Cold Frame Guide Canada
DIY plans, build vs buy, materials, what to grow, daily venting, winter use. The highest-leverage tool for extending a Canadian growing season — 4–8 weeks at each end, and true winter harvests in zones 5+.
Cold frame Canada: a wooden box with a hinged polycarbonate lid that traps solar heat. Protects to −7°C unprotected, −12°C insulated. Extends growing season 4–8 weeks at each end. Grow kale, mâche, claytonia, overwintering spinach, lettuce, brassica seedlings. Build DIY for $80–150 (cedar + twin-wall polycarbonate) or buy pre-built $250–600 (Lee Valley, Home Depot). Site facing south, sheltered from north wind. Vent on sunny winter days — closed frames hit 30°C+ inside on clear days even at −10°C air.
Why a Cold Frame Is Canada's Best Garden Investment
Every Canadian gardener knows the frustration: a 5-month growing season, a long winter staring at frozen ground, and the gap between October's last harvest and June's first one filled with grocery-store produce. A cold frame closes about half of that gap at a fraction of the cost of a greenhouse. For roughly $80–150 in materials and a Saturday afternoon, you gain 4–8 weeks at each end of the growing season and — in zone 6+ — the ability to harvest fresh greens all winter.
The physics is simple. Solar gain through transparent lid + reduced heat loss through insulated walls + thermal mass of soil = a microclimate one to two zones warmer than the surrounding garden. A cold frame in Calgary (zone 3b) provides effective zone 5 conditions inside. In Toronto (zone 6b) it's zone 7–8 capable. In Vancouver (zone 8b) it carries crops right through winter with zero effort beyond venting and watering.
Build vs Buy: What Each Costs in Canada
DIY ($80–150)
- Cedar 2×10 lumber: $40–60
- 2×4 lumber for corner posts & lid frame: $15–25
- Twin-wall polycarbonate sheet (1.2 m × 0.6 m): $30–50
- Hinges, screws, hardware: $15–25
- Build time: 2–4 hours
- Best for: custom sizing, learning, gardeners with basic tools
Pre-built kit ($250–600)
- Lee Valley Tools (Canadian, ships nationally): several models $250–600
- Juwel Biostar (German-made, sold at Home Depot Canada): $300+
- Local garden centres: seasonal stock May–July, mixed quality
- Build time: 30–90 min assembly
- Best for: no-tools households, gardeners who want it now
Salvaged ($20–50)
- Free Facebook Marketplace or curb-found old windows for the lid
- Salvaged 2×6 or 2×10 from a torn-down deck or shed
- $20–50 for screws and hinges
- Best for: lowest cost, gardeners with good materials-scouting instincts
- Watch: single-pane glass loses heat fast in zone 3–4; double-pane is much better
Lid Material Choices (Ranked for Canada)
| Material | Cost | Insulation | Lifespan | Notes |
|---|---|---|---|---|
| Twin-wall polycarbonate (6–8 mm) | $30–80 | Excellent | 10–15 years | ⭐ Modern Canadian standard; insulating air channels, snow-tolerant, won't shatter |
| Salvaged double-pane window | Free–$40 | Excellent | 15+ years | ⭐ Best free option; matches polycarbonate for insulation; heavy — build strong frame |
| Salvaged single-pane window | Free–$20 | Fair | 10+ years | Cheapest; lower insulation = less useful in zone 3–4; OK for zone 5+ |
| Acrylic / plexiglass | $40–100 | Good | 5–8 years | Yellows in UV faster than polycarbonate; scratches easily; not recommended over polycarbonate |
| 6-mil greenhouse poly | $10–15 | Poor | 1–2 years | Cheapest option; becomes brittle and cloudy quickly; replace annually |
Where to Site a Cold Frame
Sloped lid (lower at front, higher at back) catches the low winter sun. South or southeast best. North-facing or shaded sites are pointless — the cold frame needs every minute of winter sun it can get.
Against the south wall of a building or fence, or behind a wind-blocking hedge. Cold prevailing winds suck heat out of any enclosure; a windbreak preserves the captured solar gain.
Cold frame floor at or slightly above surrounding soil. Wet feet kill cold-frame plants faster than cold does. In wet zones, build on a slight gravel-bed foundation.
Near the house if possible. You'll use it 10× more if it's a 10-second walk for a salad than a 5-minute trek across snow. Many Canadians site cold frames against a south-facing deck-skirt or garage wall.
