Culture
Best grown in slightly acidic, humusy, moderately fertile, medium moisture, well-drained soils in full sun. Full sun is needed for best flowering. Can be somewhat invasive (rampant growth plus rooting surface runners). This vine needs space and a sturdy support structure on which to grow. It can be slow to establish. Although this vine may produce flowers by the second or third year after planting, it may take longer. Pruning for this wisteria should be kept to a minimum (e.g., pruning back tips after bloom or in winter). Improper pruning may overly stimulate vegetative growth at the expense of flowers. Consult a pruning guide for specifics on the initial training of wisterias and the types of pruning that can or should be done. Root pruning in late fall may stimulate flowering for the following spring. An application of superphosphate in early spring can also help stimulate flowering. Choose growing sites wisely because plants dislike being transplanted.
Noteworthy Characteristics
Wisteria macrostachya, sometimes commonly called Kentucky wisteria, is a deciduous vine that is native to the southcentral U.S., ranging from Louisiana and Texas north to Kentucky, Illinois, Missouri and Oklahoma. In Missouri it is primarily found in the southeastern Bootheel region in low swampy woods and along the borders of swamps, bayous and low streams (Steyermark). It is very similar in appearance to Wisteria frutescens, American wisteria, and was once designated as W. frutescens var. macrostachya, but is noted by some as having a longer inflorescence. It is a less aggressive grower than some of the other wisterias, but is well noted for its excellent winter hardiness and ability to produce flowers in USDA Zones 3-4. It grows to 15-25’ and features 6-12” long racemes of fragrant, pea-like, blue flowers that bloom in June. Flowers bloom somewhat simultaneously on the racemes thus producing a dramatic floral display. Flowers give way to pendant, velvety, bean-like seed pods (4-5” long) which ripen in autumn and may persist into winter. Stems twine counterclockwise and are clad with compound, odd-pinnate, deep green leaves (each leaf typically with 9 ovate leaflets). Over time, the stems of this vine become twisted and trunk-like.
Genus name honors Caspar Wistar (1761-1818), professor of anatomy at the University of Pennsylvania.
‘Blue Moon’ may bloom up to three times in a growing season once established, producing beautiful, foot-long racemes of fragrant, pea-like lavender-blue flowers.
Problems
Although susceptible to a number of foliage-chewing insects and fungal diseases, none are significant. Failure of vines to produce flowers may be attributable to a number of causes including death of flower buds in winter, too much shade, plants too young (especially seed grown ones), improper pruning or overfertilization.
Uses
Plan ahead when planting this vine. It must be sited and trained only on sturdy structures which will be able to support the considerable weight of the mature vine. This is an excellent vine for large, sturdy, freestanding arbors, pergolas, posts, trellises, fences or terrace walls, and can be particularly effective when grown near or above patios where the flowers can be enjoyed in season.