A cold frame is a box with a window on top. That is the whole idea. There is no heater, no power, no plumbing, and yet it will hand you spinach in February and let you start hardening off transplants a month before your neighbours dare.
It is also the cheapest season extension you can build. A salvaged storm window and four boards will do it, and the payback is measured in weeks of harvest at both ends of the year.
What you will find below:
A cold frame is a shallow, usually bottomless box with a transparent lid, heated by nothing but the sun. Soil and framing absorb heat during the day and release it overnight, and the lid stops wind stripping that warmth away.
Here is the honest number. University of Missouri Extension puts the inside temperature at generally no more than 5 to 10 F (3 to 6 C) above outside, with a mat or blanket thrown over on a cold night adding a few degrees more. That sounds trivial. It is not, because those few degrees sit exactly at the boundary between a hardy crop surviving and a hardy crop turning to mush.
The wind protection matters as much as the heat. So does the reduction in freeze-thaw cycling, which is what actually destroys overwintered roots. Add it together and practitioners describe a decent frame as roughly a zone and a half warmer than the open garden.
| Structure | Protection | Best For |
| Floating row cover | 2 to 8 F (1 to 4 C) by fabric weight | Light frost, insect exclusion, whole beds |
| Cold frame | 5 to 10 F (3 to 6 C), plus wind shelter | Winter salad, hardening off, small area |
| Low tunnel | Similar to a frame, over a whole row | Row crops, 15 to 30 in (38 to 76 cm) high |
| High tunnel | Substantially more, walk-in scale | Serious production, higher cost |
| Hotbed | Frame plus added heat from manure or cable | Warm-season starts, germination |
Sources: University of Missouri Extension G6965, University of Nebraska Extension G2246, Penn State Extension, Season Extenders
On season length, Iowa State University Extension gives two to four weeks at each end of the season, while University of Nebraska Extension puts spring planting three to five weeks earlier. Call it a month either side, more in a mild winter, less in an exposed site.
Iowa State's specification is the one to copy because it is built around your arms rather than around a picture. No more than 3 ft (0.9 m) front to back, so you can reach the far corner without climbing in. About 12 in (30 cm) high at the front, 18 in (46 cm) at the back.
That 6 in (15 cm) of difference is not decoration. The slope tilts the glazing toward a low winter sun so more light gets in, and it sheds rain and snow instead of pooling. Face the open slope south. Tuck the high back against a wall, fence or garage if you have one, which cuts heat loss on the shaded side.
Find your lid first
Everything else is sized to it. An old storm window, a shower door, a sheet of twinwall polycarbonate. Glass gives the best light and the most weight. Twinwall polycarbonate insulates better, weighs almost nothing and will not shatter when a branch lands on it.
Build the box to match
Untreated cedar or larch lasts years in soil contact without chemistry you would rather not have near salad. Avoid anything treated with creosote. Painting the outside white reflects light and slows decay; painting the inside walls black helps them absorb and re-radiate heat.
Site it on free-draining ground, facing south
Standing water under a frame in February is a bigger problem than cold. Leave it bottomless so roots reach real soil and the ground can act as your thermal mass.
Hinge the lid and add a prop
Hinges at the back, a notched stick or a length of chain to hold it at two or three vent positions. This is the part people skip and then regret in the first week of March.
Insulate the perimeter
Bank soil, straw bales or leaves against the outside walls. Iowa State specifically recommends this to flatten out temperature swings, which is the thing that damages winter crops more than absolute cold.
Budget: roughly $40 to $80 in lumber and hardware if the lid is salvaged, and a weekend afternoon. Manufactured units start well above that and rarely fit an existing bed as neatly.
Why This Works: Catching and Storing Energy
Nothing in a cold frame generates heat. It collects sunlight that was already falling on that patch of ground, stores it in soil and timber, and slows the rate at which it leaves. That is the same principle behind a south-facing wall that ripens peaches, a pond that moderates frost, and a stone path that keeps a bed warm after dark. Once you notice it, you start reading your whole garden for places where energy is arriving and nothing is catching it, which is a habit that runs right through permaculture design.
A cold frame does four different jobs, and most people only use it for one.
Autumn and winter harvest. Sow in late summer or early autumn so plants are near full size before the light fades. Spinach, mache, claytonia, arugula, tatsoi, leaf lettuce, kale, parsley and scallions all hold through winter under a frame in much of the country.
