Greenhouse
A greenhouse extends your growing season at both ends, protects from frost, and lets you grow heat-loving crops (tomatoes, cucumbers, peppers, basil) in climates that would otherwise be marginal. Even a small cold frame can add 4–6 weeks in spring and autumn.
Why a greenhouse?
Start sowing 4–6 weeks earlier in spring — seedlings grow faster in warm, protected air.
Extend autumn harvests of peppers, eggplants and tomatoes until the first hard frost.
Grow winter greens (mâche, spinach, Asian greens) almost year-round in mild climates.
Reliable crop from heat-lovers even in cool, wet summers — no more tomato blight from rain on leaves.
A dedicated space for seedlings, cuttings and overwintering tender herbs like rosemary or lemon balm.
Types to consider
Cold frame
Low, unheated box with a hinged glass or polycarbonate lid. Cheapest entry point — great for hardening off seedlings, winter greens, and an early spring head-start. Fits neatly over a square-foot bed.
Lean-to
Attached to a south-facing house wall. Benefits from the wall's thermal mass (releases heat at night), uses less material, and is easy to wire for electricity or water.
Free-standing
Classic greenhouse, placed anywhere with good light. More expensive but the most flexible — you can design the inside exactly for square-foot beds.
Polytunnel
Hooped frame covered with greenhouse foil. Huge cubic volume for little money, but foil needs replacing every 4–6 years and insulation is weaker.
Glazing: glass vs polycarbonate vs foil
Glass
Best light transmission (90%+), long-lasting (30+ years), easy to clean. Heavy, fragile, expensive. Use toughened safety glass where possible.
Twin-wall polycarbonate
Double-walled sheet with air pockets — much better insulation than single glass. Lightweight, shatterproof, diffuses light (less leaf burn). Scratches over time, UV coating degrades after ~15 years.
Greenhouse foil (PE/EVA)
Cheapest. Light, easy to replace. Poor insulation, torn by wind or hail. Good for seasonal polytunnels, less for year-round use.
Choosing a spot
Full sun — minimum 6 hours of direct light in winter. South or south-east orientation is ideal.
Shelter from strong wind (hedge, wall) — but not shaded by it.
Level ground, or prepare a level pad. A tilted greenhouse pools water and stresses the frame.
Access to a water tap and (ideally) power — one extension cord trip through the garden gets old fast.
Keep 50 cm clear around all sides for maintenance, cleaning and cold-frame-lid swing.
Square-foot gardening inside
A greenhouse and SFG are a natural fit — the dense, organised planting of SFG maximises the expensive square meters under glass.
Raised beds fit perfectly: 1.2 m wide, accessible from both sides (or 60 cm if only one side).
Leave a central path of 60–80 cm so you can kneel comfortably.
Use the bed edges as staging surfaces for seed trays and small pots.
Tall crops (indeterminate tomatoes, cucumbers, climbing beans) go to the north end so they don't shade the rest.
Add a vertical trellis against the back wall — cucumbers and tomatoes grow up, not out.
Ventilation — the #1 thing most beginners get wrong
A closed greenhouse in June can hit 50 °C in an hour and cook every plant inside. Ventilation is not optional.
Roof vent + door opening creates a chimney effect that pulls hot air out. Open both whenever it's above 20 °C outside.
Automatic window openers (paraffin-powered, no electricity needed) are worth their weight in gold — they open between 16–24 °C automatically.
Aim for a total vent area of at least 20% of the floor area.
In very hot weather, leave the door open all day and close only at dusk.
Watering, humidity and shade
Water in the morning so leaves dry before evening — humid greenhouse + wet leaves at night = fungal heaven.
Water the soil, not the leaves. Drip lines or a watering can with a narrow spout work well.
Damping down (spraying the floor with water on hot days) cools the air and raises humidity for tomatoes and cucumbers.
From late May, apply shade paint or shade netting to the south-facing glass — 30–50% is usually enough.
What to grow — when
Think of the greenhouse in two seasons: warm-loving crops in summer, cold-hardy crops in winter.
Warm season (April–October)
Tomatoes — indeterminate varieties trained up a string
Cucumbers — climbing, on a trellis
Peppers, chili, eggplant — heat-lovers that struggle outdoors
Basil and other tender herbs as a understory
Cold season (October–March)
Mâche (lamb's lettuce) — winter-hardy, sweetest after frost
Winter spinach, Asian greens (pak choi, mizuna, mustard)
Winter lettuce varieties ('Winter Density', 'Arctic King')
Overwintering garlic and broad beans for an early spring harvest
Build / set-up tips
Foundation: concrete strips or a railway-tie frame — don't skip it, or the first storm will lift the whole structure.
Floor: brick, paving slabs or weed fabric with gravel — bare soil attracts slugs and makes cleaning impossible.
Install a water butt to collect rainwater from the roof — soft water is better for plants than tap water.
A thermometer with min/max memory tells you what actually happens overnight.
Run a basic 230 V circuit for frost-protection heating and lighting — even 1 kW keeps a small greenhouse above freezing.
Add staging (shelves) for seedlings at waist height, leaving floor space for the big crops.
Rule of thumb: spend half the budget on the frame/foundation, half on ventilation and watering. A well-ventilated polytunnel outperforms a poorly-ventilated glasshouse every time.