Cold Frames & Season Extension: Stretching Alberta's Short Growing Season

How a cold frame works

A cold frame is a bottomless insulated box, typically 30–60 cm tall at the back and sloping down toward the front, with a glass or polycarbonate lid angled to capture low-angle sunlight. On a sunny day in April, the lid traps solar heat; internal temperatures easily reach 15°C even when outdoor air is barely above freezing. At night, the thermal mass of the soil inside holds heat far better than open ground.

The practical effect: plants inside a cold frame are growing in conditions equivalent to a zone warmer than the surrounding garden. For Alberta, that’s the difference between “technically possible but risky” and “reliably productive.”

Three build options, cheapest to nicest

1. Straw bale + old window (under $30)

The minimalist build: arrange four straw bales in a rectangle directly on your garden bed, set an old window or polycarbonate sheet on top as a lid. Straw walls provide insulation; the dirt floor supplies thermal mass. Lasts one or two seasons as the bales break down (then mulches your garden for free). Ideal for first-time cold framers.

2. Scrap lumber box (~$75)

Build a sloped wooden box from cedar fencing or scrap 2×8s: ~120 cm wide, 80 cm deep, 50 cm tall at the back, 30 cm tall at the front. Hinge an old window on top. Stake into the ground or sit on a prepared bed. Lasts 5–10 years with minimal maintenance.

Use rot-resistant wood (cedar is best, pressure-treated is fine for outside but don’t let soil touch the treated wood). Paint the interior white or line with reflective foil to bounce more light onto plants.

3. Polycarbonate kit or purpose-built greenhouse ($200–500)

For serious season extension, twin-wall polycarbonate panels provide better insulation than single-pane glass. Pre-made cold frame kits from garden centres typically run $200–350. If you’re planning to grow year-round, a small walk-in greenhouse is the logical upgrade.

Placement matters more than build quality

• South-facing location. Full sun all day. Even 2 hours of afternoon shade cuts effectiveness dramatically.
• Against a south or west building wall if possible. The wall stores daytime heat and releases it at night — effectively doubling your thermal mass.
• Slope the lid at 30–40 degrees toward the south. Maximizes winter sun capture.
• Protected from prevailing winds. In Alberta, that usually means a windbreak on the west or northwest side — fence, hedge, or house wall.
• Avoid low-lying spots. Frost settles in hollows; a cold frame 30 cm higher than surrounding ground stays warmer on still nights.

Seasonal calendar: what to grow when

March (start 3–4 weeks before last frost)

• Direct-sow hardy greens: spinach, mache, arugula, claytonia, baby kale
• Pre-sprout potato eyes for early planting
• Hardening off indoor-started brassicas (broccoli, cabbage, cauliflower) for 7–10 days

April – early May

• Lettuce, radish, green onion direct-sown
• Hardening off tomatoes, peppers, herbs before main planting
• Overwintered spinach and kale reaching full size

Mid May – early June

Cold frame becomes a warm nursery. Prop the lid fully open on warm days or remove entirely. Use for:

• Sprouting sweet potato slips
• Starting cucumbers and melons a few weeks early for transplant after June 1
• Protecting tender transplants from late frosts

June – August (prop lid open or remove)

Use the bed as a regular raised bed. Or reserve it for heat-loving crops: peppers, eggplant, melons, basil — they’ll produce longer in Alberta inside a frame with the lid removed. The box still provides wind protection and warmer soil.

September – October (season extension end)

• Late kale, spinach, arugula, lettuce reaching maturity protected from frost
• Last tomatoes ripening inside the frame with lid closed at night
• Start winter spinach and mache for overwintering (will size up briefly, then go dormant under snow)

November – February

A well-built cold frame with extra insulation (straw bales around the outside, or a second layer of bubble wrap inside the lid) can keep spinach and mache alive all winter in Alberta. They won’t grow much in December–January — day length is too short — but they’ll hold size and resume growth by mid-February. Brush snow off the lid on sunny days; the sun warms the frame even in deep winter.

The cooking problem — and how to solve it

The #1 cold-frame failure: plants roasting on a sunny April day when the owner is at work. On a cloudless afternoon, even with outdoor air at 5°C, a closed frame can hit 35°C quickly. Solutions:

• Prop the lid on warm days. A 5–10 cm opening vents heat without losing all warmth. Leave it propped whenever daytime high is above 10°C.
• Automatic vent openers. A cylinder filled with wax expands as it warms and lifts the lid automatically. $30–50, battery-free. Worth every penny.
• Indoor/outdoor thermometer. Monitor frame temperatures from inside the house; you’ll quickly learn when you need to vent.

Beyond cold frames: other season extenders

• Row cover / floating row cover (Reemay, Agrofabric).Lightweight fabric laid directly over plants or supported on hoops. Cheap, flexible, adds about 3 °C of frost protection. Ideal for quick coverage during a cold snap.
• Low tunnels / mini hoop houses. Flexible hoops over a row covered with clear plastic or row cover. Similar protection to a cold frame at longer/wider scales, though without the thermal mass benefit.
• Walls-O’-Water / tomato teepees.Cylinders of water-filled tubes surround a single plant. Release stored heat at night. Excellent for getting tomatoes started 3–4 weeks early.
• South-facing stone walls.Nothing you build — but if you have one, it’s free season extension. Plant against it for noticeably earlier warm-up and later first frost.