A cold frame is a bottomless box with a clear lid, set on the soil and angled toward the sun. It has no heater and no power. It works by trapping the sun’s heat during the day, holding the ground’s heat at night, and cutting the wind, which is what does most of the damage to plants in cold weather. Used well, it buys two to four weeks at each end of the season and lets you harvest salad greens in December in zone 6.
The part most guides skip is that a cold frame is not a way to grow things in winter. It is a way to hold things through winter. Plants in a frame need to be close to full size by the time the days get short, because once daylight drops under about ten hours, almost nothing grows, frame or no frame. Everything about using one well follows from that.
How much a cold frame actually buys you
On a clear night a closed, well-sealed frame typically runs several degrees F above the outside air, and more if the sides are banked with soil, leaves or straw bales. It also stops wind entirely, and wind chill on wet leaves is what kills lettuce long before the thermometer says it should. The first frost guide puts a cold frame in the same class as a low tunnel: roughly one to two zones of protection for what is inside.
For planning, that works out to about two weeks on your fall sow-by dates and about two weeks earlier in spring. The table below uses exactly that: the open-ground dates come from the site’s planting engine, counted back from each zone’s average first frost, and the frame columns add two weeks.
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Cold frame planting dates by zone
| Zone | First frost | Lettuce, open ground | Lettuce, in a frame | Kale, in a frame | Radishes, in a frame | Spinach to overwinter | Spring lettuce in a frame |
|---|---|---|---|---|---|---|---|
| 3a | Sep 15 | by Jul 27 | by Aug 10 | by Aug 14 | by Sep 8 | Aug 4 to Aug 18 | from Apr 3 |
| 3b | Sep 18 | by Jul 30 | by Aug 13 | by Aug 17 | by Sep 11 | Aug 7 to Aug 21 | from Apr 1 |
| 4a | Sep 22 | by Aug 3 | by Aug 17 | by Aug 21 | by Sep 15 | Aug 11 to Aug 25 | from Mar 29 |
| 4b | Sep 28 | by Aug 9 | by Aug 23 | by Aug 27 | by Sep 21 | Aug 17 to Aug 31 | from Mar 24 |
| 5a | Oct 3 | by Aug 14 | by Aug 28 | by Sep 1 | by Sep 26 | Aug 22 to Sep 5 | from Mar 19 |
| 5b | Oct 8 | by Aug 19 | by Sep 2 | by Sep 6 | by Oct 1 | Aug 27 to Sep 10 | from Mar 14 |
| 6a | Oct 13 | by Aug 24 | by Sep 7 | by Sep 11 | by Oct 6 | Sep 1 to Sep 15 | from Mar 9 |
| 6b | Oct 18 | by Aug 29 | by Sep 12 | by Sep 16 | by Oct 11 | Sep 6 to Sep 20 | from Mar 4 |
| 7a | Oct 25 | by Sep 5 | by Sep 19 | by Sep 23 | by Oct 18 | Sep 13 to Sep 27 | from Feb 27 |
| 7b | Nov 1 | by Sep 12 | by Sep 26 | by Sep 30 | by Oct 25 | Sep 20 to Oct 4 | from Feb 20 |
| 8a | Nov 8 | by Sep 19 | by Oct 3 | by Oct 7 | by Nov 1 | Sep 27 to Oct 11 | from Feb 11 |
| 8b | Nov 15 | by Sep 26 | by Oct 10 | by Oct 14 | by Nov 8 | Oct 4 to Oct 18 | from Feb 1 |
In zone 9 and warmer a cold frame is rarely needed for greens, and its main use is protecting tender seedlings through the odd freeze. In zone 3 the fall columns land in August, which is why northern growers who want winter greens usually pair the frame with a row cover inside it.
Spinach is the exception to the “finish before the short days” rule. The overwintering window is about getting a small, established rosette in the ground four to six weeks before the frost. It stalls through winter, and in a frame it restarts weeks before open-ground spinach would, which makes it the single best crop for a cold frame in zones 4 to 7.
