Many vegetables and flowers are grown in greenhouses in late winter and early spring, when it is still too cold to grow plants outside. Then these plants move into the soil outside as the weather warms up. Greenhouses are used to grow crops in cold countries such as Canada. The largest group of greenhouses in the world is in Leamington, Ontario (in Canada), where about 200 acres (0.8 km²) of tomatoes are grown in glass greenhouses.