Practice

Cold Frame

Also known as: coldframe

A small, low, transparent-topped structure used to grow cold-hardy crops in seasons too cold for open-ground production, and to harden off indoor-started seedlings before transplanting. Typically a wooden or stone frame 12–24 inches tall, slope-tilted south, with a glazed lid (glass, polycarbonate, or recycled storm window). One of the most ancient season-extension structures, in continuous use in temperate gardens for at least 400 years. Eliot Coleman's *Four-Season Harvest* established it as the foundational tool of modern cold-climate winter gardening.

A box, a transparent lid, a south-facing tilt, and a piece of ground. The cold frame is the simplest piece of garden infrastructure that meaningfully extends the season — by roughly one hardiness zone in winter protection and 4–6 weeks at either end of the growing season.

Why it works

The transparent lid lets sunlight in and traps the resulting heat. On a sunny winter day, the air inside a cold frame can reach 70°F+ when outside air is below freezing. At night the frame holds residual heat in the soil and air, buffering against the temperature drop. Over a season, this gradient is enough to grow lettuce, spinach, mâche, and other cold-hardy crops through the worst months in zones 5–7.

A workable build

Materials:

Sides: untreated cedar, hemlock, or recycled lumber. Stone, brick, and concrete block also work and are more durable.
Slope: the lid should tilt 30–45° toward the south (in the northern hemisphere) to maximize winter sun capture. Standard: back wall 24 inches tall, front wall 12 inches.
Lid: an old storm window, a sheet of polycarbonate, or twin-wall plastic. Hinged at the back for easy opening.
Ground: placed directly on prepared soil or set into a sunken pit. Frames sunk 12–18 inches below grade have substantially better cold performance.

A typical home-scale frame: 3 feet wide × 6 feet long. Holds about 18 square feet of growing space — enough for serious winter salad production for a household.

How it gets used through the year

Late winter / early spring: starts cold-hardy seedlings (lettuce, peas, brassicas) for early transplanting
Spring: hardens off indoor-started tomato, pepper, basil seedlings before they go to the garden
Summer: covers removed; used as a propagation bed or for shade-loving lettuce in a partial-shade location
Fall: extends harvest of late crops (lettuce, scallions, herbs) by 4–6 weeks past first frost
Winter: cold-hardy greens (spinach, mâche, claytonia, kale) for harvest under cover

Venting

The major operational discipline: a closed cold frame on a sunny winter day can overheat to lethal temperatures. Open the lid (or prop it partially) when daytime air temperatures rise above 40°F (4°C); close before sunset. Automatic temperature-activated openers exist and are highly recommended for any gardener who isn’t home at midday.

Where it sits in this wiki

The cold frame is the foundational structure in [[season-extension|season-extension]] and a working tool in any [[home-gardening|home garden]] in temperate climates. Larger structures (low tunnels, hoop houses) extend the same principle to bed and field scale; the cold frame is where the practice begins.

Sources

Eliot Coleman, Four-Season Harvest (Chelsea Green, revised 1999)
Niki Jabbour, The Year-Round Vegetable Gardener (Storey, 2011)
The Garden Cottage Diaries: My Year in the 1790s (period-accurate cold-frame use)

Rooted in life.

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Practical

shares approach with

Season Extension the cold frame is the foundational season-extension structure; everything else extends or scales the same logic

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