Orchid trees (Bauhinia spp.) are tropical flowering trees highly prized for their shade-casting branches and beautiful flowers. Although not an orchid, the flower is a modified example of a pea flower that forms a more familiar, pea-like seed pod. There are hundreds of species, but fewer than 50 species are widely cultivated worldwide in frost-free gardens.
Classification and Nomenclature
Although called "orchid trees" these plant species are not orchids at all, but members of the pea family, Fabaceae. Their flowers resemble orchids, but the seed pods that ensue reveal its inclusion in the pea family.
Within Fabaceae, orchid trees are grouped into the subfamily Caesalpinioideae. Characteristics of the subfamily include flowers with bilateral symmetry and with one petal that looks like a lip, which is why they sometimes looks like orchid blossoms. Some plant taxonomists place plants in this subfamily into their own family, Caesalpiniaceae.
Orchid trees belong to the genus Bauhinia. A genus is a grouping of very similar and related species. There are between 250 and 350 species of Bauhinia, including plants that become trees, shrubs and vining climbers.
Bauhinia species are native only to the frost-free, warm tropical regions of the globe. This includes Sub-Saharan Africa, Central America, northern South America and much of southern and southeastern Asia. They are found in dry savannas or moist forest habitats.
Although tolerant of light frosts and mild bouts of subfreezing temperatures, orchid trees will be killed to the ground and resprout annually if freezes are prolonged or severe. Long, year-round growing seasons ensure plants reach their full size of tree-like proportions. In general, orchid trees are best grown in USDA winter hardiness zones 9 through 15, although species exist that are much less tolerant of winter chill, best grown only in zones 12 and higher.
Orchid trees have bi-lobed leaves, having two fused ovals that resemble the hooves of livestock. They are arranged alternately on the stems and branches. Plants may be evergreen or in many cases, deciduous, losing some or all foliage in the dry weather of winter. Leaves range in color from light green to jade green, some with a silvery or blue-green cast.
Flowers are irregular with five petals that are yellow, white, pink, orange or violet in color. Orchid tree species can have star-like blossoms with ruffled petals or one petal that is lip-like, or have thin petals with long, stringy stamens. Petal shapes vary from oval to distinctly club-, spoon- or spade-like.
Seeds are contained in pods. As they mature and dry, the pods become dark brown and brittle and snap open to project the seeds a fair distance from the mother plant.
Popular Ornamental Species
Perhaps the most famous orchid tree species is the Hong Kong orchid (Bauhinia x blakeana), with vivid, prolific flowering of fuchsia-pink in winter and spring. It does not produce seeds, a reason many gardeners favor it, unlike other species that drop seeds and often have many saplings around their trunks. Other ornamentally important species include the spiny orchid tree (Bauhinia aculeata), dwarf white orchid tree (Bauhinia acuminata), pompom orchid tree (Bauhinia divaricata), red bauhinia (Bauhinia galpinii), pink butterfly tree (Bauhinia monandra), autumn orchid tree (Bauhinia purpurea), semla gum (Bauhinia semla), St. Thomas tree (Bauhinia tomentosa) and mountain ebony (Bauhinia variegata). Mountain ebony is notorious for shedding seeds and is regarded as environmentally invasive in many warm regions, including Hawaii and Florida in the United States.