Simple DIY Build Plan (1.2 × 0.6 m)
Materials: cedar or untreated softwood 2×10 (for walls); cedar 2×4 (corner posts + lid frame); 1.2 m × 0.6 m sheet of 6 mm twin-wall polycarbonate; 3 brass or stainless hinges; 3-inch deck screws; weather washers for polycarbonate fasteners; a wooden prop stick for venting.
Steps:
- Cut the back wall to 1.2 m × 30 cm tall, the front wall to 1.2 m × 20 cm tall, and two trapezoid side walls 60 cm wide × 30 cm at the back sloping down to 20 cm at the front.
- Screw the four walls together at the corners using 2×4 corner posts as backing (deck screws through wall boards into the post).
- Build the lid frame from 2×4s sized to match the box top with a 1–2 cm overhang on the front and sides.
- Attach the polycarbonate sheet to the lid frame using screws with weather washers (don't overtighten — polycarbonate needs room to expand/contract with temperature).
- Hinge the lid to the back wall with 3 hinges evenly spaced.
- Place on level ground (or a raised bed) with the front facing south. Fill the inside with 30 cm of finished compost-amended garden soil.
- Cut a wooden stick to act as a prop for venting. A piece of 2×2 cut to about 40 cm with a notch on one end works well.
What to Grow in a Canadian Cold Frame
🥴 Winter-hardy greens
Kale (Winterbor, Red Russian, Westlandse Winter), mâche (corn salad), claytonia (miner's lettuce), overwintering spinach (Tyee, Giant Winter), arugula, Asian greens (mizuna, tatsoi), overwintering parsley. The reliable winter producers in zone 5+ cold frames.
🥬 Lettuces (zone 6+)
Winter Density, Salad Bowl, Black-Seeded Simpson, Marvel of Four Seasons, Rouge d'Hiver. Cold-tolerant cut-and-come-again lettuces that produce through fall and into winter in milder zones.
🌿 Spring jump-start
Seedlings of broccoli, cabbage, kale, lettuce, onions started in March/April for transplanting outdoors when last-frost passes. Cold frame seedlings are hardier than indoor-only seedlings — they "harden off" naturally.
🥕 Carryover roots
Late carrots, parsnips, leeks, scallions sown in fall finish maturing slowly through winter under cold-frame protection. Pull as needed all winter. The cold frame doubles as cold storage for crops you don't dig up.
❌ Skip these
Tomatoes, peppers, basil, eggplant, cucumbers, squash — don't try heat-loving crops. Insufficient winter sun and no source of active heat. Cold frames are for cold-tolerant species only.
❄️ Overwintering seedlings
Sow sprouting broccoli (Purple Sprouting), overwintering onions (Walla Walla overwinter type), and walking onions in early September; they survive winter as small plants in the frame and produce in March–April — weeks before any spring-sown crop.
The Daily Discipline: Venting
The #1 cold-frame failure mode is cooking your crops, not freezing them. A closed cold frame on a sunny winter day with −10°C air outside can hit 30°C+ inside. Greens wilt and bolt; brassica seedlings stretch and weaken; lettuce gets bitter. 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 late afternoon to capture the day's heat for the cold night.
If you can't check the frame daily, install an automatic vent opener — Lee Valley sells these for $50–80. A wax cylinder expands with heat and opens the lid at a preset temperature (usually 21°C), then closes it as temperatures drop. Nearly mandatory if you travel or have a busy work schedule.
Cold Frame Use by Season
Direct sow spinach and arugula by mid-February in zone 5+. Start brassica + lettuce seedlings indoors and move to the cold frame after first leaves emerge for hardening-off.
Direct sow lettuce, radishes, peas. Continue hardening off summer crops indoors. By mid-May the cold frame is producing while open-garden beds are barely starting.
Leave the lid open or remove entirely. Cold frame is unnecessary in summer; the space can be used as a regular raised bed or left fallow with a cover crop.
Direct sow mâche, claytonia, overwintering spinach, kale, parsley in early September. Replace the lid in late September. Harvest fall greens until they slow in late December.
Plants hold dormant during short days. Harvest as needed; water sparingly. Days lengthen by mid-January and active growth resumes. The fresh-greens lifeline through Canadian winter.
Pair Your Cold Frame With…
Common Cold Frame Questions
How is a cold frame different from a hoop house or polytunnel?
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.
Do I need a heater in my cold frame?
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.
What happens if it gets really cold (e.g. −25°C)?
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.
Can I use a cold frame to start tomatoes?
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.
Plan Your Cold Frame
Find your first fall frost date, pick a south-facing spot, build or buy a 1.2 m × 0.6 m frame, and you're set for a 4–8 week season extension at both ends — and true winter harvests in zone 6+.