Ferns are some of the oldest plants in the world—they've been thriving for 300 million years and grow in an astonishing array of environments. As houseplants, they've been in cultivation for centuries. Worldwide, the American Fern Society estimates there are about 12,000 species of ferns, ranging from cold hardy to tropical, and ranging in size from miniature to the monstrous tree ferns of New Zealand and Australia.