The function of a greenhouse is to control humidity, light, moisture and temperature, allowing gardeners to grow plants before and after a normal growing season. Greenhouses can be large structures made of glass or small units made from heavy-duty plastic stretched over a frame.
Large greenhouses also allow gardeners to store potting materials near the garden. Although greenhouses provide protection from the elements, gardeners must use fans, vents and heaters to ensure that the structures do not become too warm or cold. Larger greenhouses provide warmer conditions than smaller units, as they draw heat from a bigger patch of ground.