Why Season Extension Changes Everything
A 4-week extension on each end of the season (spring and fall) effectively gives a zone 5 garden the production window of a zone 6 or 7 garden. For gardeners in shorter-season climates, this is the highest-return investment per dollar spent in the whole garden.
Cold Frames: The Classic Low-Tech Option
A cold frame is essentially a bottomless box with a transparent lid — traditionally old window sashes laid over a wooden frame. The trapped air inside warms from solar gain and holds heat overnight. A simple cold frame can protect plants down to 10–15°F overnight if the day was sunny (solar charging the soil mass inside). Build the frame with back wall taller than front wall (slope toward south) to maximize sun angle in spring and fall.
Simple cold frame construction: Two 8-foot 2x8 cedar or white oak boards ripped at an angle for the side walls, two full-height back and low front pieces, and an old window or 4mm poly panel on top. Cost with found windows: under $40. See also our cedar potting bench build for working with similar materials.
Low Tunnels: More Coverage, Less Permanence
Low tunnels use bent wire hoops (10-gauge wire or EMT conduit) to support floating row cover (reemay) or clear poly film over a bed. They're quicker to set up and take down than cold frames and cover full beds rather than a fixed box. Row cover provides 4–8°F of frost protection. Clear poly provides 10–15°F but requires active ventilation on warm days.
Ventilation: The Most Important Management Task
The greatest risk with cold frames and poly tunnels is overheating. On a 40°F sunny day, interior temperatures inside a poly cold frame can exceed 90°F — enough to kill seedlings in hours. Prop cold frame lids open on any sunny day above 40°F. For low tunnels with poly, lift the edges on warm days. Automated ventilation openers (wax-cylinder actuators) open cold frame lids automatically at a set temperature — worth the $30–40 investment for gardeners who can't check daily.
What to Grow in Cold Frames and Tunnels
Spring: Start lettuce, spinach, arugula, radishes, and brassica seedlings 6–8 weeks earlier than outdoor planting date. Start seeds indoors, transplant to cold frame to harden off and get ahead of the season. Fall: Extend production of lettuce, spinach, kale, chard, arugula, and root vegetables 4–6 weeks past normal first frost. Winter: In zones 6+, a cold frame can maintain production of spinach, mache, kale, and claytonia through the cold months.
FAQ
How many degrees does a cold frame add?
10–15°F warmer than outdoor temperatures overnight, assuming sunny days to charge the soil. A zone 5 cold frame effectively provides zone 6–7 nighttime minimums during the shoulder seasons.
What is the best glazing material for a cold frame?
Old single-pane glass windows are the traditional choice and transmit the most light. Polycarbonate twin-wall panels are lighter, safer (no breakage), and provide some insulation value. Clear 4mm greenhouse poly film is cheapest but degrades in UV within 2–3 years.
Can I overwinter vegetables in a cold frame in zone 5?
Yes — spinach, mache (corn salad), kale, arugula, and claytonia can all survive zone 5 winters in a cold frame. They go semi-dormant in the coldest months and resume growth in February–March.
How do I anchor a low tunnel in wind?
Push wire staples or U-pins through the row cover into the soil at 2-foot intervals along each side. Alternatively, lay boards or pipes along the edges. The most secure method is burying the fabric edges in a shallow trench on the windward side.
Do I need to water inside a cold frame?
Yes — the glazing blocks rain, so plants inside depend entirely on you for water. Check moisture every 2–3 days. In spring and fall, cold temperatures slow evaporation significantly, so plants typically need less water than in summer.