Brassica and lettuce transplants can move from the windowsill into the frame weeks before they could go in the ground, and the frame is the easiest possible place to toughen up warm season starts.
- Shelters overwintering pots and winter sown containers. Perennial divisions, cuttings and jugs of winter sown seed get protection from the worst wind and freeze and thaw cycling.
What it does not do is grow crops quickly in midwinter. In December and January, short days limit growth far more than cold does, so the goal is to plant early enough that crops reach near full size before the days get short, then use the frame to hold them. The timing and how much season a frame adds compared with row cover and low tunnels are laid out in our guide to what to plant in October. A cold frame also does not replace a frost plan for the rest of the garden, which our page on how to protect plants from frost covers.
Pick the size before anything else
The size of a cold frame is set by two limits: how far you can reach, and the size of the glazing you have.
Depth, front to back, should be 3 to 4 feet at most. You tend a cold frame from the front, kneeling or bending over, and you need to reach the back row without climbing in. Three feet is comfortable for most people. Four feet is the maximum, and only for a frame you can reach from both the front and the back.
Length can be whatever the site allows. Longer frames are simply built in sections, with one lid per section. Keeping each lid at 3 to 4 feet wide keeps it light enough to lift one handed.
Height depends on what grows inside. Low greens need only 8 to 12 inches of headroom at the front. Seedlings in trays, overwintered pots and kale need more, around 12 inches in front and 18 to 24 inches in back.
| Frame size (outside) | Lids | Good for | Notes |
|---|---|---|---|
| 3 x 3 ft | 1 | A single salvaged window, a few flats of seedlings | The easiest first build. Heats and cools fastest |
| 3 x 6 ft | 2 | Winter greens for a household, plus spring seedling flats | The most useful all round size |
| 4 x 8 ft | 2 or 3 | Sits on top of a standard raised bed | Match the raised bed exactly so the frame can go on and off by season |
| 3 x 12 ft or longer | 4 or more | Serious winter salad production | Build in 3 or 4 foot sections so each lid stays manageable |
Bigger frames hold temperature better. A tiny frame has little air and soil mass, so it swings hot and cold quickly. A 3 x 6 foot frame is noticeably steadier than a 3 x 3, and it is still small enough to vent by hand.
If you are building around a salvaged window or door, measure the glazing first and build the box to fit it, with the lid overhanging the walls by about an inch on the front and sides so rain runs off the outside of the box.
The lid angle
The lid slopes down from a tall back wall to a short front wall. That slope does three things: it tilts the glazing toward the low winter sun, it sheds rain and snow, and it keeps condensation running down the glass instead of dripping on the plants.
The ideal angle for capturing midwinter sun is steep, often 45 degrees or more, but a very steep lid makes a tall back wall, a big volume of air to heat and a frame that loses heat quickly. Practical cold frames use a compromise between 15 and 35 degrees, steeper the farther north you are.
| Latitude (roughly) | Example areas | Suggested lid angle | Back wall rise over a 3 ft deep frame | Lid length front to back |
|---|---|---|---|---|
| 30 to 34°N | Gulf Coast, north Florida, central Texas | 20° | About 13 in | About 38 in |
| 35 to 39°N | Tennessee, North Carolina, Oklahoma, northern California | 25° | About 17 in | About 40 in |
| 40 to 44°N | Pennsylvania, Ohio, Illinois, Colorado, New York | 30° | About 21 in | About 42 in |
| 45°N and north | Minnesota, northern Wisconsin, Maine, northern Oregon, Washington | 35° | About 25 in | About 44 in |
"Back wall rise" is how much taller the back wall is than the front wall. So at 40°N, a frame with a 12 inch front wall wants a back wall of about 33 inches for a full 30 degree lid. Plenty of good frames use a shallower slope than the table suggests, especially when the back wall is built from stacked boards. A 2x12 front and two stacked 2x12s in back, on a 3 foot deep frame, gives about 17 degrees, which works well in the South and is perfectly usable farther north. Anything under about 10 degrees starts to collect snow and pooled rain on the lid.
