Greenhouse & Cold Frame Construction
Season extension is the difference between subsistence and abundance. A cold frame adds 4-8 weeks to both ends of your growing season. A greenhouse can provide fresh food year-round, even in climates with harsh winters. Neither requires sophisticated technology—farmers have used both for centuries.
Cold Frames
A cold frame is the simplest season-extension tool: a bottomless box with a transparent lid, placed over garden soil. Sunlight enters through the lid, heats the soil and air inside, and the box traps that heat.
Basic Construction
Materials:
- Back wall: 40-50cm tall (boards, logs, stacked stone, or earthbags)
- Front wall: 20-30cm tall (the slope directs the lid toward the sun)
- Side walls: tapered from back height to front height
- Lid: transparent material in a frame that sits on top
Orientation: The lid faces south (Northern Hemisphere). The sloped lid angle should approximately match your latitude for maximum solar gain.
Size: Standard: 120×180cm (comfortable reach from the front). Don’t build wider than you can reach to the back row—you’ll avoid stepping on plants and compacting soil.
Lid materials (best to worst):
- Old window sashes (glass) — excellent insulation and light transmission. Fragile
- Polycarbonate panels (salvage from greenhouses, skylights) — tough, good insulation. UV-stable
- Plastic sheeting over a frame — cheap, easy to replace. Tears in wind. 1-2 season lifespan
- Oiled muslin or cotton — traditional, moderate light transmission. Needs re-oiling regularly
Hinge the lid at the back so it opens upward for ventilation. A prop stick holds it open at varying heights.
The Hotbed: Manure-Heated Cold Frame
A hotbed is a cold frame with a built-in heater: decomposing horse or livestock manure generates heat for 6-8 weeks, warming the soil from below.
Construction:
- Excavate 45-60cm below the cold frame footprint
- Fill with fresh horse manure (with bedding straw) to 30cm depth
- Water thoroughly and let heat for 3-4 days until internal temperature reaches 50-60°C
- Cover with 15-20cm of good garden soil or compost
- Plant into the soil layer. The manure below provides bottom heat; the cold frame lid provides top insulation
A hotbed can start seedlings 8-12 weeks before the last frost. This gives your garden an enormous head start.
Greenhouse Structures
Lean-To vs. Freestanding
Lean-to greenhouse: Built against the south wall of an existing building. The building wall provides thermal mass, wind protection, and structural support. Heat can flow between the greenhouse and building (open a door or window between them). The building benefits from passive solar heat gain through the greenhouse.
Advantages: Less material needed, shared wall reduces heat loss, building acts as thermal mass. Best for: Cold climates, small-to-medium scale, attached to a home for convenient access.
Freestanding greenhouse: A separate structure with glazing on all or most sides. Gets more total light but loses more heat.
Advantages: Can be any size, maximum light exposure, can be oriented independently. Best for: Larger operations, mild climates, community growing.
Frame Construction
The frame must support glazing weight, wind loads, and occasional snow loads while minimizing shadow-casting structure.
Wood frame: Most accessible. Use rot-resistant species (cedar, locust, oak) or treat with linseed oil. Typical: 10×10cm posts at 120-180cm spacing, with lighter rafters (5×10cm) supporting the glazing.
Bamboo frame: Excellent where available. Lash joints with cordage or wire. Bamboo’s natural taper creates a nice curved profile. Replace every 3-5 years.
Salvaged metal: Conduit, rebar, or pipe bent into hoops creates a tunnel greenhouse (hoop house). Cover with plastic film. Fast, cheap, effective. The standard commercial design pre-collapse.
Glazing
Salvaged glass is the premium option. It transmits ~90% of light, insulates better than plastic, and lasts indefinitely. But it’s heavy, fragile, and requires careful framing. Seal panes into frames with putty (linseed oil + chalk powder) or silicone (if salvageable).
Plastic film (polyethylene) is the most practical option for large greenhouses:
- 6-mil greenhouse-grade film is ideal
- Standard hardware-store poly works but degrades faster in UV (1 season vs 3-4 for greenhouse film)
- Double-layer inflation (two sheets with an air gap blown between them by a small fan or wind catcher) dramatically improves insulation
Oiled cloth: Historically used before glass became affordable. Heavy cotton or linen, saturated with linseed oil, becomes translucent. Transmits ~60% of light. Must be re-oiled annually. A viable last resort if no glass or plastic is available.
Climate Control
Thermal Mass
A greenhouse without thermal mass overheats during the day and freezes at night. Add mass to store daytime heat:
- Water barrels — the most effective greenhouse thermal mass. Paint black and place along the north wall. 200 liters of water per square meter of glazing is a good target
- Stone or brick walls — the north wall of a lean-to, or a stone knee wall inside the greenhouse
- Dark-colored floor — stone, brick, or dark gravel absorbs and stores heat
Ventilation
Overheating kills plants faster than cold. A closed greenhouse in full sun can reach 50°C+ in minutes. Ventilation is critical.
Manual: Open doors, windows, or lift cold frame lids when temperature exceeds 25-30°C. Close in late afternoon to trap heat for the night.
Passive automatic: A simple wax-cylinder vent opener (salvage from garden supply) expands as temperature rises, pushing open a vent window. No electricity needed.
Rule of thumb: Total vent area should equal 15-25% of the floor area for adequate air exchange.
Growing in the Greenhouse
A greenhouse changes what you can grow and when:
Winter crops (no supplemental heat needed in mild climates):
- Lettuce, spinach, mâche, claytonia
- Kale, chard, Asian greens
- Radishes, turnips (small varieties)
- Herbs: cilantro, parsley, chives
Spring start (in hotbed or heated greenhouse):
- Tomato, pepper, eggplant seedlings
- Brassica starts (cabbage, broccoli)
- Onion and leek starts
- Flower seedlings for the garden
Summer use:
- Heat-loving crops in short-season climates: tomatoes, peppers, melons, cucumbers
- Open vents fully—the greenhouse becomes a rain shelter and wind block
Succession planting: Start a new flat of lettuce or greens every 2-3 weeks for continuous harvest. As one crop finishes, another is ready. Never let greenhouse space sit empty—every empty square meter is wasted solar energy.
Pest Management
Greenhouse environments favor both plants and pests:
- Aphids — encourage ladybugs, lacewings. Spray with diluted soap (compatible with greywater-recycling)
- Whitefly — yellow sticky traps (smear yellow-painted boards with salvaged petroleum jelly)
- Fungal disease — improve ventilation, avoid overhead watering, space plants for airflow
- Slugs — copper barriers (strip of copper wire around raised beds), beer traps
- Screen vents with fine mesh to prevent insect entry while allowing airflow