Orchids (Orchidaceae) grow naturally throughout the United States, but epiphytic orchids -- which grow on trees -- are subtropical or tropical plants. Only trees in locations with warm winters can serve as hosts for epiphytic orchids. In one way, though, epiphytic orchids can grow in trees for part of the year in cooler climates.
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Matching Orchids with Tree Hosts
Attach orchids to trees that will let some sunlight reach them. Sand live oak (Quercus geminata), which is hardy in U.S. Department of Agriculture plant hardiness zones 7 through 10, can be an orchid host. Sweet orange tree (Citrus sinensis), hardy in USDA zones 9 through 11, is also a suitable home for orchids. A tree’s bark does not need to have a rough texture, but orchids tend to attach themselves more easily to rough bark than to smooth bark.
Choose orchids that thrive in the amount of light exposure available. For example, Cattleyas (Cattleya spp.) and moth orchids (Phalaenopsis spp.), both hardy in USDA zones 10 through 12, grow best in shady areas.. Some other orchids need sunnier conditions, where they get direct sunlight for six or more hours each day. Those orchids include the cultivar Vanda "Miss Joaquim" (Vanda x "Miss Joaquim"), hardy in USDA zones 10b through 11.
Making the Connections
Tie or glue orchids to trees when the orchids begin to grow new roots. The timing varies from orchid to orchid. For instance, Cattleya skinneri should be attached to a tree in January or February because this is when it begins to root. It continues to grow new roots until it blooms in March and April. Cattleya bowringiana begins to grow new roots in early fall or September to October, right after it finishes flowering. So place it on a tree branch or trunk in late summer, and spread its roots.
Do not put sphagnum moss or osmunda fiber between an orchid and a tree. It will stay too wet, preventing the orchid’s roots from adhering to the tree and causing rot. One way to secure orchids to trees is by using cotton thread or string. The cotton will deteriorate naturally while the orchids grow roots that affix to the trees. The string can be wrapped over an orchid's roots and around a tree branch or trunk to hold the orchid securely.
Alternatively, a hot-glue gun can be used to apply a drop of glue to the bottom of the orchid's pseudobulb or rhizome. Press the orchid to the tree for a few seconds, until the glue sets.
Watering the Orchids
Orchids in trees often do fine on their own once they become established, but they should be watered regularly until that time and during dry periods even after they are well-established. Water orchids by misting them. Mist the orchids in the morning with air-temperature water. Distilled water or rainwater is best for orchids because tap water often contains excessive amounts of salt and chemicals that can damage orchids.
How often orchids need to be misted depends on their species and the weather conditions. Moth orchids and Vanda species (Vanda spp.) may need to be watered every morning when the weather is dry. All orchids need to be watered more often than normal during hot, dry conditions. Always mist them in morning so they will dry before temperatures cool in the evening. Check them a few hours after misting to ensure water isn’t pooling in the plants' crowns, which is where the roots and stems join. If water pools there, then gently brush it out by hand.
Fertilizing the Orchids
Orchids growing in trees should be given fertilizer. Their fertilizer requirements vary by species and time of year but, in general, they should be given 20-20-20 orchid fertilizer diluted to one-half strength once each week. A common half-strength dilution rate is ½ teaspoon of the fertilizer mixed with 1 gallon of water. Use distilled or rainwater. Pour the fertilizer-water mixture into a spray bottle, and spray it on orchids' roots and leaves 30 minutes after you gave the plants a regular watering.
Hanging Orchids in Cool Climates
If you live in an area that has cool or cold winters, then attach epiphytic orchids to a slab of cork bark, cypress tree branch, or redwood or wire basket, following the same procedure used to attach orchids directly on trees. Hang the cork bark, tree branch or basket from a tree branch outdoors when temperatures warm to above 50 degrees Fahrenheit. Water and fertilize the orchids as you would if they were growing on trees. Move the orchids indoors when temperatures begin to drop below 50 degrees Fahrenheit.