Most cultivated orchids are native to the tropics. In their natural habitat, they attach themselves to the bark of trees, or the surface of other plants. Their thick, white roots are specially adapted to absorb moisture and dissolved nutrients. Because these tropical orchids usually grow high in the trees, rather than on the forest floor, they are accustomed to good air circulation and plenty of light. They prefer a 12-hour day, all year-round, and require a high intensity of light — about the same as midsummer conditions in temperate regions.

Orchids are usually grouped into two broad categories that characterize their growth habits. Monopodial orchids have a single, upright stem, with leaves arranged opposite each other along the stem. The flower stem appears from the base of the uppermost leaves. Orchids with this growth habit include the phalaenopsis and vandas.

The more common growth habit is sympodial. These orchids grow horizontally, sending out new shoots from the old rhizome. Leaves and flower scapes form at the top of the new shoots. Many sympodial orchids form pseudobulbs, which are swollen shoots that store water and nutrients to help the plant survive periods of prolonged drought. Sympodial orchids include cattleya, cymbidium, oncidium and dendrobium.

Orchids can also be classified by their native habitat, which gives an indication of the temperature, moisture and light levels they prefer. Orchids native to the humid tropics, such as phalaenopsis and paphiopedilum, prefer daytime temperatures of 73° to 85°F, with 80 to 90 percent humidity. They are happiest in an east or southeast window where the light is not too intense.

Warm-climate orchids, including cymbidiums and dendrobiums, are accustomed to an average temperature of 55° to 70°F, a steady supply of moisture, and good air circulation. They are generally happy in a south-facing window, though they may need a little shading during high summer.