Materials for the box
The walls hold heat, block wind and support the lid. Almost anything solid works, and each material has trade offs.
| Material | Lifespan | Insulation | Best use | Notes |
|---|---|---|---|---|
| Rot resistant boards, such as cedar, 2 inches thick | Many years | Good | Permanent or movable frames | Easy to cut into angled sides. Untreated cedar is the common choice around food crops |
| Standard lumber, 2 inches thick | Several years | Good | Movable frames | Seal the outside with exterior paint. Paint the inside white to bounce light onto plants |
| Modern pressure treated lumber | Many years | Good | Permanent frames | Current treatments are considered acceptable in many food gardens, but many growers line the inside with plastic or choose untreated wood instead |
| Concrete block or brick | Decades | Fair, but excellent heat storage | Permanent frames against a wall | The blocks soak up daytime heat and release it overnight. Fill the cores with soil or sand |
| Straw bales | One or two seasons | Excellent | Fast, temporary frames | Stack bales in a rectangle and lay a window across. Compost the bales in spring |
| Rigid foam board | Several years if covered | Excellent | Adding insulation to the north and end walls | Protect it from sunlight with a board facing, since UV breaks it down |
| Salvaged pallets | A few years | Poor unless lined | Temporary frames | Use only pallets stamped HT, meaning heat treated. Avoid any stamped MB, which were treated with methyl bromide |
Two quick upgrades make any wooden frame work better. Line the inside of the north wall with rigid foam or stack straw bales against the outside of it, since the north wall loses the most heat and gains almost nothing from the sun. And paint the inside of the back and side walls white or line them with reflective material, which bounces light down onto the plants instead of letting dark wood absorb it.
Glazing choices
The lid covering, called glazing, has to let in as much light as possible while slowing heat loss. Here is how the common options compare.
| Glazing | Light transmission | Heat retention | Weight | Lifespan | Notes |
|---|---|---|---|---|---|
| Old glass window sash | High, around 90 percent | Fair | Heavy | Decades if not broken | The classic free lid. Test old painted sashes for lead before handling, and wet scrape rather than sand |
| Twin wall polycarbonate, 6 to 8 mm | Around 80 percent, diffused | Best of the common options | Light | 10 years or more | Diffused light prevents hot spots. Put the UV coated side up and tape the open flutes at the ends |
| Single wall corrugated polycarbonate | High, around 90 percent | Fair | Light | Many years | Rigid and nearly unbreakable. Seal the corrugations at the ends with foam closure strips |
| Acrylic sheet | High | Fair | Light | Several years | Scratches easily and can crack when cut or drilled in cold weather |
| Greenhouse film, 6 mil, on a wood frame | High when new | Poor as a single layer | Very light | One to four seasons | Fast and simple. Double it with an air gap for better insulation |
| Clear shower door or storm door glass | High | Fair | Very heavy | Decades | Tempered glass cannot be cut, so build the box to the door size |
Twin wall polycarbonate is the best all round choice for a new build. The air channels between its two layers insulate far better than a single pane, it is light enough to lift easily, and it will not shatter under snow or a dropped tool. Seal the top edge of the flutes with aluminum tape and the bottom edge with vented tape, so moisture can drain out without dirt and insects getting in.
A salvaged window is the best choice for a first frame built from what you have. Old wooden sashes are heavy, and single pane glass holds heat less well, but they last a long time and the only real work is finding one. Old single pane glass is not tempered and breaks into sharp shards, so wear gloves, keep the frame where children and pets do not play, and never climb on it. Build the box to fit the window rather than cutting the glass.
Where to put a cold frame
Siting decides more about a cold frame's performance than materials do.
Face the lid south. The low side of the frame should face due south, or within about 20 degrees of it. A frame facing southeast warms up earlier in the morning, which helps after cold nights. A frame facing southwest runs hotter in the afternoon.
