Cold Frame Guide Canada

Size and access. A cold frame is small (1–2 m²), low to the ground, accessed by lifting the lid. A hoop house or low tunnel is 3–5 m² covered with greenhouse plastic stretched over PVC or metal hoops, accessed by lifting one end of the plastic. A polytunnel or hoophouse is walk-in (3+ m wide, 6+ m long, 2+ m tall) covered with greenhouse plastic. They all use the same physics; the difference is scale and convenience. Cold frame: best for households with limited space and modest goals (winter salads, spring seedlings). Polytunnel: best for serious gardeners growing significant winter food. Most home gardeners get most of the benefit from a single cold frame, then upgrade later if they want more.

For cold-tolerant crops (kale, mâche, claytonia, spinach, parsley): no. The frame's passive solar gain plus the insulating effect of the lid is sufficient. For tender seedlings during a cold spring snap: a soil-heating cable or a single electric heat mat under the seedlings can prevent damage in zone 3–4 if a polar vortex hits. Don't try to heat the whole frame — the heat loss through the walls and lid is far too high to maintain warm temperatures with any economic amount of electricity. If you want serious heating, you need a small greenhouse, not a cold frame.

Crops freeze. Most cold-tolerant species (kale, mâche, parsley) survive being frozen solid for a few days and resume growth when temperatures recover. To buy 5–10°C of additional protection during cold snaps, lay an old blanket, an extra layer of polycarbonate sheet, or 2–3 layers of floating row cover INSIDE the frame (over the plants) at night. Remove during the day for light. Pack straw bales against the outside walls of the frame for additional insulation in extreme zones. In zone 3 mid-winter, accept that some growth stops during the coldest 4–6 weeks — the cold frame keeps plants alive but not necessarily growing.

Sort of. You can use the cold frame to "harden off" tomato seedlings started indoors before they go to the open garden — a 7–10 day stay in the cold frame in May acclimatizes them to outdoor sun and wind. But you can't grow tomatoes to maturity in a cold frame; they need consistent 20°C+ nights and full sun, both of which a cold frame can't reliably provide. For tomatoes in fall to extend the harvest, a polytunnel works; a cold frame doesn't.

A cold frame is a small unheated enclosure with transparent (or translucent) lid that traps solar heat during the day and holds it through cold nights — essentially a mini-greenhouse without doors. Typical Canadian dimensions: 90–120 cm wide × 60–120 cm deep × 30 cm tall at the front sloping to 45 cm at the back. The sloped lid maximizes south-facing winter sun. Daytime temperatures inside reach 15–25°C even when outdoor air is well below freezing; nighttime temperatures stay roughly 5–8°C warmer than outside ambient. Cold frames protect to about -7°C unprotected and can be insulated with straw bales or floating row cover inside for protection to -12°C. They extend the season at both ends — spring start dates move 4–6 weeks earlier, and fall harvest extends 6–10 weeks longer. In zones 6+ they enable true winter harvests of kale, mâche, claytonia, and overwintering spinach.

Hardy cool-season crops thrive in a Canadian cold frame from October through April. The reliable winter crops: kale (Winterbor, Red Russian, Westlandse Winter), mâche (corn salad), claytonia (miner's lettuce), overwintering spinach (Tyee, Bloomsdale Long Standing), arugula, Asian greens (mizuna, tatsoi), and overwintering parsley. In zone 7+: lettuce (Winter Density, Salad Bowl), green onions, swiss chard, leeks. Less hardy but worthwhile in zones 5–6: bok choy, fall-sown radishes, fall-sown carrots that finish maturing slowly through winter. For spring use: start seedlings of brassicas (broccoli, cabbage, kale), lettuce, and onions 3–4 weeks earlier than you could outdoors. Don't try to grow heat-loving crops (tomatoes, peppers, basil) in a cold frame — there's not enough sun in winter and they require active heat. Cold frames are for cold-tolerant species only.

Build vs buy depends on budget, materials availability, and woodworking skill. DIY pros: $50–150 for a 1.2 × 0.6 m frame using untreated cedar lumber and a salvaged window or new polycarbonate sheet; sized exactly to your space; you can scale up or modify; satisfaction. DIY cons: requires 2–4 hours of work, basic tools (drill, saw, screwdriver), and access to materials. Pre-built pros: arrives ready to assemble; consistent build quality; insulated kits available for cold zones. Pre-built cons: $200–500 typical for a comparable size; sizes are fixed; shipping costs for Canadian buyers can be high. Canadian retailers: Lee Valley Tools (Ottawa-based, ships nationwide, several models $250–600), Home Depot (Juwel Biostar polycarbonate $300+), garden centres carry seasonal stock May–July. For zone 3–4 Prairie use, build double-glazed (two layers of polycarbonate or a polycarbonate-over-glass sandwich) to get the insulation needed.

