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Because of the wide variety of orchid species, it's impossible to generalize about their growing habits and preferences. It all depends on which kind of orchid you're dealing with whether it prefers to grow in the air, on a rock, in bark or moss or in the soil.
Orchids are divided into three different groups.
Epiphytes are the most familiar orchids for most people. They are 'air growers', taking their nutrients from the air, though the usually grow on and in trees. Unlike many plants that grow in trees, epiphytic orchids are not parasites and don't harm the plants on which they grow. Cattleya, cymbidium and dendrobium are epiphytes, as are most of the orchids that are commonly cultivated.
Grow epiphytes in: baskets, mounted on cork, or in loose sphagnum moss or other planting media
Lithophytes grow on rock surfaces. Like epiphytes, they take nothing from their growing surface, using it only as somewhere to anchor their roots and keep from blowing away. Instead, they get their nutrients from the air. Australian rock orchid and eria grandis are lithophytes.
Grow lithophytes in baskets, on cork or on gravel or other planting media.
Terrestrials are orchids that anchor themselves in soil, but unlike most plants that root themselves on the ground – but not in the soil. Terrestrial orchids grow in the humus – the rich, loose top layer of soil that is made up of decaying leaves and vegetable matter. Lady slippers and some kinds of cymbidium are terrestrial orchids.
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