Cold Frames: Winter Greens Without a Greenhouse

A cold frame is a box with a clear lid, and it is the highest return on effort in the entire season-extension category. Sixty dollars of lumber and an old storm window buys you spinach in January in places where the ground is frozen solid two feet away. It requires no power, no fan, no permit and no foundation, and if you get bored of it you can move the whole thing with two people.

The catch is a habit, not a cost. A cold frame on a sunny February afternoon will cook everything inside it if nobody opens the lid. That single point is the difference between people who use cold frames for twenty years and people who try one for a season.

What a cold frame actually does

It does three things at once. It traps solar gain during the day. It blocks wind, which is a bigger deal than most people realize — desiccating winter wind kills more overwintered greens than cold does. And it holds a shallow envelope of air over soil that is itself giving off stored heat, which is why a frame sunk a few inches into the ground works markedly better than one sitting on top of it.

The practical effect is roughly one USDA zone of protection. Add a layer of row cover draped directly over the plants inside the frame and you get close to two zones. That is enough to keep hardy greens alive and harvestable through a northern winter.

What it does not do is make plants grow in midwinter. Once daylight drops below about ten hours — early November through early February around 44 degrees north — growth essentially stops regardless of temperature. The frame is a holding pen, not a factory. Everything you plan to eat in January has to be nearly full size by the first week of November, which means seeding in late August and early September. Miss that window and the frame sits there full of seedlings that will do nothing until March.

Building one that lasts

• Size: 3 feet deep by 6 feet wide is the classic and it works because you can reach the back without climbing in. Do not build deeper than you can reach.
• Slope: back wall around 18 inches, front around 12, giving a south-facing pitch. That sheds rain and snow and catches the low winter sun closer to square. A steeper pitch catches more winter light at the cost of headroom.
• Glazing: an old wooden storm window is the traditional answer and the prettiest. Twin-wall polycarbonate insulates better, weighs nothing and does not shatter. Salvaged single-pane glass is free but heavy and unforgiving of a dropped lid.
• Lumber: untreated cedar or hemlock. Avoid pressure-treated stock in contact with soil you are eating from. A plain pine frame will last five or six seasons, which is fine.
• Siting: against the south wall of the house or a garage is best — free radiant heat, wind shelter, and you will actually walk past it. Otherwise, a spot with unobstructed southern exposure and good drainage.
• Sink it: dig the frame 4 to 6 inches into the ground and berm soil against the outside of the walls. Free insulation and better soil contact.

Hinge the lid at the back and add a handle at the front. A prop stick with notches cut at three heights costs nothing and is the manual version of a vent control.

Venting: the part that decides whether this works

A closed cold frame on a clear day at 40 degrees outside will be in the 90s inside by late morning. Spinach that has hardened off for freezing weather does not enjoy 95 degrees, and bolting or heat damage in February is a genuinely common way to lose a winter crop.

The rule most growers settle on: prop the lid when the outside temperature is above roughly 40 degrees and the sun is out, and close it by mid-afternoon so the frame recharges heat before dark. On overcast days below freezing, leave it shut.

If you travel, work irregular hours, or simply know yourself, buy a wax-cylinder automatic vent opener. They run roughly $30 to $40 at the time of writing, use no electricity at all, and open and close on temperature by themselves. It is the single best purchase in this project and it converts the frame from a daily obligation into something that just sits there working.

What to plant, and when

Crop	Sow by	Winter behavior
Spinach	Late Aug – early Sep	The benchmark. Survives hard freezes, resumes in Feb.
Mache (corn salad)	Early Sep	Toughest thing in the box. Nearly unkillable.
Claytonia	Early Sep	Very hardy, mild, keeps producing in low light.
Arugula	Early Sep	Hardy, sharper flavor in cold. Regrows fast in spring.
Tatsoi and mizuna	Late Aug	Reliable, quick, good cut-and-come-again.
Scallions and leeks	Midsummer	Hold in place all winter; dig as needed.
Carrots	Midsummer	Overwinter in place and sweeten in the cold.

Lettuce is the crop people try first and it is the wrong choice — most varieties are marginal below the mid-20s. Spinach and mache do the same job and shrug off weather that flattens lettuce.

Winter management, in four lines

Water very sparingly — plants in cold soil use almost nothing, and a wet frame invites rot. Harvest at midday when the leaves have thawed; frozen leaves picked at dawn turn to mush. Brush heavy wet snow off glass so the weight does not break it, but leave a light dusting, which insulates. And expect the plants to look dead at 15 degrees and be perfectly fine by noon. That is what cold-hardy greens do; do not pull them.

The honest limits

Volume is small. A 3-by-6 frame is 18 square feet, which is a salad or two a week for a couple in midwinter, not a food supply. Scale by adding frames rather than by building one enormous one you cannot reach into.

And the calories are negligible. This is fresh green food in a season when you would otherwise have none, and that is a real quality-of-life item — but it is not the winter food plan. The winter food plan is what came out of the garden in October and went into the cellar. A cold frame is the fresh side dish next to it, which is exactly the same relationship a passive solar greenhouse has to storage, at one twentieth the cost.

My recommendation

Build two small ones rather than one large one, put them against the south wall of the house where you will see them from a window, and buy the automatic opener before you buy anything else. Seed them in the last week of August. If that all works, build two more next year. This is the rare garden project where the second year costs less than the first and produces twice as much, because by then you know your own sowing dates.

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Frequently Asked Questions

How cold can it get inside a cold frame?

Roughly one zone warmer than outside on a still night, more if the frame is sunk into the ground and bermed. Inside temperatures below freezing are normal and expected — the crops listed above are chosen precisely because they tolerate it.

Can I add a heat source?

You can, and then it is a hotbed rather than a cold frame. The traditional version uses a foot of fresh horse manure under the soil, which composts and gives off heat for weeks. Electric soil cables work too. Neither addresses the light limitation in midwinter.

Do I need to water in winter?

Rarely. Once a month on a mild day is often plenty. Overwatering in cold soil causes far more losses than drought does at that time of year.

Will a cold frame work in zone 4 or colder?

Yes, with the row-cover layer inside and a well-bermed frame. Growers in northern New England and the upper Midwest do it routinely. What changes further north is the sowing deadline — it moves earlier, because your ten-hour-daylight window closes sooner.

What to Read Next

• Passive Solar Greenhouse: What It Really Grows
• The Root Cellar: What It Is and Who Can Have One
• All our growing guides
• Heat and cold, from wood stoves to frozen pipes

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