Cold Frames: Extend Your Growing Season by Weeks

How a Cold Frame Works

A cold frame is the simplest possible season extension device: a transparent lid over a shallow growing bed, using the sun to trap heat. Unlike a greenhouse, which relies on structural volume and active air exchange, a cold frame is essentially a solar collector that sits directly on the ground. The enclosed air mass warms during the day and releases heat slowly at night.

The key measurement is the temperature differential: on a sunny March day when outdoor temperature is 45°F, a well-designed cold frame can reach 75-85°F inside. At night, if the lid is insulated, the interior stays 15-20°F warmer than the outside air. That differential is what lets you grow cold-hardy greens weeks before and months after your frost dates.

The physics is straightforward: shortwave sunlight passes through the transparent lid, strikes the dark soil inside, and converts to longwave infrared radiation. The transparent lid allows shortwave in but blocks longwave from escaping — the greenhouse effect, in its simplest form.

What You Can Grow

Not every crop tolerates a cold frame. The key is matching your crops to the temperature range your cold frame actually produces:

Tomatoes, peppers, and other warm-season crops do not work in a cold frame without supplemental heating. The frame stays too cold at night. Save those for the greenhouse or high tunnel.

Three Designs We Have Built

Design 1: Basic Wood Frame (Budget)

Cost: $45 | Time: 2 hours | Difficulty: Easy

This is the build we recommend starting with. Four boards of 2x12 lumber (cedar or pine), cut to length and screwed together at the corners. A sheet of 6-mil plastic film over the top, held down by furring strips. No hinges — just lift the whole lid to access the bed.

Dimensions: 4 feet wide, 8 feet long, 12 inches deep at the back, 8 inches deep at the front (the slope helps shed rain and captures more sunlight). We use this design for overwintering spinach and as a nursery for early spring transplants.

Design 2: Cedar Cold Frame with Polycarbonate Lid (Recommended)

Cost: $125 | Time: 4 hours | Difficulty: Medium

This is our primary unit and the one we use most heavily. Cedar 2x6 lumber for the walls (naturally rot-resistant), with a hinged polycarbonate lid. Polycarbonate is the right material for the lid — it transmits 85-90% of available light, is virtually unbreakable, and lasts 10+ years.

We added a 2-inch layer of rigid foam insulation around the exterior walls. This is the upgrade that matters most: on cold nights, the insulation prevents the soil from freezing and keeps the internal temperature 8-12°F warmer than an uninsulated frame.

Design 3: Straw Bale Cold Frame (Free)

Cost: $0 | Time: 30 minutes | Difficulty: Trivial

If you have straw bales, you have a cold frame. Arrange four to six bales in a rectangle, set an old window or polycarbonate sheet on top, and you have a growing space that holds heat remarkably well. The straw provides both the walls and the insulation. We use this for late fall extension (harvesting kale through December) and for hardening off transplants in spring.

The limitation is lifespan: after one winter, the straw bales are soaked and starting to compost. But they cost nothing and the concept is immediately deployable.

Materials and Cost Comparison

Temperature Performance Data

We logged internal and external temperatures hourly for 12 months using a digital data logger. Here are the key findings:

The data tells a clear story: the cold frame adds 10-15°F at night year-round. In spring and fall, the daytime highs inside reach temperatures that would stress or bolt lettuce — we vent by opening the lid on warm days. In winter, the night differential is what keeps plants alive.

During December and January, when overnight lows hit 22°F or below, we add an additional layer of insulation over the polycarbonate at night: a moving blanket or piece of old carpet. This raises the night differential to 18-20°F, keeping the interior above freezing.

Seasonal Use Calendar

This is our actual planting and harvest calendar for the cold frame:

• February: Direct sow lettuce, spinach, and arugula. They will germinate once soil temperature reaches 40°F.
• March: First harvest of overwintered spinach. Transplant lettuce seedlings started indoors.
• April: Begin hardening off tomato and pepper transplants in the frame during the day.
• May: Frame is too warm for greens; use for transplant hardening only.
• June-September: Shade the frame with 50% shade cloth; grow heat-tolerant varieties or let the frame rest.
• October: Direct sow winter lettuce, spinach, arugula, mache.
• November: Transplant kale and chard for winter harvest.
• December-February: Harvest as needed; frame may go dormant in very cold spells.

Ventilation Management

Every cold frame needs a way to release excess heat. On a sunny March day, interior temperatures can hit 90°F even when it is 45°F outside. That heat stress causes lettuce to bolt and spinach to flower. The fix is simple: open the lid.

We prop the lid open 4-6 inches when temperatures inside exceed 75°F. A simple wooden prop (a lath or dowel) holds the lid at the desired opening. Automatic vent openers ($25-40) are worth considering if you are not home during the day: they use a wax cylinder that expands with heat to push the lid open, and close as it cools. We installed one on our primary frame and it eliminated the daily check during shoulder seasons.

Building Tips

• Face south. Orient the frame to capture maximum winter sun. A 10-15° slope toward the sun improves light capture by 15-20% over a flat frame.
• Use darker soil or mulches inside. The soil surface inside the frame should be dark (black landscape fabric works well) to maximize heat absorption. Light-colored or mulched soil absorbs less solar energy.
• Insulate the north wall heaviest. The north wall gets no direct sun and loses the most heat. Double-layer the insulation on that side.
• Check daily in shoulder seasons. Spring and fall weather swings wildly. A cold frame that is perfect at 8 AM can be dangerously hot by noon. Daily attention is required.

More Growing Guides

• DIY Greenhouse Guide — if cold frames are not enough for you
• Best Off-Grid Crops — crops that produce heavily with minimal input
• Food Preservation Guide — store what you grow
• Backyard Chickens — add protein to your homestead