Greenhouse Basics: Choosing, Setting Up, and Using a Home Greenhouse

What a Greenhouse Actually Does

A greenhouse is not magic — it cannot make plants grow in any condition. What it does is extend the growing season by capturing solar energy, raising the interior temperature above ambient, and protecting plants from wind, rain, and pests. A typical unheated greenhouse in a temperate climate provides 4–8 weeks of additional growing at each end of the season — plants can be started 4–6 weeks earlier in spring and continue growing 4–6 weeks later in autumn. With supplemental heating, the greenhouse becomes year-round.

The trade-offs are real. A greenhouse is a significant investment, requires daily monitoring (especially for temperature on sunny days), and creates a封闭 environment that can amplify pest and disease problems if not managed carefully. For gardeners who already grow a substantial vegetable garden or who want to grow plants outside their climate zone, a greenhouse is transformative. For casual gardeners, a cold frame or row cover may provide most of the benefit at a fraction of the cost.

Choosing a Greenhouse Material

The three main greenhouse covering materials — glass, polycarbonate, and polyethylene film — each have distinct advantages and price points. The choice depends on your budget, climate, and how permanent you want the structure to be.

Greenhouse covering materials compared:

• Glass — the traditional material, beautiful and long-lasting (50+ years), with excellent light transmission. Heavy, fragile, expensive ($2,000–10,000 for a small greenhouse), and requires a proper foundation. Best for permanent installations in areas without hail risk.
• Polycarbonate (twin-wall) — the modern standard. Lightweight, impact-resistant, good insulation (the twin-wall construction traps an air layer), and moderately priced ($500–3,000). Light transmission is slightly lower than glass but the insulation advantage more than compensates in most climates. Lifespan of 10–20 years.
• Polyethylene film — the cheapest option, used for high tunnels and hoop houses. A single layer of UV-stabilized poly film costs $50–200 for a 3 x 6 meter structure. Lifespan is 3–5 years before the film needs replacement. Light transmission is good but insulation is poor unless double-layered with inflation. Best for seasonal use rather than year-round growing.

For most home gardeners, polycarbonate is the right choice. It balances cost, durability, insulation, and ease of installation. Twin-wall polycarbonate in 6 mm thickness provides enough insulation for year-round growing in most temperate climates with minimal supplemental heat. Look for panels with UV protection on the exterior surface — without it, the polycarbonate yellows and becomes brittle within 3–5 years.

Siting and Orientation

The position of your greenhouse determines how much sun it receives and how easy it is to use. In the northern hemisphere, orient the long axis east-west so the south-facing wall captures maximum winter sun. The greenhouse should receive at least 6 hours of direct sun in winter — more is better. Avoid sites shaded by buildings or evergreen trees; deciduous trees that lose their leaves in winter can provide welcome summer shade without blocking winter sun.

Consider access too. A greenhouse that is far from the house gets used less than one that is steps from the back door. You will visit it daily during the growing season, often in cold or rainy weather, so a paved path is worth the investment. Proximity to water and electricity is also important — running a hose is fine for summer but a frost-proof spigot inside the greenhouse is essential for winter use.

Ventilation: The Most Overlooked Essential

More greenhouses fail from overheating than from freezing. On a sunny day, even in winter, the interior temperature of an unventilated greenhouse can reach 30–40°C when the outside temperature is only 10°C. Plants suffer irreversible damage above 35°C, and many die at 40°C. Ventilation is not optional — it is the most critical system in any greenhouse.

Provide ventilation through a combination of roof vents (which let hot air escape by convection) and side vents or louvered windows (which let cool air in). The total vent area should be 20–30% of the floor area for manual vents or 10–15% for automated vents. Install automatic vent openers (cylinders filled with wax that expands as temperature rises, opening the vent without electricity) on at least the roof vents — these are inexpensive ($40–60 each) and prevent catastrophic overheating even if you are not present.

For summer cooling, add an exhaust fan on one end of the greenhouse and intake louvers on the opposite end. The fan should be sized to exchange the entire volume of greenhouse air every 1–2 minutes. In hot climates, evaporative cooling pads on the intake side can lower the incoming air temperature by 5–10°C, though they only work in dry climates (evaporative cooling is ineffective in humid conditions).

Heating for Year-Round Use

An unheated greenhouse in a temperate climate maintains interior temperatures 3–5°C above ambient on sunny days and 1–2°C above ambient on cloudy days and nights. This is enough to extend the season but not enough for true year-round growing in cold climates. For year-round use, supplemental heating is required.

Electric heaters are the simplest option for small greenhouses — a 1500W electric heater can maintain a 3 x 4 meter greenhouse 8–10°C above ambient at a cost of $1–2 per day in electricity. Propane and natural gas heaters are more economical for larger greenhouses but require ventilation to remove combustion products. Wood-fired and pellet heaters are options for off-grid greenhouses but require frequent attention. Whatever the heat source, install a thermostat to control it — leaving a heater running unregulated can cook plants when the sun comes out.

Pest and Disease Management

The closed environment of a greenhouse is both a blessing and a curse for pest management. Pests cannot easily colonize from outside (good), but once established, they have ideal conditions to multiply rapidly and no natural predators to control them (bad). The key is prevention: inspect all plants before bringing them into the greenhouse, quarantine new plants for 2 weeks, and remove any infested plant material promptly.

Common greenhouse pests include aphids, whiteflies, spider mites, fungus gnats, and thrips. Each has specific biological controls available — Encarsia formosa for whiteflies, predatory mites for spider mites, Bacillus thuringiensis for fungus gnat larvae. Release biological controls at the first sign of pests, before populations explode. Sticky yellow traps placed among the plants catch flying adults and serve as an early warning system.

Fungal diseases thrive in the humid greenhouse environment. Maintain good air circulation with horizontal airflow fans running constantly — this also strengthens plant stems and reduces fungal spore germination. Water at the base of plants rather than overhead, and water in the morning so foliage dries before nightfall. Remove and destroy any diseased plant material rather than composting it, as greenhouse compost piles rarely get hot enough to kill pathogens.