Overwintering for an early spring. Small plants sitting dormant under glass explode into growth in late February while open ground is still waking up. This is where the real yield gain is.
Hardening off. Tomatoes, peppers and annuals coming out of the house need a gradual introduction to wind and direct sun. Iowa State makes the point that a frame turns this from carrying trays in and out for a fortnight into closing a lid at night.
Seed starting. Cool-season crops such as lettuce, spinach and radish go in two to four weeks earlier than open ground. For anything that needs a winter chill instead of protection, the frame is the wrong tool: those species want cold-moist stratification, not warmth.
One expectation to reset. Between roughly early November and early February in the northern half of the country, daily light drops below about 10 hours and plants effectively stop growing. Eliot Coleman calls this the Persephone period.
Nothing you plant in December will size up in December. A cold frame in deep winter is a refrigerator holding a crop you already grew, and that is exactly how to plan it: get the growth done in autumn, then harvest slowly through the dark.
Cooking Is More Likely Than Freezing
The classic cold frame failure is not a hard freeze. It is a sunny 45 F (7 C) day in March with the lid shut, when the inside climbs past 85 F (29 C) by mid-morning and bolts or kills a crop that would have shrugged off a frost. Iowa State is explicit: vent above 85 F (29 C), close before sunset, and check even on cloudy days. If you are away from the house on weekdays, an automatic wax-cylinder opener at around $40 to $60 is not a luxury, it is the difference between a working frame and a dead one.
Get Our Free Companion Planting Chart
Join 10,000+ gardeners getting weekly tips on what to plant together, soil health, and permaculture techniques.
Send Me the ChartWater rarely and early in the day. Plants under glass in winter use very little, and wet foliage going into a cold night invites botrytis. Water the soil, not the leaves, and do it in the morning so foliage dries before dusk.
Brush snow off the lid. Insulating snow sounds appealing until you realise it blocks the light your crop needs, and heavy wet snow can crack a glass sash.
Watch for tenants. A warm, sheltered box with fresh greens is exactly what voles want in January. If you see runs, set traps early rather than losing the lot.
And if you want a frame to do more than hold ground, add heat and it becomes a hotbed. Traditionally that meant a deep layer of fresh manure below the growing bed, releasing warmth as it broke down, which is the same decomposition process behind a compost heap put to work under your feet. Electric soil cables do the same job with a switch.
Key Takeaway
Build it 3 ft (0.9 m) deep at most, 12 in (30 cm) front and 18 in (46 cm) back, facing south, and solve venting before you solve anything else. A frame you cannot vent on a working Tuesday will cook its contents long before winter gets a chance at them.
It extends the growing season at both ends without power or plumbing. Iowa State Extension puts the gain at two to four weeks in spring and autumn, and Nebraska Extension at three to five weeks earlier for spring planting. It also gives you a place to harden off transplants, overwinter hardy greens for an early spring crop, and shelter tender perennials.
Missouri Extension puts the typical differential at no more than 5 to 10 F (3 to 6 C) above the outside air, with an insulating mat over the top on a cold night adding a few degrees more. Practitioners often describe the effect as roughly a zone and a half of protection, because wind shelter and reduced freeze-thaw cycling matter as much as the raw temperature gain.
Cool-season crops. Spinach, mache, claytonia, arugula, tatsoi, leaf lettuce, kale, parsley, scallions and carrots all perform well. Warm-season crops such as tomatoes and peppers only belong in a frame temporarily, while they harden off before planting out.
Salvage the lid and build the box to fit it. An old storm window or shower door plus four lengths of untreated cedar, hinges and screws will come in around $40 to $80. Straw bales with a window laid on top work as a no-build version for a single season.
Overheating first, by a wide margin, followed by poor drainage under the frame, condensation and fungal disease from evening watering, snow blocking light, and rodents moving in. All five are management issues rather than design failures.
Yes, and it is often the easiest route. Build a sloped collar that sits on the existing bed walls, or make the frame the same footprint as the bed so the lid rests on the top edge. Keep the same rules: no more than 3 ft (0.9 m) front to back, higher at the back than the front, facing south.
Ready to Grow Smarter?
Get our free 20-page beginner's guide to backyard food forests. Two printable worksheets, a year-by-month food-forest calendar, and a curated reading path.
Read the Free Guide