Learn & Understand

Trapping the Sun: The Greenhouse and Its Effect

A greenhouse works a small miracle: it stays warm inside even on a cold, sunny day, letting gardeners grow tender plants out of season. The physics behind it — sunlight in, heat trapped — is the same principle that regulates the temperature of the whole planet, and it gave that planetary process its name.

Warmth From Glass

A greenhouse lets in sunlight, which warms the soil, plants, and surfaces inside. Those warmed surfaces radiate heat back, but the glass or plastic covering slows that heat from escaping and blocks the wind that would carry it away. Sunlight enters freely; warmth leaves reluctantly. The result is an interior that grows and stays far warmer than the air outside.

From Orangery to Glasshouse

The idea is old. Wealthy Europeans built “orangeries” — heated buildings with large windows — to keep citrus and tender exotics alive through winter, displays of both botany and status. As glass grew cheaper, the glasshouse spread, letting gardeners extend seasons, start plants early, and grow crops far from their native climates. The greenhouse democratized growing warmth.

The Planet's Greenhouse

The same principle warms the Earth. Certain gases in the atmosphere let sunlight through but trap the heat the surface radiates back, keeping the planet far warmer than it would otherwise be — the “greenhouse effect,” named directly for the glasshouse. A modest natural version makes Earth habitable; strengthening it by adding more of those gases is what drives climate change.

Fighting Heat Loss

On a cold night, a greenhouse still loses heat through its covering, so growers in cold climates must add warmth. How much depends on the structure's surface area, the temperature it must maintain against the cold outside, and crucially the insulating quality of its covering — a single film loses heat far faster than double glazing. Choosing the covering is choosing the heating bill.

Sizing the Heat

To estimate greenhouse heating needs, use the Greenhouse Heating Requirement Calculator. Extend the outdoor season alongside it with the Growing Season Length Calculator, and time indoor starts with the Seed Starting Date Calculator.