How to Build a Cold Frame for Winter Salad Greens

Your fall greens look great in October, then a 24 degree night in late November turns half the bed into translucent mush. A cold frame is the cheapest fix I know for that problem, and if you build a cold frame correctly it will keep spinach, lettuce, mache and scallions harvestable through most of winter in zones 5 through 7. Build it wrong and you get either a frozen box or a solar oven that cooks seedlings by noon.

This is the design I have used on our market plots for years, with real dimensions, a lumber list, and the venting habits that matter more than the box itself.

What a cold frame actually does

A cold frame is a bottomless box with a clear lid set directly on the soil. It does not create heat. It traps the heat the ground stored during the day and blocks the wind that strips that heat away at night. On a clear night, a well sealed frame typically stays 6 to 10 degrees warmer than the open air, and the soil inside stays unfrozen weeks longer than the ground around it.

That gap sounds small, but it is the difference between spinach that freezes solid and spinach that simply stops growing and waits. Most cold hardy greens survive 15 to 20 degrees if they are protected from wind and drying, which is exactly what a frame provides.

If you already run a simple low tunnel to extend the growing season, think of the cold frame as its sturdier cousin: smaller, more insulated, and better for the coldest six weeks of the year.

How to build a cold frame step by step

Size and materials

Make the frame 3 feet deep and 6 feet wide. Three feet is the longest reach most people manage without kneeling on the bed, and 6 feet matches common lumber and polycarbonate sizes, so you cut less.

• Two 2x10 boards, 8 feet long, untreated or cedar (back wall and sides)
• One 2x6 board, 8 feet long (front wall)
• Four 2x2 corner posts, 12 inches long
• One sheet of 8 mm twin wall polycarbonate, cut to about 3 feet 4 inches by 6 feet 2 inches
• Two strap hinges, 3 inch exterior screws, and weatherstripping foam
• A 1x2 frame to stiffen the lid edges

Expect to spend $110 to $160 on materials at current lumber prices. Old storm windows are cheaper, often free, but glass breaks, is heavy to lift fifty times a winter, and often carries lead paint on the sash. I stopped using them after one cracked under a heavy snow.

Assembly

1. Cut the back wall from a 2x10 at 6 feet. Cut the front wall from the 2x6 at 6 feet.
2. Rip the side pieces on an angle so they run from 9.25 inches tall at the back to 5.5 inches at the front. That slope sheds rain and tilts the lid toward the low winter sun.
3. Screw all four walls to the 2x2 corner posts.
4. Build the lid frame from 1x2, attach the polycarbonate with washered screws, and hinge it to the back wall.
5. Run weatherstripping along the top edges of the box so the lid seals when closed.

Placement

Face the low side due south, or as close as your site allows. Set it against a south facing wall or fence if you have one; the stored heat from a wall adds a few more degrees. Avoid low spots where cold air pools on still nights.

Venting matters more than insulation

Here is where most first year frames fail. On a sunny 40 degree day, the air inside a closed frame can pass 90 degrees by early afternoon. Lettuce bolts, spinach wilts, and you lose the moisture balance that keeps leaves tender.

My rule is simple: if the outside temperature will pass 40 degrees and the sun is out, prop the lid 4 to 6 inches by mid morning. Above 50 degrees, open it fully. Close it again by 3 p.m. in December so the soil captures heat before sunset.

If you cannot be home during the day, buy an automatic vent opener. These wax filled cylinders open the lid as the temperature rises and close it as it falls, with no power needed. They cost about $35 to $50 and save a lot of cooked crops.

What to grow, and when to plant it

The trick most growers learn late is that a cold frame protects plants, but it does not make them grow much in deep winter. Below about ten hours of daylight, growth nearly stops. That means your crops need to be close to full size before the short days arrive.

Shift those dates two weeks earlier per zone colder and two weeks later per zone warmer. If you want a steady supply rather than one big wave, the approach in succession planting without a spreadsheet works fine for frames too; just make each sowing smaller.

Why I do not insulate the box with bales

A lot of guides suggest banking straw bales around the frame. I tried it for two winters and quit. The bales held moisture against the wood, rotted the lower boards within three seasons, and became a winter hotel for voles that ate the spinach roots from below.

A better choice for very cold snaps is a layer of floating row cover laid directly on the plants inside the frame. That second layer adds roughly 4 to 6 degrees of protection, weighs nothing, and comes out in seconds on warm days. On nights forecast below 10 degrees, throw an old blanket over the closed lid and pull it off in the morning.

Frequently asked questions

How much warmer is a cold frame than outside?

On a clear, calm night, a sealed cold frame usually runs 6 to 10 degrees warmer than outdoor air. On a sunny day it can be 30 to 50 degrees warmer, which is why venting matters. Adding row cover inside gains a few more degrees at night.

What is the best material for a cold frame lid?

Twin wall polycarbonate is the best all round choice. It insulates better than single glass, does not shatter, and is light enough to open daily. Clear plastic film works as a budget option but usually needs replacing every one or two seasons.

Can you grow vegetables in a cold frame all winter?

You can harvest all winter, but very little grows from late November to mid January in most of the northern United States because the days are too short. Plant early enough that crops are near full size by the end of October, then treat the frame as living storage.