A cold frame is the most cost-effective season extension tool available — a simple box of wood sides and a salvaged window costs under $20 and reliably extends your season 4-6 weeks on each end. In zones 3-7, where cold frames have the most impact, that means spinach from March instead of May and kale still producing in November instead of stopping at first frost.
How Cold Frames Work
A cold frame traps solar energy during the day and retains heat at night. A well-built frame maintains interior temperatures 10-15°F warmer than ambient air — enough to protect cold-hardy crops through the low 20s°F. For maximum solar gain, tilt the lid toward the south at your latitude plus 15 degrees (a gardener at 45° latitude tilts to approximately 60°).
Building Your First Cold Frame
The simplest cold frame uses four wooden sides and a salvaged window. Back height: 12-18 inches. Front height: 6-8 inches (the slope sheds rain and increases solar gain). Salvaged storm windows are ideal glazing — heavy, double-paned, and free or nearly free. Twin-wall polycarbonate is lighter and better insulating but degrades with UV over time.
Zone-by-Zone Season Extension
Zone 3: Spring extension from late April to mid-March (5-6 week gain); fall from mid-September to late October. Zones 4-5: Spring to late March; fall to late November with N-50 row cover inside the frame. Spinach and mâche overwinter reliably. Zones 6-7: The sweet spots — spring planting begins in February or late January. Overwintering spinach, arugula, kale, and lettuce is consistent and reliable; fall harvest extends to December and beyond.
What to Grow in a Cold Frame
Best crops: spinach (sow September for spring harvest), mâche/corn salad (germinates in cold soil, harvested through winter in zone 5+), arugula (faster than spinach), kale (Siberian and Red Russian — sweeten with cold), lettuce (Winter Density and Forellenschluss for best cold tolerance), and radishes (French Breakfast in early spring, 3-4 weeks before outdoor planting).
Ventilation: Your Most Important Task
Cold frames overheat rapidly — even in winter, the interior can reach 80-90°F on a clear February day when outside temperatures are in the 30s. Vent daily on any day when temperatures rise above freezing by propping the lid open a few inches. Automatic thermostatic vent openers are worth the investment if you cannot monitor during the day; set them to open at 45-50°F.
Frequently Asked Questions
- How much colder can a cold frame handle vs. row cover?
- A glass cold frame maintains 10-15°F above ambient. N-70 row cover provides 4-6°F. A cold frame is 2-3 times more protective, especially in wind where fabric cover loses much of its value.
- Can I start seedlings in a cold frame?
- Yes for cool-season crops — a cold frame excels at hardening off transplants and direct-seeding 4-6 weeks ahead of outdoor conditions. It does not get warm enough for tomatoes or peppers until the outdoor season is already underway.
- Do I need to water cold frame crops in winter?
- Yes, but far less than summer crops. Check weekly; water when soil is dry 1 inch down. Water on sunny mornings so foliage dries before evening.
- Can cold frames help with zone pushing?
- Yes, for overwintering borderline-hardy perennials (figs, rosemary, artichokes). The 10-15°F buffer is meaningful when combined with heavy root mulching.