Wisteria is a genus of about ten species of woody climbing vines native to the eastern United States and the East Asian states of China, Korea, and Japan. Aquarists refer to the species Hygrophila difformis, in the genus Hygrophila, as water Wisteria.
Wisteria vines climb by twining their stems either clockwise or counter-clockwise round any available support. They can climb as high as 20 m above ground and spread out 10 m laterally. The world's largest known Wisteria vine is located in Sierra Madre, California, measuring more than an acre in size and weighing 250 tons.
The leaves are alternate, 15 to 35 cm long, pinnate, with 9 to 19 leaflets. The flowers are produced in pendulous racemes 10 to 80 cm long, similar to those of the genus Laburnum, but are purple, violet, pink or white, not yellow. Flowering is in the spring (just before or as the leaves open) in some Asian species, and in mid to late summer in the American species and W. japonica. The flowers of some species are fragrant, most notably Chinese Wisteria. The seeds are produced in pods similar to those of laburnum, and, like that genus, are poisonous.
The genus was named after Dr. Caspar Wistar (1761 - 1818), a professor of anatomy at the University of Pennsylvania. As a consequence, the name is sometimes given as "Wistaria", but the spelling Wisteria is conserved under the International Code of Botanical Nomenclature.
Wisteria, especially Wisteria sinensis, is very hardy and fast-growing. It is considered an invasive species in certain areas. It can grow in fairly poor-quality soils, but prefers fertile, moist, well-drained ones. It thrives in full sun to partial shade.
Wisteria can be propagated via hardwood cutting, softwood cuttings, or seed. However, seeded specimens can take decades to bloom; for that reason, gardeners usually grow plants that have been started from rooted cuttings or grafted cultivars known to flower well. Another reason for failure to bloom can be excessive fertilizer (particularly nitrogen). Wisteria has nitrogen fixing capability (provided by Rhizobia bacteria in root nodules), and thus mature plants may benefit from added potassium and phosphate, but not nitrogen. Finally, wisteria can be reluctant to bloom because it has not reached maturity. Maturation may require only a few years, as in Kentucky Wisteria, or nearly twenty, as in Chinese Wisteria. Maturation can be forced by physically abusing the main trunk, root pruning, or drought stress.
Wisteria can grow into a mound when unsupported, but is at its best when allowed to clamber up a tree, pergola, wall, or other supporting structure. Whatever the case, the support must be very sturdy, because old wisteria can grow into immensely strong and heavy wrist-thick trunks and stems. These will certainly rend latticework, crush thin wooden posts, and can even strangle large trees. Its pendulous racemes are best viewed from below.
Wisteria flowers develop in buds near the base of the previous year's growth, so pruning back side shoots to the basal few buds in early spring can enhance the visibility of the flowers. If it is desired to control the size of the plant, the side shoots can be shortened to between 20 and 40 cm long in mid summer, and back to 10 to 20 cm in the fall. The flowers of some varieties are edible, and can even be used to make wine. Others are said to be toxic. Careful identification by an expert is strongly recommended before consuming this or any wild plant.