The ten hour rule
Once daylight falls below about ten hours, growth all but stops, even in a frame that stays above freezing. Eliot Coleman, who popularized winter harvesting in Maine, calls this stretch the Persephone period. Where it falls depends on latitude, not zone:
| Latitude | Example cities | Days under about 10 hours |
|---|---|---|
| 45°N | Minneapolis, Portland (Oregon) | early November to early February |
| 42°N | Chicago, Boston | about Nov 7 to Feb 2 |
| 40°N | Philadelphia, Denver, Columbus | about Nov 11 to Jan 29 |
| 35°N | Albuquerque, Memphis, Charlotte | about Nov 27 to Jan 13 |
| 33°N | Atlanta, Dallas | early December to early January |
| 30°N | Houston, Austin, Jacksonville | never drops below 10 hours |
Two practical consequences. First, what you harvest from a frame in December is mostly what grew in October, so the sow-by dates above are deadlines, not suggestions. Second, harvest outer leaves rather than whole plants during the short days, because a cut-and-come-again plant with most of its leaves left regrows fast once February arrives, and a stump does not.
Venting: the mistake that kills more crops than cold
A closed cold frame on a sunny day is a solar oven. With the outside air at 40 degrees and full sun on the lid, the inside can pass 90 degrees by noon, which is enough to wilt and scorch lettuce and spinach that would have shrugged off a frost.
The working rules:
- Keep greens under about 70 degrees inside. They grow best from 45 to 65.
- On a sunny day above about 40 degrees outside, prop the lid an inch or two.
- Above 50, open it wide or take it off.
- Close it in mid afternoon, while the soil still holds the day’s heat, not at dusk after it has gone.
- On overcast days it can often stay shut, which is why a min/max thermometer is worth having: it tells you what actually happened while you were out.
Water less than you think. Evaporation almost stops in winter, and wet soil in a closed frame invites rot and mold. Water on a mild morning, only when the top inch of soil is dry, and never late in the day.
Where to put it and how to build one
Face the low side south, with the lid sloping toward the sun. Against a south-facing wall of the house is the best spot you have: the wall stores heat, blocks north wind, and radiates warmth back at night. Avoid low spots where cold air pools on still nights.
Keep it small. A frame about three feet deep and four to six feet long can be reached from the front without climbing in, and a low frame has less air to heat. Height should be about 18 inches at the back and 12 at the front for greens.
A simple DIY build uses ordinary lumber and one good lid:
- Sides from 2x10 or 2x12 boards, untreated if you are growing food in it, with the side pieces cut on a diagonal so the back is about six inches taller than the front.
- A lid of twin-wall polycarbonate screwed to a light wooden frame, hinged along the back. An old window works too, but single glass insulates less, and old painted sash can carry lead paint, so do not sand or scrape it over your bed.
- A notched prop stick for venting, or a wax-cylinder opener.
- Soil or straw banked against the outside walls once the weather turns cold.
For very cold nights, throw an old blanket or a heavy frost blanket over the closed lid and take it off in the morning. The row cover and frost blanket comparison covers fabric weights, and a lightweight row cover laid directly over the plants inside the frame adds a second layer that is worth another few degrees.
The picks
If you would rather buy than build, these are the pieces that matter, in the order most people need them.
| Pick | Best for | Price | Link |
|---|---|---|---|
| Outsunny Wooden Cold Frame Greenhouse Best starter frame | A first frame on a patio or at the end of a bed | $$ | Check price |
| Juwel Easy Fix Cold Frame Best for real winters | Zones 4 to 6, where the frame has to hold through January | $$$ | Check price |
| Automatic Vent Opener The part that saves the crop | Anyone who is not home at noon on a sunny day | $ | Check price |
| Digital Min/Max Thermometer Cheapest useful upgrade | Learning what your frame actually does | $ | Check price |
| Twin-Wall Polycarbonate Panel For a DIY lid | Building your own frame to fit a bed | $$ | Check price |
Outsunny Wooden Cold Frame Greenhouse
Best for: A first frame on a patio or at the end of a bed
A wooden box with a hinged clear lid, sized for one person to reach across. It is the shape a cold frame should be: low, sloped toward the sun, and small enough to close up on a cold evening in ten seconds.
