Aerial roots are typically thick, white or silvery and grow from the orchid plant outward, rather than down into the soil. They might also be seen growing outside of the pot in which an orchid is planted. They should not be cut off or buried in soil. Most orchids have aerial roots and are called epiphytic orchids, meaning the orchids grow on other plants, often trees, in the wild. But the orchids are not parasites and do not receive any nourishment from or harm the plants to which they're attached. Epiphytic orchids are commonly found in tropical areas on high tree branches, an adaptation to overcome the lack of sunlight on the ground in areas of dense vegetative growth.
Orchids belong to one of the largest, most diverse flowering-plant families, the Orchidaceae, with more than 25,000 known species and an additional 100,000 hybrids. One reason why orchids are so popular with collectors is the shear variety of orchid characteristics -- in plant parts, sizes and colors -- due to orchids adapting to different climates and growing conditions. Roots that enable some orchids to grow on trees represents one unusual adaptation.
Orchid aerial roots evolved so they bind the plant to its support material, such as a tree branch. They also provide water and nutrition for the orchid. A multi-layered, water-retaining, sponge-like covering on aerial roots, called the velamen, absorbs waters from surrounding air -- particularly from dew and rainfall -- and helps to give roots structure and prevents water loss. Aerial roots produce a sticky substance that allows them to adhere when they come into contact with a surface. Trying to remove a root damages it.
Unlike terrestrial orchids, which have short roots that grow downward and get nutrients from the soil, epiphytic orchids receive most of their nutrients from decaying organic matter, including bird droppings, that accumulates around their root zone and on their leaves. However, the tip of the aerial roots is green and provides the orchid with some, though not adequate, amounts of photosynthesis.
Air and Water Quality
Orchid roots require open air and will rot if they become waterlogged. Because of their roots' unique qualities and growing conditions, orchid growers must also pay attention to air and water quality. Unclean air damages aerial roots. Smoke, including cigarette smoke, can damage orchid roots, and those living in areas with polluted air might need to choose orchids that are most tolerant of poor air quality. Softened water should not be used for orchids because its salt damages roots; use distilled water or collected rainwater, or purchase pure spring water. Orchids vary by species and ornamental conditions regarding the amount of water they require.
Common epiphytic orchids include Cattleya, Laelia, Dendrobium and Phalaenopsis. Epiphytic orchids can be grown on supports such as wood, a coconut husk or in a pot with a growing medium, but the best support varies according to the different genera and environmental conditions. In a pot, epiphytic orchids require an orchid mix -- such as those made of coarse bark chunks or a black fiber of roots from the osmunda fern -- or a similar material that doesn't absorb much moisture and has good aeration. If you're using supports such as wood or a tree, the roots must come into contact with the material and remain stable so they can grow onto the material. Orchids prefer humidity levels between 40 and 60 percent, which most homes readily supply. If humidity is low, such as during the winter, mist orchid leaves every morning or use a small humidifier. Blackened root tips is a sign that too much fertilizer has been used.
- University of Guam Cooperative Extension: Growing Orchids on Guam
- University of Vermont Extension: Growing Orchids Indoors
- Smithsonian Gardens: Orchid Fact Sheet
- University of Oklahoma Department of Botany and MicrobiologyLaboratory 27: Magnoliophyta -- Vegetative Morphology IV
- Florida State University Nikon MicroscopyU: SMZ1500 Fluorescence Image Gallery -- Orchid Aerial Root
- AboutOrchids.com: Orchids Prefer High Humidity
- Photo Credit Peter Macdiarmid/Getty Images News/Getty Images