Give it full winter sun. The site needs 6 or more hours of direct sun in December, not in June. Winter shadows are long: at about 40°N, a 6 foot fence throws a shadow roughly 12 feet long at noon in late December, so a spot that is sunny in summer can be shaded all winter. Our guide to full sun vs partial sun vs partial shade has the shadow lengths for each season and latitude, and it is worth checking your spot in November before you build a permanent frame there.
Put the back against a south facing wall if you can. A house foundation, garage or masonry wall on the north side of the frame blocks the wind and radiates stored heat into the frame overnight. It is the single best upgrade available.
Block the north and west wind. If there is no wall, a fence, hedge, a row of straw bales or a berm of soil on the north and west sides makes a clear difference.
Choose well drained ground. A frame in a low spot collects cold air and standing water. Build on level or gently sloping ground and, if drainage is poor, raise the soil inside a few inches above the surrounding grade.
Keep it close to the house. You will visit the frame almost daily to vent it. A frame near the back door gets vented. A frame at the far end of the yard gets forgotten on the one sunny day it matters.
How to build a cold frame, step by step: a 3 x 6 foot plan
This version uses 2x12 boards for the walls and two twin wall polycarbonate lids. It gives a lid angle of about 17 degrees, which you can steepen by adding a ripped board to the back wall.
Cut list
| Part | Material | Quantity | Size |
|---|---|---|---|
| Front wall | 2x12 board | 1 | 72 in long |
| Back wall | 2x12 board | 2, stacked | 72 in long |
| Lower side walls | 2x12 board | 2 | 33 in long, to fit inside the front and back walls |
| Upper side wedges | 2x12 board | 1 board ripped corner to corner | 33 in long, makes 2 triangles |
| Corner cleats | 2x4 board | 4 | Cut to the inside height at each corner |
| Center lid rest | 2x4 board | 1 | Front to back across the middle of the frame |
| Lid frames | 2x2 boards | 4 long pieces, 4 short pieces | Sized to make two lids about 36 in wide and 38 in long |
| Glazing | Twin wall polycarbonate, 6 to 8 mm | 2 sheets | Cut to the outside size of each lid frame |
| Hardware | Exterior screws, 4 strap hinges, 2 handles, aluminum tape, vented tape, foam weatherstrip, 4 hook and eye latches | As needed |
Assembly
- Build the back wall. Stack the two 72 inch 2x12s on edge and join them with two cleats screwed across the inside face. This gives a back wall about 22 1/2 inches tall.
- Attach the corner cleats. Screw a 2x4 cleat inside each end of the front and back walls. The side walls fasten to these cleats, which keeps screws from splitting the board ends.
- Fit the lower side walls. Set the 33 inch side boards between the front and back walls and screw them into the cleats. You now have a rectangular box 11 1/4 inches tall all the way around.
- Add the wedges. Rip a 33 inch 2x12 diagonally from one corner to the opposite corner to make two long triangles. Set one on top of each side wall with the tall end at the back, and screw it to the back wall cleat and to the side wall below with a short cleat or mending plates. The sides now slope from the front wall up to the back.
- Install the center rest. Screw a 2x4 across the top of the frame from front to back at the midpoint, following the slope, so the inside edges of both lids have something to sit on.
- Build the lids. Screw the 2x2s into two rectangular frames. Add a middle crosspiece to each if you are using lighter glazing or you get heavy snow.
- Attach the glazing. Tape the flute ends of the polycarbonate, aluminum tape at the top edge and vented tape at the bottom edge, then screw the sheet to the lid frame using screws with rubber washers. Drill slightly oversized holes so the sheet can expand and contract with temperature.
- Hinge the lids to the back wall. Two strap hinges per lid, mounted along the top edge of the back wall, let each lid swing up and back.
- Weatherstrip the top edges of the walls with foam tape so the closed lids seal without gaps.
- Add handles and latches. A handle on the front edge of each lid, and two hook and eye latches holding the front edge of each lid to the front wall, so wind cannot lift a closed lid.
