A greenhouse is a building made mostly of glass or clear plastic where plants are grown in controlled conditions. The transparent walls and roof let sunlight stream in to warm the soil and air inside, while keeping cold wind and harsh weather out. This creates a warm, protected environment where gardeners can grow tomatoes in winter, start seedlings before spring, or cultivate tropical plants in cold climates.

Commercial greenhouses can be enormous structures covering acres, producing flowers, vegetables, and herbs year-round. A backyard greenhouse might be small enough to hold just a few dozen plants.

Greenhouses work through a simple but clever principle. Sunlight passes easily through the glass, warming everything inside. That warmth tries to escape back out as heat, but glass doesn't let heat pass through as readily as light. The result? The inside stays warmer than the outside, sometimes dramatically so. On a cold March day, a greenhouse interior might feel like May.