Greenhouses are mini-environments--limited, controllable and flexible. The air may be both warmer and more moist than that outside, which is better for many plants. Greenhouses are also valued for the shelter they provide from the rain, their even temperature, and even for the shade that can be created under their roofs. A greenhouse can serve different purposes depending on your part of the country.
Plants That Need Warmth
A common reason for growing plants in greenhouse is for warmth, either in winter or in summer. Any plant that isn't hardy in your area can often be grown in a pot with this extra protection, though generally tropicals, such as tree ferns, gardenias, poinsettas, tender orchids and bird-of-paradise plants are the favorites.
Plants that need extra heat in areas with cool summers include eggplants, green and hot peppers, cucumbers, melons and tomatoes. Basil often does better in pots in a greenhouse, too. Look for varieties of vegetables bred specially for greenhouse culture, especially those such as lettuce and other greens that will provide food throughout the winter.
Plants That Need Even Temperatures
Many seedlings and tiny plants grow better in the controlled environment of a greenhouse, even when they are hardy enough to grow outside once mature. Annuals can be started earlier in the year than if they were seeded directly outdoors, bulbs such as freesias, florist's cyclamen and dahlias can be potted and pushed into blooming earlier, and perennials can be grown to planting size more quickly.
Plants That Need Protection From Rain
In areas where winters are wet, plants from desert areas such as agave and cactus may die from root rot even though they are cold hardy. In a greenhouse, they can be given exactly the right soil, usually sandy or gravelly potting mix, and just enough water to keep them going through the winter. Lantanas, for instance, valued for their drought-tolerance and brilliant heads of flowers, are easier to overwinter if kept on the dry side in a protected area.
Plants That Need Shade and High Humidity
Fuchsias and tuberous begonias are plants that like a humid, partially shaded environment and, in areas with dry winds, do better in a shaded greenhouse. Many people plant vines to cover the south side of their greenhouses in summer, but you can also buy shade cloth and roll it down over the roof to protect the plants inside from too much light.
Many of the plants grown indoors do quite well in the humid environment of a shaded or partially shaded greenhouse, being native to the thick undergrowth of the tropics. The light will generally be stronger than inside a greenhouse, so you can grow plants such as brugmansia, mandevilla and New Guinea impatiens as well.