Cold Frame Construction: Extend Your Season by 6 Weeks

A cold frame is the simplest, cheapest season extension tool. It is essentially a bottomless box with a transparent lid that traps solar heat, creating a microclimate 10-20°F warmer than the surrounding air. With a well-built cold frame, you can start seeds 4-6 weeks earlier in spring, grow salad greens through winter, and harden off seedlings without shock.

After building and using cold frames for 15 years in USDA zone 5, I have refined designs that cost under $50, require no carpentry skills, and last 10+ years. This guide covers construction, planting strategies, and management through all seasons.

How Cold Frames Work

Cold frames operate on three principles:

• Solar heat gain: The transparent lid allows sunlight to enter. The soil and interior surfaces absorb this energy and radiate it as heat.
• Insulation: The enclosed space prevents wind from stripping away heat. Soil acts as thermal mass, storing heat during the day and releasing it at night.
• Heat trapping: The angled lid creates a greenhouse effect. Short-wavelength solar radiation enters easily. Long-wavelength infrared radiation (heat) has difficulty escaping.

A cold frame with 6 inches of soil inside maintains temperatures 10-15°F above ambient on sunny days. On cloudy days, the difference is 5-8°F. At night, the stored soil heat keeps the interior 5-10°F warmer than outside air.

Design Options

Option 1: Reclaimed Window Frame (Easiest, $0-20)

An old wooden window with glass panes makes the perfect lid. The frame provides structure; the glass transmits 90% of light.

1. Source an old window (check Craigslist, Habitat for Humanity ReStores, or demolition sites). Avoid windows with lead paint.
2. Build a box from scrap lumber or cinder blocks to fit the window dimensions. The box should be 8-12 inches tall in back, sloping to 6-8 inches in front (angled toward the sun).
3. Place the window on top. Prop open with a stick or purchase automatic vent openers ($25) that lift the lid when temperatures exceed 70°F.

Cost: $0-20 if using scavenged materials. Lifespan: 5-10 years depending on wood quality.

Option 2: PVC and Plastic Hoop (Lightweight, $30-50)

PVC pipes bent into hoops create a tunnel frame covered with greenhouse plastic.

1. Cut ½-inch PVC pipes to 6-foot lengths. Insert ends into rebar stakes driven 2 feet into the ground.
2. Space hoops 2-3 feet apart.
3. Cover with 6-mil greenhouse plastic. Secure with PVC clips or sandbags.
4. Bury the plastic edges 4-6 inches deep to prevent wind from lifting it.

Cost: $30-50. Lifespan: 3-5 years (plastic degrades in UV). Advantage: Lightweight and portable.

Option 3: Straw Bale Cold Frame (Insulated, $20-40)

Straw bales stacked to form walls with a window or plastic lid on top. The straw provides excellent insulation (R-value 1.5 per inch).

1. Arrange 8-10 straw bales in a rectangle (3×6 ft interior space).
2. Place a window or Plexiglas sheet on top, angled toward the south.
3. Fill the interior with 6-8 inches of compost or soil.

Cost: $20-40 for straw bales. Lifespan: 1 season (straw decomposes into compost). Advantage: At season's end, spread the straw as garden mulch.

Option 4: Cinder Block and Glass (Durable, $40-60)

Cinder blocks form permanent walls. A sheet of glass or Plexiglas serves as the lid.

1. Stack cinder blocks two high in a rectangle (4×6 ft).
2. Fill block cores with soil for thermal mass.
3. Top with a glass or Plexiglas sheet, hinged at the back for access.

Cost: $40-60. Lifespan: Indefinite. Advantage: Permanent, durable, and holds heat better than wood.

What to Grow in Cold Frames

Spring (February-April)

Start cold-hardy crops 4-6 weeks before last frost:

• Lettuce and spinach (sow in February for April harvest)
• Radishes (25-30 days; sow every 2 weeks)
• Green onions (sow in March for May harvest)
• Kale and collards (transplant seedlings in March)
• Peas (sow in February for May harvest)

Fall (September-November)

Extend the season for summer crops and start winter crops:

• Salad greens (sow in September for November harvest)
• Spinach (overwinters in cold frames; harvest through December)
• Carrots (sow in August; harvest through November)
• Beets (sow in August; harvest greens and roots through October)

Winter (December-February)

With additional insulation, cold frames grow hardy crops through winter:

• Spinach (survives to 10°F under glass)
• Kale (becomes sweeter after frost)
• Mâche (corn salad; extremely cold-hardy)
• Claytonia (miner's lettuce; survives to 0°F)

Temperature Management

The biggest challenge with cold frames is preventing overheating. On a sunny day in March, interior temperatures can reach 100°F if the lid is closed.

Ventilation Rules

• Above 70°F: Prop lid open 2-3 inches.
• Above 80°F: Open lid fully.
• Above 90°F: Remove lid entirely or provide shade cloth.
• Below 40°F: Close lid completely. Add insulation (blankets, straw) at night.
• Below 25°F: Add 6-12 inches of straw or old blankets over the lid at night. Remove in morning.

Automatic Vent Openers

A $25 automatic vent opener uses wax-filled cylinders that expand when heated, lifting the lid. When temperatures drop, the wax contracts and the lid closes. These devices eliminate the daily chore of manual venting and prevent catastrophic overheating if you forget.

Watering Cold Frames

Cold frames reduce evaporation, so plants need less water than open garden beds. However, the enclosed space can create humid conditions that favor fungal diseases.

• Water in the morning so leaves dry before evening.
• Water at the base, not overhead.
• Check soil moisture every 2-3 days. The top inch should be slightly moist.
• Reduce watering in winter when plants are barely growing.

Common Cold Frame Mistakes

Overheating: The #1 killer of cold frame plants. A closed cold frame on a sunny day becomes an oven. Always vent above 70°F.

Poor drainage: Cold frames with solid bottoms (like boxes on concrete) trap water. Ensure drainage holes or a gravel base.

Planting too early: Tender crops (tomatoes, peppers) cannot survive in cold frames until nighttime temperatures stay above 45°F. Wait until 3-4 weeks before last frost.

Neglecting ventilation: Humid, stagnant air causes mildew and damping-off. Prop the lid open even on cool days for air circulation.

Using the wrong orientation: A cold frame facing north gains minimal heat. Always face south in the Northern Hemisphere.

Insulating for Extreme Cold

In climates with sub-zero temperatures, add insulation:

• Straw bales: Stack around the exterior for extra insulation.
• Bubble wrap: Line the interior walls to add an air barrier.
• Water jugs: Fill gallon jugs with water and place inside. Water absorbs heat during the day and releases it at night.
• Blankets: Cover the lid at night with old wool blankets or moving blankets. Secure against wind.

A cold frame is the gateway to year-round gardening. Build one this weekend using an old window and scrap lumber. Plant spinach in September, and you will be harvesting fresh greens in January while your neighbors buy bagged lettuce at the grocery store.