A greenhouse is a glazed room for growing plants, and it can supply fresh food year-round even where winters are harsh. Choose between a lean-to built against a south wall and a freestanding structure, build the frame from wood, bamboo or salvaged metal, and cover it with glass, plastic film or oiled cloth.
A greenhouse is a cold frame large enough to walk into: a frame covered in transparent material that traps sunlight as heat.
A Greenhouse Is Three Choices: Where It Stands, What Holds It Up, And What Covers It
Season extension is the difference between subsistence and abundance, and a greenhouse takes it further than a cold frame can. A cold frame adds 4-8 weeks to each end of the season; a greenhouse can give fresh food through the winter in climates where the ground freezes. Neither needs sophisticated technology, and farmers have used both for centuries.
Every greenhouse comes down to three decisions:
- Where it stands. A lean-to uses a building’s south wall for support, shelter and stored heat; a freestanding greenhouse gets more light but loses more heat. See Lean-To vs. Freestanding.
- What holds it up. The frame must carry the glazing, the wind and sometimes snow while casting as little shadow as it can. See Frame Materials: Wood, Bamboo, Salvage.
- What covers it. Glass lasts longest and costs the most to find; plastic film is the practical choice for size; oiled cloth is the last resort. See Glazing Options.
What You Need
- A site with full winter sun, sheltered from the strongest wind, and well drained
- A frame — wood, bamboo or salvaged metal pipe
- Glazing — glass, film or cloth
- Provision for ventilation and heat storage, covered in Climate Control
Method: Plan Before You Build
Step 1: Match the scale to the need. A small lean-to attached to a home suits a household. A larger freestanding house suits a community growing at scale.
Step 2: Pick the frame to match your materials. Use what grows or lies about you: rot-resistant wood where you have it, bamboo where it grows, bent conduit or pipe where you can salvage it.
Step 3: Decide the glazing before you frame. Glass panes dictate a frame spaced to their size. Film wants a smooth, snag-free frame. Choose first, so the frame fits the covering.
Step 4: Plan ventilation before you build the walls. Vents cut into a finished frame are awkward. Mark where the doors, side vents and ridge vent go.
Common Mistakes
- Building closed with no vents — a closed greenhouse in full sun can exceed 50 °C and kill the crop. Overheating is a faster killer than cold.
- Ignoring wind and snow — a greenhouse roof is a structure. See Wind Loads and Live Loads.
- Choosing glazing last — the frame spacing then does not fit the panes or the film.
Safety
Heat, glass and falling snow
An unventilated greenhouse reaches temperatures that cause heat illness within minutes for anyone working inside. Broken glass and a roof that sheds snow suddenly are the other two hazards; stand clear of the eaves after a heavy fall.
What’s Next
- Learn next: Lean-To vs. Freestanding — the first decision
- Combine with: Climate Control — keeping the interior in the plants’ range