- Set the frame in place. Level the frame on the site, pack soil around the outside base to seal gaps, and fill the inside with loose, compost rich soil or set trays directly on a gravel floor for seedlings.
Venting: the job that keeps plants alive
A closed cold frame traps solar heat very efficiently. On a clear winter day, even when the air outside is near freezing, the inside can climb into a range that wilts greens and scorches seedlings within an hour or two. Overheating kills more plants in cold frames than cold does.
Know your target range. Winter greens are happiest with daytime temperatures inside the frame between about 45 and 70°F. Seedlings and spring transplants do well between about 60 and 75°F during the day. Anything above 80°F is too hot for either.
Put a thermometer inside. A simple min and max thermometer, hung at plant height and shaded from direct sun, tells you what happened while you were away. That record teaches you more about venting your particular frame than any rule.
Use a simple venting routine.
- Cloudy days near or below freezing: leave the lid closed.
- Sunny days in the 30s: crack the lid a couple of inches by late morning.
- Sunny days in the 40s: prop it open several inches.
- Days in the 50s and warmer, sunny or not: open it wide, or remove the lid for the day.
- Close it by mid afternoon, around 2 or 3 p.m. in winter, so the frame captures the last warm air before the temperature drops.
Consider an automatic vent opener. These non electric openers use a cylinder of wax that expands as it warms, pushing a piston that lifts the lid. Most can be adjusted to start opening somewhere between about 60 and 75°F. They are ideal for anyone who is away during the day, as long as the lid is light enough for the opener's rated lift. Lightweight polycarbonate lids suit them well; heavy glass sashes usually do not.
Water sparingly and in the morning. A closed frame loses very little moisture, and cold, wet soil and damp air invite rot and mildew. Water only when the top inch of soil is dry, early in the day, so leaves dry before the lid closes. Our guide to damping off explains why seedlings in cool, wet, still air are especially at risk.
Building a lid prop
Every cold frame needs a way to hold the lid open at several heights and a way to stop the wind from throwing it over backward.
A notched stick prop is the classic answer. Take a piece of 1x3 or 2x2 about 16 to 20 inches long. Cut shallow notches into one edge at roughly 2, 4, 8 and 12 inches from the bottom. Stand the stick on the front wall, or fasten it to the side wall with a single screw so it pivots, and rest the front edge of the lid in whichever notch gives the opening you want. Two inches is a winter crack, 12 inches is a warm day.
A stepped block works too. Cut a short 4x4 into a stair step shape with steps at 2, 4 and 6 inches. Set it on the front wall under the lid and turn it to choose the height.
Add a stop so the lid cannot flip. A lid swung fully open is a sail. Fasten a short length of chain or a strong strap from the inside of the lid to the back wall, so the lid can open to just past vertical and no farther. Without it, the first gust can tear the hinges out.
Latch it when closed. The hook and eye latches from the build keep a closed lid from lifting in a storm, and a brick on each lid is a fine backup in a windy site.
Adding heat retention for the coldest nights
In zones 3 through 6, a few extra steps help a frame through the deepest cold.
- Bank the outside walls. Pile soil, shredded leaves or straw bales against the outside of the walls, especially the north and ends.
- Add thermal mass inside. Dark colored gallon jugs of water along the back wall absorb heat during the day and release it overnight. Leave room so they do not shade the plants.
- Cover the lid on the coldest nights. An old blanket, a moving pad or a sheet of rigid foam laid over the closed lid at dusk holds in more heat. Remove it every morning so plants get light.
- Add an inner layer. A piece of floating row cover laid directly over the plants inside the frame adds another layer of protection for very little effort.
- Make it a hotbed. The traditional way to heat a frame is fresh horse manure mixed with straw, packed 12 to 18 inches deep under 6 inches of soil. As it composts it warms the soil from below for several weeks, which is ideal for early spring seedlings. Let the initial heat spike pass before planting.
