Build a Simple Cold Frame for Fall and Winter Greens

A cold frame is one of the cheapest ways to stretch your garden into the colder months. It is basically a bottomless box with a clear lid that catches the low sun, holds the daytime warmth, and keeps frost off the plants inside.

You do not need a greenhouse or a big build. A frame made from salvaged lumber and an old window can be put together in an afternoon, and it can give you fresh salad greens weeks after the rest of the garden has gone to sleep.

This article covers what a cold frame does, how to build a simple one, where to put it, how to keep it from cooking your plants, and what to grow inside.

What a cold frame actually does

A cold frame is passive solar. The clear lid lets sunlight in during the day, and the soil and the box hold that warmth through the night. The result is a pocket of air a few degrees warmer than the garden around it, with frost kept off the leaves.

It is not a heated structure, and it will not grow tomatoes in January. What it does well is turn a fall of cold nights into several more weeks of harvest, and it gets spring seedlings started earlier.

A cold frame is best for:

• Cold-hardy greens like kale, spinach, arugula, and mache
• Leafy crops that bolt when it gets hot, since cool fall days keep them tender
• Root crops that tolerate frost, like carrots and radishes held in the ground
• Hardening off seedlings you started indoors
• Starting a few hardy seeds in early spring

It will not do much for:

• Warm-season crops like tomatoes, peppers, or eggplant past their season
• Anything that needs real heat through a deep winter freeze
• Plants that would suffer from a muggy, closed-in space

The honest promise is a longer shoulder season, not a four-season tropical garden.

What you need to build one

The classic cold frame is a rectangle with the back slightly higher than the front, so the lid slopes toward the sun. The bottom stays open, resting right on the soil, because the ground is part of the insulation.

Here is a simple shopping and salvage list:

• The lid. An old window, a storm door, or a sheet of clear polycarbonate or greenhouse plastic. This is the most important piece.
• The frame. Untreated lumber, salvaged boards, cinder blocks, bricks, or even stacked straw bales. You want the sides tall enough for the crop and the back a few inches higher than the front.
• A hinge. Two or three simple hinges let you prop the lid open to vent.
• A stick or block. Used to hold the lid open on warm days.

If you do not have an old window, a simple wood frame covered with a sheet of clear plastic can work as the lid, though it is less durable.

Choosing the spot

Placement matters more than fancy materials. A cold frame needs as much winter sun as it can get.

Look for a spot that:

• Faces south, with the sloped lid angled toward the sun. This is the single most important detail, since the winter sun stays low in the sky.
• Gets full sun, not shade from a house, fence, or tree.
• Is out of the prevailing wind. Next to a south-facing wall or foundation is often ideal, since the wall also radiates some warmth at night.
• Has well-drained soil, so the inside does not turn to mud.

Set the frame into the ground a few inches if you can. Burying the base a little keeps cold wind from blowing underneath and makes the whole thing sturdier.

Putting it together

The exact dimensions are flexible and should match the lid you have on hand. The main thing is that the back is taller than the front.

A simple build in a few steps:

1. Build a rectangle with the back about 6 to 10 inches taller than the front, sized to fit your lid.
2. Attach the clear lid to the back edge with hinges so it opens upward and forward.
3. Set the frame in place, over the bed, and press the bottom into the soil so it is sealed.
4. Prop the lid open with a stick whenever you need to vent.
5. For cold nights, you can lay an old blanket, a row cover, or a piece of burlap over the lid to add a little more insulation.

A frame roughly 3 by 6 feet is a good size for a family bed of greens. Bigger is fine too, but keep it narrow enough that you can reach every plant from the edge.

The part people forget: ventilation

The biggest beginner mistake is leaving a cold frame closed on a sunny day. Even in the middle of winter, a closed frame in full sun can heat up far more than you would expect, sometimes well into the 70s or 80s. That heat can wilt seedlings, cause soft, weak growth that later frost damage, or outright cook tender leaves.

The simple rule is to open the lid whenever the sun is out:

• On a sunny day, prop the lid open several inches.
• Close it back up in the afternoon before the heat is lost at dusk.
• On cloudy or truly frigid days, leave it closed.
• A quick way to check: the inside should feel no hotter than a mild, pleasant day, not like a hothouse.

If you are away all day and cannot babysit the lid, a stick placed under the edge to keep it cracked is safer than leaving it sealed shut.

What to plant

Cold frames shine with crops that tolerate cold and taste better after a chill. Timing matters more than the crop choice, since the plants need time to get established before the worst cold arrives.

Fall and winter favorites for a cold frame:

• Kale, which gets sweeter after frost
• Spinach, arugula, and other salad greens
• Mache and claytonia, which handle cold especially well
• Leaf lettuce and cold-tolerant loose-leaf mixes
• Scallions and hardy onions
• Carrots and radishes, left in the ground and pulled as needed

Plant these in late summer or early fall so they are established before deep cold. In a zone 7 garden, mid-to-late September is often still a reasonable time to sow a quick crop of hardy greens, though the frame buys you the weeks they need to mature.

The plants grow slowly once days get short and cold, so do not expect rapid growth through winter. The cold frame mainly keeps them alive and harvestable, holding them in good shape until you pull them.

A few last notes

A cold frame is a low-cost, low-drama way to keep a garden useful longer. It does not demand much once it is built, just a bit of daily attention to the lid on sunny days.

The other advantage is community: a neighbor with an old window they no longer need is often happy to pass it along, and a frame you built from salvaged parts fits the spirit of using what is already around.

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— C. Steward 🥕