Few vines add such graceful beauty and scent to landscapes as wisteria. Two types of wisteria are commonly grown in the United States, Japanese wisteria (Wisteria floribunda) and Chinese wisteria (Wisteria sinensis). Their flowers offer differing impacts. Japanese wisteria provides longer sprays of pink or lavender blossoms that open gradually from the spray's base. Chinese wisteria presents its violet-blue flowers in one big show. In either case, gardeners will need patience for blossoms to appear.
Wisteria is usually slow to establish during its first one or two years of growth. Once established, it often grows rapidly. According to Purdue Extension, wisteria's attractively gnarled vines can grow up to 10 feet per year under optimal growing conditions. Wisteria produces a lot of foliage first and needs to fully mature before producing flowers, which can take 15 years or more.
According to Washington State University Extension, it is best not to plant wisteria from seeds. Plants growing from seeds require the most time to reach maturity and blossoming, and they may never bloom at all, due to genetic variability of seeds. Plant wisteria from own-root cuttings or grafted plants, instead. Purchasing named cultivars also helps to ensure abundant flowering at maturity, according to Ohio State University. Even root-cuttings or grafted cultivars will likely need five to 10 years before they bloom.
To promote blooming, Ohio State University offers some recommendations: Plant wisteria in a location receiving at least six to eight hours of full sun daily. Provide a trellis or other support that will last at least 20 years. If the structure collapses after a few years, the plant may require severe cutting back for placement on a new structure and blooming may not occur for at least two years. The soil should be well-draining but of average fertility to promote reproductive maturity at the earliest. Fertilise once a year -- it is best to do this in early spring, with a fertiliser relatively low in nitrogen and relatively high in phosphate. If plants seem reluctant to bloom, provide a heavy application of superphosphate (0-20-0) in early spring, at 1.36 to 2.27kg. per 100 square feet. When the plant is established, at approximately 5 years of age, Purdue Extension advises pruning it back hard on an annual basis, in late spring or early summer, leaving any short lateral shoots, which may produce blooms. Wisteria can be pruned back to back to three or four buds to keep the plant manageable and reinvigorated. Remember that wisteria can grow so fast that in some areas it is considered an invasive plant.
Although it may sound dangerous, the University of Illinois Extension recommends root pruning. Root pruning can limit wisteria's ability to absorb nitrogen, which encourages flowering rather than foliage growth. It also stresses the plant, which can bring about flowering. The extension advises root pruning in late fall by inserting a sharp spade about 18 inches deep at least 3 to 4 feet away from the main trunk, all the way around, slicing off the roots that extend outward.