As temperatures drop and most gardeners pack away their tools, savvy growers know a secret: cold frames and row covers can extend your growing season by weeks or even months in both directions. These simple, low-tech solutions create microclimates that protect plants from frost while harnessing the sun’s warmth—no electricity required.
Understanding Cold Frames
A cold frame is essentially a bottomless box with a transparent lid, usually glass or plastic. It acts like a miniature greenhouse, trapping solar heat during the day and insulating plants from cold night air. The temperature inside a cold frame can be 10-30°F warmer than outside, depending on sun exposure and construction.
Traditional cold frames use old windows as lids, but modern versions might feature polycarbonate panels, which are lighter and won’t shatter. The frame itself can be wood, concrete blocks, straw bales, or even repurposed materials like old doors or shower doors.
Cold Frame Design Basics
- Orientation: Face the glass toward the south to maximize sun exposure
- Angle: Slope the lid at 35-45 degrees to catch low winter sun
- Size: Standard dimensions are 3×6 feet, but adapt to your available windows
- Height: Back wall 18-24 inches, front wall 12-14 inches
- Ventilation: Include props or automatic vent openers to prevent overheating
Building a Simple Cold Frame
You don’t need carpentry skills to build an effective cold frame. Here’s a straightforward approach using common materials:
Materials Needed
- Old window or storm door (determines your frame size)
- Four 2×12 boards (two for sides, two for front/back)
- Wood screws
- Two hinges
- A prop stick or automatic vent opener
Assembly Steps
- Cut side boards at an angle so back is taller than front
- Screw the four boards together to form a box
- Attach the window to the back board with hinges
- Position in your garden facing south
- Bank soil around the outside for extra insulation
For an even simpler version, stack straw bales in a rectangle and lay an old window on top. This provides excellent insulation and can be disassembled after the season.
Managing Your Cold Frame
The biggest challenge with cold frames is temperature management. On sunny days, even in winter, internal temperatures can soar above 80°F, cooking your plants. On frigid nights, they can still drop below freezing.
Ventilation
Open the lid when temperatures inside reach 70°F. Automatic vent openers, which use wax cylinders that expand with heat, are invaluable—they’ll open and close the lid even when you’re not home. Manual gardeners should check their frames at least twice daily during variable weather.
Insulation
For extra cold nights, cover the glass with old blankets, burlap, or rigid foam insulation. Remove coverings in the morning to let light in. Some gardeners add jugs of water inside the frame—they absorb heat during the day and release it slowly at night.
What to Grow in Cold Frames
Cold frames excel at growing cold-hardy vegetables and hardening off transplants:
Fall and Winter Growing
- Leafy greens: Spinach, lettuce, arugula, mache, claytonia
- Asian greens: Bok choy, tatsoi, mizuna
- Root vegetables: Carrots, beets, radishes (plant in late summer)
- Alliums: Scallions, garlic greens
- Herbs: Parsley, cilantro, chervil
Spring Use
- Starting seeds 4-6 weeks earlier than outdoor planting
- Hardening off indoor-started transplants
- Growing early salad crops
- Protecting tender perennials
Row Covers: Flexible Season Extension
Where cold frames are stationary, row covers bring protection directly to your garden beds. These lightweight fabrics drape over crops, creating a warm, protected environment while allowing air, light, and water to penetrate.
Types of Row Covers
Floating row covers (also called garden fabric or Reemay) are spunbonded polypropylene that lies directly on plants or is supported by hoops:
- Lightweight (0.5 oz/sq yd): 2-4°F frost protection, 85% light transmission. Good for pest exclusion and light frost.
- Medium weight (1.25 oz/sq yd): 6-8°F frost protection, 70% light. Excellent all-purpose choice.
- Heavy weight (2.0 oz/sq yd): 8-10°F frost protection, 50% light. Best for deep cold but limits growth.
Plastic covers (polyethylene film) provide more heat but require ventilation and don’t breathe:
- Clear plastic: Maximum light and heat, must ventilate
- White plastic: Diffuses light, less overheating risk
- Slitted plastic: Self-ventilating but less insulating
Installing Row Covers
Row covers can float directly on plants or be suspended on supports:
Floating Method
Simply drape the fabric over your bed, leaving slack for plant growth. Secure edges with rocks, boards, soil, or landscape staples. Plants push the fabric up as they grow. This works well for low crops and pest protection.
Supported Method (Low Tunnels)
Create hoops from wire, PVC pipe, or flexible fiberglass rods spaced every 4-5 feet. Drape fabric over hoops and secure edges. This prevents fabric from crushing plants and allows better air circulation.
Quick Hoop Materials
- #9 wire: Flexible, rust-resistant, reusable for years
- ½” PVC pipe: Inexpensive, insert ends into rebar sleeves
- EMT conduit: More rigid, good for windy areas
- Fiberglass rods: Very flexible, won’t kink
Combining Methods for Maximum Protection
The most protected gardens layer these techniques:
- Base layer: Dark mulch absorbs heat during the day
- First cover: Lightweight row cover directly on plants
- Second cover: Plastic tunnel over the row cover
- Cold frame: For starting seeds and hardiest crops
This layered approach can keep salad greens growing through Zone 5 winters, and in milder climates, you may never stop harvesting.
Seasonal Strategies
Fall Season Extension
Install row covers in mid-fall before the first frost. Cool-weather crops actually improve in flavor after light frosts. Keep covers in place through Thanksgiving and beyond—many greens survive well into December or even January with protection.
Spring Jump-Start
Set up cold frames and tunnels in late winter to warm the soil. Start cold-hardy seeds inside cold frames 6-8 weeks before last frost. Move transplants to covered beds 4 weeks before last frost. Remove covers gradually as temperatures warm.
Summer Pest Protection
Lightweight row covers exclude insects without trapping heat. Cover susceptible crops like brassicas (to block cabbage moths) and squash (to prevent vine borers) immediately after planting. Remove only for pollination if needed.
Troubleshooting Common Problems
Overheating: More plants are lost to overheating than cold. Ventilate cold frames when temperatures exceed 70°F. Use lighter weight covers as temperatures rise.
Condensation and disease: Trapped moisture can promote fungal problems. Ensure good air circulation and remove covers on dry, sunny days to air out beds.
Wind damage: Secure covers thoroughly. Use heavier materials for edges. Consider trenching edges into soil for permanent installations.
Animal intrusion: Mice and voles love the protected environment. Keep covers tight to the ground and consider hardware cloth barriers.
Return on Investment
A simple cold frame costs $50-100 in materials and can add 2-3 months of production annually. Row covers run $20-50 for enough material to cover 100 square feet and last 3-5 seasons with care. Compare this to buying organic winter greens at $8-12 per pound, and these tools pay for themselves quickly.
More valuable than the money saved is the satisfaction of harvesting fresh salads in December, starting tomato seedlings while snow still covers the ground, and extending your relationship with the garden through all four seasons. With cold frames and row covers, winter becomes just another growing season.
