The question of whether an orchid is an air plant is common, due to the plant’s unique biology and the popular use of the term. While many orchids are classified as epiphytes, meaning they grow on another plant for support, the common household “air plant” often refers to the Tillandsia genus, a different group entirely. Understanding the biological classification of orchids reveals why most cultivated varieties are technically air plants, even though their care differs greatly from their Tillandsia cousins.
Defining Epiphytes: What Exactly Is an Air Plant?
An epiphyte is a plant that grows harmlessly upon another plant, typically a tree, using it solely for physical support. These plants are often called “air plants” because they do not root in soil. Instead, they derive moisture and nutrients from the surrounding air, rain, and accumulated debris. Epiphytes are distinct from parasites, which penetrate the host plant’s tissues to extract resources and cause harm. Common examples include mosses, ferns, bromeliads, and a large portion of the orchid family.
The Epiphytic Nature of Most Orchids
Approximately 70 to 75 percent of all orchid species exhibit an epiphytic growth habit, thriving high up on tree trunks and branches in tropical and subtropical regions. This elevated position allows them to access more sunlight in dense forest canopies. They have developed specialized aerial roots that serve two main functions: anchorage to the host tree and the rapid absorption of water and minerals. These roots are designed to cope with a highly xeric environment, where they are frequently drenched but dry out quickly due to air movement.
The outer layer of the aerial root features a spongy, multilayered tissue composed of dead cells called the velamen radicum. This velamen acts like a sponge, rapidly absorbing atmospheric moisture, dew, and nutrient-laden rainwater before it evaporates. The velamen also helps reduce water loss from the living inner cortex and provides mechanical protection. Once water is absorbed, it is transferred to the inner root structure, which is protected by a waterproof layer, allowing the plant to conserve moisture.
Terrestrial and Lithophytic Orchids: The Exceptions
While the majority of orchids are epiphytes, the family Orchidaceae is remarkably diverse, including significant minorities with different growth habits. Terrestrial orchids, which account for about 25 percent of all species, grow in the ground, rooting in humus and soil like typical plants. Examples include some species of Cypripedium (Lady Slipper Orchids) and Paphiopedilum. These ground-dwelling varieties often develop underground structures like tubers or corms to store energy and survive dry periods.
A much smaller group, comprising less than 5 percent of species, are lithophytic orchids, which grow on rock faces and cliffs. These plants anchor their roots into rock crevices, drawing nourishment from decaying organic matter and debris. Although their location is different, lithophytes absorb nutrients from runoff water similarly to their epiphytic relatives. These exceptions demonstrate the adaptability of the orchid family.
Practical Differences in Care
The biological differences between epiphytic orchids and the common Tillandsia air plants translate directly into different care requirements for the home grower. Epiphytic orchids, such as Phalaenopsis, are typically grown in a coarse potting medium composed of bark chips, charcoal, or sphagnum moss. This medium is not soil but rather a simulation of the tree bark and debris pockets where their roots would naturally anchor and hold moisture. The velamen-covered roots require frequent watering followed by a rapid drying period, simulating the tropical wet-dry cycle.
In contrast, Tillandsia air plants absorb almost all their water and nutrients through specialized scales on their leaves, called trichomes. Their roots serve almost exclusively for physical attachment and can often be trimmed without harming the plant. Care for Tillandsia often involves a weekly routine of fully submerging the plant in water for a period, followed by ensuring complete air-drying to prevent rot. Therefore, while both plants are classified as epiphytes, the orchid’s specialized root system necessitates a moist, well-draining medium, whereas the Tillandsia thrives with no medium at all.