Pros
- Right height for greens, so there is little dead air to heat
- Hinged lid props open for venting
- Simple screw-together assembly
Cons
- Thin panels insulate less than twin-wall polycarbonate
- Softwood needs a coat of exterior oil to last
- Too light to leave unanchored in a windy spot
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Juwel Easy Fix Cold Frame
Best for: Zones 4 to 6, where the frame has to hold through January
Twin-wall polycarbonate traps a layer of air in the panel itself, which is what separates a frame that holds spinach through a January cold snap from one that only buys a few weeks in October.
Pros
- Twin-wall panels insulate much better than single glazing
- Aluminium frame does not rot
- Hinged lids prop open for venting
Cons
- Costs noticeably more than a wooden kit
- Diffused glazing means you cannot see the plants clearly
- Still needs banking or a blanket below about 10 degrees F
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Automatic Vent Opener
Best for: Anyone who is not home at noon on a sunny day
A wax-filled cylinder expands as it warms and lifts the lid with no power at all. A closed frame on a sunny 40 degree day can pass 90 degrees inside by lunchtime, and this is what stops you cooking a bed of lettuce while you are at work.
Pros
- No wiring, batteries or app
- Opening temperature is adjustable
- Fits most hinged frame lids
Cons
- Has a maximum lid weight, so check it against your frame
- Slow to react to a sudden cloud break
- The cylinder wears out after several seasons
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Digital Min/Max Thermometer
Best for: Learning what your frame actually does
It records the highest and lowest temperature since you last reset it. Two weeks of readings tell you more about when to vent and when to cover than any rule of thumb, including the ones on this page.
Pros
- Shows the overnight low you slept through
- Shows the midday high you missed
- Costs about the same as a packet of good seed
Cons
- Cheap probes drift, so check it against a known thermometer
- Needs shading from direct sun to read air temperature
- Batteries fail in deep cold
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Twin-Wall Polycarbonate Panel
Best for: Building your own frame to fit a bed
The glazing is the only part of a cold frame that is hard to improvise well. A sheet of twin-wall polycarbonate cut to size and screwed to a simple wooden lid gives you the insulation of a premium frame on a box you built from lumber.
Pros
- Light, shatterproof and easy to cut
- Insulates far better than an old single-pane window
- UV-coated sheets last many years outdoors
Cons
- Cut edges need taping to keep dirt and algae out of the flutes
- Flexes on a wide lid unless it has a center support
- The UV-coated side has to face out
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Cold frame vs low tunnel vs greenhouse
A low tunnel is hoops with fabric or plastic over a bed. It covers more ground for less money and is easier to scale, but it holds less heat, catches more wind, and has to be opened by hand. A cold frame holds heat better per square foot, stands up to snow if the lid is strong, and is the easiest to vent. A greenhouse lets you walk in and grow tall crops, but an unheated one gives roughly the same protection as a frame at many times the cost, and its large volume of air cools faster on a clear night than a frame’s does.
For a home gardener who wants salad in December and spinach in March, a frame or two is the right size of investment.
Spring: the second season most people forget
In late winter the frame becomes a head start. Sow lettuce and spinach in it two weeks ahead of your open-ground dates (the last column of the table), and use it as the halfway house for seedlings you started indoors. A frame with the lid propped a little more each day is the easiest way to run a hardening off schedule, and it saves the daily carrying of trays in and out.
If you do not have a frame this year, November is not too late to set one up for spring. The November planting guide lists what is still going in the ground this month, and your crop pages have the spring dates to plan against.