Twin-wall polycarbonate is the modern Canadian standard. The 6 mm or 8 mm twin-wall (also called multi-wall) variety has insulating air channels that hold heat far better than glass — and it doesn't shatter in heavy snow or wind. Cost: $30–80 per 1.2 × 0.6 m sheet at Lee Valley, Home Depot, or hardware stores. Lifespan 10–15 years in Canadian conditions before UV degradation reduces clarity. Old single-pane windows (free if you find them, $20–40 used) work but are heavier, easier to break, and lose heat faster. Old double-pane windows are the best free option if you can find them — match polycarbonate for insulation at zero cost. Greenhouse plastic (6-mil clear poly) is the cheapest at $10–15 per frame but lasts only 1–2 years before becoming brittle and cloudy. Acrylic plexiglass works but yellows faster in UV than polycarbonate. For most home builds: twin-wall polycarbonate is the sweet spot.

Three rules. (1) South-facing exposure — the lid should face south or southeast so it catches the most winter sun. The sloped top (lower at the front, higher at the back) maximizes the sun angle in winter when the sun is low in the sky. (2) Wind protection — site against the south side of a building, fence, or hedge if possible. Cold prevailing winds (north-westerly in most of Canada) suck heat out of any enclosure; a windbreak holds the captured solar gain. (3) Good drainage — the cold frame floor should be on the same level or slightly raised above surrounding soil so water doesn't pool inside during fall rains or spring snowmelt. Wet feet kill cold-frame plants faster than cold does. Bonus: site near the house if possible — you'll use the cold frame far more often if it's a 10-second walk to harvest a salad than a 5-minute trek across a snowy yard. Many Canadian gardeners build their cold frame as part of a south-facing deck-skirt or against a south-facing garage wall.

The single most important daily task: vent on warm sunny days. A closed cold frame on a clear winter day can reach 30–40°C inside — enough to cook the crops you're trying to grow. Open the lid (or prop it partially open with a wooden stick) any time outdoor air rises above 5–7°C and the sun is shining. Close before evening to capture the day's heat. Automatic vent openers (Lee Valley sells these for $50–80) use a wax cylinder that expands with heat and opens the lid at a preset temperature, then closes it as temperatures drop — these are nearly mandatory if you can't check the frame daily. In zone 3–4 mid-winter, add insulation: lay an old blanket or 2–3 layers of floating row cover INSIDE the frame at night (remove during the day for light), or pack straw bales against the outside walls. Snow on the lid is actually beneficial in deep winter — adds insulation and the plants don't need light when soil is frozen anyway. Brush snow off when temperatures rise above freezing and plants need light again.

Cold frames have three distinct use seasons. Spring jump-start (March–April): start seedlings of brassicas, lettuce, onions, leeks 3–4 weeks earlier than you could in open ground; direct sow radishes and spinach in March in zones 5+ for April harvest. Fall extension (September–November): cold frame extends the harvest of summer-planted greens, late carrots, leeks, and brassicas by 6–10 weeks. The cold frame is the difference between fresh lettuce ending in late September vs continuing into early December. True winter harvest (December–February): for zones 6+, the cold frame supports living winter crops — mâche, claytonia, kale, overwintering spinach, parsley. Sow these in early September so they reach 50% of mature size before short December days; they then hold dormant through deep winter and resume growth in late January as days lengthen. In zone 3–4, winter is mostly a holding phase: harvest what's there and replant in mid-February.

A simple 1.2 × 0.6 m cold frame for a Canadian backyard takes 2–4 hours and costs $80–150 in materials. Materials: untreated cedar or pressure-treated 2×10 lumber (don't use pressure-treated lumber that contacts soil where you'll grow food — use cedar for the bottom course, PT only for upper if needed); 2×4 lumber for corner posts and lid frame; 1.2 × 0.6 m sheet of 6mm twin-wall polycarbonate; brass or stainless hinges (3 along the back edge); deck screws (3 inch); a wooden prop stick for venting. Build steps: (1) Cut the 2×10 boards to your dimensions; the back wall is 1.2 m × 30 cm tall, the front wall is 1.2 m × 20 cm tall (sloping down toward the south), side walls are trapezoids 60 cm wide × 30 cm at back × 20 cm at front. (2) Screw together at the corners using 2×4 inner corner posts as backing. (3) Build the lid frame from 2×4s sized to match the box top with a slight overhang. (4) Attach polycarbonate sheet to the lid frame using screws with weather washers. (5) Hinge the lid to the back wall. (6) Place on level ground or raised bed, fill with 30 cm of finished compost-amended soil. (7) Cut a wooden prop stick for venting. Free plans from Lee Valley, Mother Earth News, and Eliot Coleman's books cover variations.

Five common mistakes. (1) Not venting on sunny winter days — the #1 killer of cold frame crops is overheating, not cold. Open the lid any time outdoor air rises above 5°C with sun. (2) Siting in shade — a cold frame in winter needs every minute of sun it can get; even half-day shade dramatically reduces effectiveness. North-facing or shaded sites are pointless. (3) Using untreated single-pane glass alone in zone 3–4 — heat loss is too high; use twin-wall polycarbonate or double-glazed glass for cold-winter zones. (4) Watering too often in winter — cold frame soil dries far slower than open-garden soil because of the enclosure. Check moisture with a finger before watering; water only when the top 5 cm feels dry. Overwatering causes root rot and damping-off in cool conditions. (5) Trying to grow heat-loving plants — tomatoes, peppers, basil don't work in a cold frame even with the temperature lift; there's insufficient light in fall and winter and they need 20°C+ nights they won't get. Stick to cold-tolerant species.