If you find yourself wanting more heat than a frame can give, the next step up is a small greenhouse, and our guide on how to keep a greenhouse warm in winter covers insulation, thermal mass and heaters at that scale.
What to grow in your cold frame
Late summer and fall sowings for winter harvest: spinach, mache, claytonia, arugula, hardy lettuce varieties such as Winter Density and Arctic King, scallions, kale, mustard, tatsoi and parsley. Sow them early enough to reach near full size before daylight drops below about 10 hours a day, then harvest a few leaves at a time through winter. Our guide on how to grow spinach covers the most dependable winter crop in detail.
Late winter and early spring starts: broccoli, cabbage, kale and lettuce transplants can move from indoors to the frame about four to six weeks before your last frost. Check your date with our frost dates tool. A bottomless frame set over a rhubarb crown or an asparagus row in late winter also warms the soil and brings the first spears and stalks on early, and our guide to perennial vegetables by zone covers those crops.
Spring hardening off: warm season seedlings such as tomatoes and peppers go into the frame for their final adjustment, with the lid opened a little more each day until it stays open day and night. The day by day schedule is in our guide to hardening off seedlings.
Winter sown containers and overwintering pots: milk jugs of winter sown seed, perennial divisions and hardwood cuttings all benefit from the shelter. So do potted rosemary and lemon verbena in zones where they are not reliably hardy, and our perennial herbs by zone guide shows where that line falls. Our page on winter sowing explains which seeds work best that way.
Frequently asked questions
What is the best size for a cold frame?
Three feet deep and 6 feet long is the most useful size for most gardens. Three feet front to back lets you reach the back row without climbing in, and 6 feet split into two lids gives enough growing space for winter greens plus spring seedling flats while keeping each lid light enough to lift. Build longer frames in 3 or 4 foot sections, one lid per section.
What angle should a cold frame lid be?
Between about 15 and 35 degrees, steeper the farther north you live. A 20 degree slope works well near the Gulf Coast, around 30 degrees suits the middle of the country, and 35 degrees helps in the northern states where the winter sun is lowest. Avoid slopes under about 10 degrees, which collect rain and snow on the lid.
Which direction should a cold frame face?
South, with the low front wall facing due south or within about 20 degrees of it. Place it where it gets 6 or more hours of direct sun in December, ideally with the tall back wall against a south facing house or garage wall, which blocks north wind and radiates stored heat overnight.
What is the best material for a cold frame lid?
Twin wall polycarbonate is the best all round glazing for a new build. It insulates better than single pane glass, diffuses light to prevent scorching, is light enough to lift and vent easily, and will not shatter. An old glass window is the best choice if you want to build from salvaged materials, as long as you check old painted sashes for lead paint.
How do I build a cold frame from salvaged materials?
Stack straw bales in a rectangle about 3 feet deep, with the back row one bale higher than the front, and lay an old window or storm door across the top. Or build a simple box from leftover boards to fit a salvaged window, or from heat treated pallets lined with plastic. The key features stay the same: a clear lid sloped to the south, walls that block wind, and a way to prop the lid open.
When should I open my cold frame?
Open it whenever the sun is shining and the outside temperature is above freezing. Crack the lid a couple of inches on sunny days in the 30s, open it several inches in the 40s, and open it fully or remove it on days in the 50s or warmer. Close it by mid afternoon to trap warm air for the night. A thermometer inside the frame is the best guide.
Do I need to water plants in a cold frame in winter?
Yes, but far less often than in summer. A closed frame loses little moisture, so check the soil every week or two and water only when the top inch is dry. Water in the morning on a mild day so leaves dry before the lid closes, because cold, wet conditions encourage rot and mildew.
Can I use a cold frame to harden off seedlings?
Yes, and it makes the job much easier. Move seedlings into the frame and open the lid a little more each day instead of carrying trays in and out. Start with a small crack, increase the opening every day or two, and leave the lid fully open day and night for the last few days before transplanting. Never leave seedlings sealed in a closed frame on a sunny day.
