Build a Simple Cold Frame Now: Grow Winter Salad Greens in Zone 7

If you want greens on the table in January, late September is the time to build a cold frame. It is one of the cheapest, simplest season-extension tools there is, and it takes the overflowing salad garden of fall and stretches it into a steady source of leaves through the coldest months.

A cold frame is basically a bottomless box with a clear, slanted lid that faces the sun. Sunlight comes in through the top and warms the soil and air inside, while the frame holds that warmth close to the plants. It does not heat the space like a greenhouse, but it does not need to. The goal in Zone 7 is to protect hardy greens from hard freezes and bitter winds, not to grow tomatoes in December.

This guide covers a cheap build that uses basic materials, how to get the angle and ventilation right, and which greens are worth growing and when to sow them.

Why now is the time

A cold frame is a fall project, and it is best done before the garden gets fully put to bed.

• You can build it over a bed that already has fall greens growing, which is the fastest payoff.
• Late September still gives you a stretch of workable weather for building and sowing.
• The frame traps whatever warmth is left in the soil, which is exactly what young greens need as nights cool down.

Put it off until November and you are building in the cold for a short season. Do it now and the frame earns its keep for months.

What you need

The cheapest cold frame is built from a few basic pieces, and you likely have most of them in a scrap pile.

• A frame of wood boards to make the sides. Old deck boards, pallet wood, or even concrete blocks work.
• A clear lid or glazing. Old storm windows are a classic choice. A sheet of clear polycarbonate or twin-wall greenhouse panel is sturdier and holds heat a little better than basic plastic sheeting.
• Hinges so the lid can open, whether it is two hinges and a prop stick or just a lid you lift off.
• A thermometer. This is not optional, because the biggest beginner mistake is overheating the frame on a sunny day.

You do not need a tall box. A front wall of 12 inches or so and a back wall of 18 to 20 inches is plenty for salad greens. Taller is not better, because a taller box holds more air that has to be warmed and is harder to keep closed tightly.

Get the angle right

A cold frame works when it catches the low winter sun, so the lid needs to slope toward the south.

• Face the tall side of the frame north and the low side south. That way the clear lid tilts down toward the sun.
• Aim for a slope of roughly 30 to 45 degrees on the lid. That catches significantly more winter sunlight than a flat top.
• Place the frame where it gets full sun, not in the shadow of a building, fence, or tree line.
• Point the open side of the frame away from the prevailing winter wind to cut down on cold gusts hitting the plants.

None of this needs to be exact. A reasonable southern slope in a sunny spot beats a precise angle in a shady one every time.

Ventilation is the real work

Almost everyone who loses plants in a cold frame loses them to overheating, not to cold. On a sunny winter day, even when it is freezing outside, the air inside a closed frame can climb far past what greens enjoy.

• Watch a thermometer placed inside the frame at plant height.
• On a mild, sunny day, prop the lid open a few inches with a stick or block to let hot air escape.
• On warm afternoons, you may need to open it fully or remove the lid entirely.
• Close the lid again in late afternoon before the temperature drops so the frame can hold in the day's warmth overnight.

A good habit is to check the frame in the morning and again in the afternoon. If it is sunny, plan to vent it. If the thermometer climbs into the 70s or higher, the lid needs to be open more.

Use a thermometer in the soil too, not just the air. Cold soil slows roots just as freezing air slows leaves, and knowing both helps you decide when to close the lid for the night.

Which greens to grow

Not everything is worth growing in a cold frame, but a handful of hardy greens do very well and keep producing through winter.

• Spinach. It is one of the most reliable cold-frame crops and can be harvested even in the worst of winter if you leave a few leaves on each plant.
• Arugula. Quick, frost tolerant, and hardy to well below freezing once it is established under cover.
• Leaf lettuce and winter lettuces. Looseleaf types and cold-hardy winter blends keep producing leaves through cold weather.
• Tatsoi and bok choy. These Asian greens are notably hardy and give you something different than lettuce every week.
• Swiss chard and mizuna also hold up well and add variety.

Rocket and mustard greens pack flavor and tolerate cold. The key is to choose greens that are cold hardy to begin with, then let the frame push them just a bit further.

When to sow

Sow your cold-frame greens as a fall crop so they are established before the worst cold arrives.

• Most of these greens germinate in cool soil and grow well in the shortened days of fall, so you can sow them through late September into October in Zone 7.
• Sow several short rows or a small bed of each green rather than one giant planting. That gives you a rotating supply instead of a single harvest.
• Give the plants a chance to size up before hard frost, because an established plant survives cold far better than a tiny seedling.
• Keep the soil a little moist but not soggy, and water in the morning so the leaves dry before night.

If your cold frame sits over a bed that already has fall greens, just cover it and keep harvesting. If you are starting fresh, sow now and the frame will carry the young plants through the fall and winter.

A simple winter routine

Once the frame is set, the daily work is not much. It is more habit than labor.

• Open the lid on sunny, mild days and close it in the afternoon.
• Crack it open a few inches on warmer fall days to keep the air from going stale.
• Keep the soil moist but not wet. The enclosed frame dries out slower than open ground, so check before you water.
• Harvest leaves a little at a time, cutting outer leaves and leaving the heart to keep producing.
• In the deepest cold, add a layer of straw or a row cover over the greens inside the frame for extra protection, then remove it on sunny days.

The whole point is a steady, small supply, not a big canning-style harvest. A cold frame that gives you a salad a week all winter is a success.

Let a neighbor borrow the idea

A cold frame is one of those projects that spreads through a neighborhood. Someone builds one, a neighbor sees how easy it is, and suddenly a few houses are eating winter salads. If you build two frames and only plant one, that is a natural thing to mention at the farm table or in a trade, because extra capacity and spare greens both have a place in a community that looks after itself.

Start with one simple frame, learn its rhythm of venting and closing, and grow from there. Winter salad is within reach, and it starts with an afternoon of work in late September.

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— C. Steward 🥕