Introduced to Western gardens in 1830 but cultivated in its native Japan for centuries, Japanese wisteria is a rampant, hardy, deciduous woody vine that blooms spectacularly in late spring. Possibly a hybrid with Chinese wisteria, the cultivar ‘Issai’ begins bearing its relatively short clusters of lilac-blue pea-flowers at an early age. The fragrant flowers are followed by dangling velvety green seedpods. The compound foliage is light green and lush. This vigorous twiner rapidly becomes a massive plant that can engulf pergolas and other garden structures.
This large, vigorous, imposing vine prefers full sun and well-drained soil. Soil that is too rich in nitrogen may promote leafage at the expense of flowers. It requires a sturdy arbor, fence or trellis for proper support. Growth is so rampant during peak season that regular pruning is required to keep it in bounds. This is not a vine for small spaces. Beware of runners invading roofing and eaves, which can eventually cause structural damage if not promptly removed. Japanese wisteria can be trained as a small tree by staking a shoot upright, removing side shoots and restraining the top shoots by pruning for several years until the stem is self-supporting. It is listed as an exotic invasive in several areas including the eastern United States.
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Select a site with full sun to light shade and well-drained, deep, moist soil.
Plant in spring or fall, spacing plants 10 to 15 feet apart. Dig a hole only as deep as the root ball and 2 to 3 times as wide. If your soil is in very poor condition, amend the soil you’ve removed from the hole with a small amount of compost. Otherwise don’t amend it at all. Carefully remove the plant from the container and set it in the hole. Fill the hole half full with soil, then water it well to settle the soil and eliminate air pockets. Let the water drain, then fill the remainder of hole with soil and water thoroughly.
Apply a layer of compost under the plant each spring, followed by a 2-inch layer of mulch to retain moisture and control weeds. Water plants during the summer if rainfall is less than 1 inch per week. To control growth and encourage flowering, prune wisteria after flowering in early summer by cutting side shoots that arise from main stems back to 6 inches. In fall, shorten these same laterals back to 2 to 4 buds. Remove shoots arising from the base of the plant as soon as they appear.