Cold Frames for Season Extension

What a cold frame does — and why it works

A cold frame is an unheated, bottomless box with a transparent lid. It traps solar heat, blocks wind, and protects plants from frost — extending the growing season by 4-6 weeks on each end without any heating costs.

The physics are simple: the transparent lid admits sunlight, which warms the soil and air inside. The soil absorbs heat during the day and releases it slowly at night. The box walls block wind, which is the fastest source of heat loss in an unprotected garden bed. The net result: the inside of a cold frame stays 10-15°F warmer than the surrounding air on most nights.

On a 20°F night, that means the interior stays around 32-35°F — cold enough to suppress growth but not cold enough to kill hardy greens like spinach, kale, and mâche. Those crops can withstand temperatures down to 20°F when their cells are acclimated.

Cold frames are not magic — they do not replace a greenhouse in the depth of winter. But in Zones 5-7, a well-built cold frame can:

• Extend fall vegetable production through mid-November or later
• Allow winter survival of cold-hardy greens in Zones 6-7
• Let you start transplants 4-6 weeks earlier than outdoor planting dates in spring
• Harden off indoor seedlings without repeatedly moving trays inside and out

Building a basic cold frame

Materials

The simplest cold frame is a rectangular box with a transparent lid. Nothing more is needed.

Frame options:

Lid options:

Recommended combination: Cedar sides + twin-wall polycarbonate lid. The polycarbonate is lighter than glass, doesn’t shatter, and provides about 80% light transmission — similar to glass.

Dimensions

A standard cold frame matches the size of an old storm window — typically 3 feet deep by 6 feet wide. A 4x8 foot cold frame is the most common DIY size and fits a standard sheet of polycarbonate.

Make the back wall (north side) taller than the front wall (south side) by 4-6 inches. This angle tilts the lid toward the sun for maximum light capture. A common design: 12-inch-tall back wall, 8-inch-tall front wall.

Basic construction steps

1. Cut four boards to form the rectangular box.
2. Screw corners together — use galvanized or stainless screws to resist rusting.
3. Attach a hinge along the top back edge of the frame for the lid.
4. Attach a prop stick (a painted wooden dowel works well) to the lid for ventilation control.
5. No bottom needed — the cold frame sits directly on the soil.

Total material cost for a 4x4 foot cedar frame with polycarbonate lid: approximately $80-150 depending on local prices.

Where to place a cold frame

Siting matters almost as much as construction.

South-facing exposure is essential. The lid needs direct winter sun for most of the day. A cold frame on the north side of a house or under a tree collects far less heat and extends the season minimally.

Against a south-facing wall or fence adds free heat. Masonry walls (brick, concrete block, stone) absorb solar heat during the day and re-emit it at night. A cold frame positioned against a south-facing masonry wall can be 2-4°F warmer inside than a freestanding frame. This advantage is significant in Zones 5-6.

Avoid low spots. Frost settles in depressions. A cold frame in a low spot where cold air pools will be colder than the surrounding garden, not warmer.

Close to the kitchen is ideal. You will use a cold frame more often — and harvest from it more reliably — if it’s convenient to reach. A cold frame at the back of the lot gets ignored; one near the back door gets used.

What to grow in a cold frame

Fall cropping (September–November in Zone 6)

Plant these in the open garden in late summer and move the cold frame over them in late September, or transplant seedlings into the cold frame directly:

• Spinach — the gold standard cold frame crop; survives 20°F+ inside a frame
• Kale and chard — very cold-hardy, productive even in slow-growth conditions
• Lettuce — grows slowly below 40°F but survives and resumes growth on warm days
• Mâche (corn salad) — one of the hardiest salad greens; thrives in cold frames
• Arugula — cold-tolerant, germinates in cool soil; a bit spicy in cold weather
• Radishes — fast (25-30 days); plant in mid-September for November harvest
• Scallions — harvest green tops through November; bulbs survive harder freezes

Seed these crops 8-10 weeks before your first frost for a well-established plant going into cold-frame protection. For timing guidance, see our succession planting guide on staggered fall plantings.

Winter survival (Zones 6-7)

In Zones 6-7, spinach, kale, mâche, and carrot tops left in the ground can survive winter inside a cold frame. The plants go nearly dormant below 40°F but resume growth when days warm in February and March. This gives early spring production with zero spring planting effort.

Spring season extension (February–April in Zone 6)

Cold frames shine in early spring. Start seeds inside the cold frame 4-6 weeks before your last frost date:

• Lettuce, spinach, arugula (direct seed, 45-55 days)
• Kale transplants (started indoors, moved to cold frame 4 weeks before last frost)
• Onion seedlings for transplant
• Peas (in Zone 6, you can direct seed peas in a cold frame in early March)

For indoor seed starting that complements a cold frame, see our how to start seeds indoors guide.

Ventilation: the most common cold frame mistake

Overheating kills more plants in cold frames than cold does. On a clear day in January, inside a cold frame can reach 80-90°F while outdoor temperatures are in the 30s. At 90°F, lettuce wilts; prolonged overheating causes tip burn and permanent damage.

Rule: Vent when inside temperature reaches 70°F.

Practical ventilation method: prop the lid open with a stick. Use different prop heights:

• 2-inch gap: mild ventilation on calm, sunny days
• 4-6 inch gap: stronger ventilation on warm days
• Fully closed: nights and cold, cloudy days

Close the lid before sunset — you want to trap daytime heat for the night.

Automatic vent openers are one of the best cold frame upgrades available. They use a wax-filled cylinder that expands with heat, opening the lid automatically when a preset temperature is reached and closing it as the day cools. They require no power and cost $25-40. For gardeners who cannot check their cold frame daily, these are essential.

Zone-by-zone cold frame calendar

“Close date” = when to stop relying on outdoor growing and start cold-frame-dependent production. “Open date” = when outdoor temperatures are reliable enough to stop using the cold frame for frost protection.

Heating a cold frame into a hotbed

A hotbed is a cold frame with added bottom heat — usually electric heating cables buried under the soil or, in the traditional method, a layer of fresh horse manure that generates heat as it composes.

Heating cables let you maintain 65-70°F soil temperatures inside the structure year-round. This turns a cold frame into a nearly full-season propagation unit, capable of germinating tropical seeds in February. The cost: roughly $1-3 per day in electricity, depending on outside temperature and cable wattage.

For most home gardeners, a cold frame without heat provides enough season extension to justify the construction effort. A hotbed is more complex and expensive, but valuable if you start hundreds of seedlings or grow heat-loving crops in a short-season climate.

Cold frames and fall garden preparation

Cold frames pair naturally with fall garden prep. While the cold frame protects your fall salad crops, the rest of the garden beds can be put to rest properly. After summer crops come out:

• Add 2-3 inches of compost
• Plant a cover crop (rye, clover) in beds that will sit empty until spring
• Continue harvesting from the cold frame through November or later

For the full fall garden wind-down routine, see our fall garden prep guide.

Cold frame quick reference

For more on frost protection methods that pair with cold frames, Cornell University Cooperative Extension covers row covers, frost cloth, and related techniques with research-backed guidance for northeastern US gardeners.