Easily grown in average, medium, well-drained soil in full sun. Becomes weedy and sparse with diminished flowering performance if not grown in full sun. Does poorly in wet conditions. In USDA Zones 5 and 6, this plant will often die to the ground in winter and therefore is often grown therein in the manner of an herbaceous perennial. Even if plants do not die to the ground in winter, they usually grow more vigorously, produce superior flowers and maintain a better shape if cut to the ground in late winter each year. Removal of spent flower spikes during the growing season will encourage additional bloom.
Buddleja davidii, commonly called butterfly bush, is a deciduous shrub that is native to thickets on mountain slopes, limestone outcrops, forest clearings and rocky stream banks in China. It typically grows to 6-12’ (less frequently to 15’) tall with a spread to 4-15’ wide when not killed back by cold winter temperatures. It is noted for its bushy habit, arching stems, showy/fragrant flowers and vigorous growth. It has escaped gardens and naturalized in the eastern U. S. plus Washington, Oregon, California and Hawaii. It has been declared a noxious weed in Oregon and Washington. Aggressive spreading has been observed in a number of eastern states including Pennsylvania, New Jersey, West Virginia, Kentucky and North Carolina.
Spike-like terminal and axillary flower clusters bloom from early to late summer, sometimes to first frost. Flowers are densely clustered in showy cone-shaped panicles from 6-18” long. In the wild, straight species flowers are lilac to purple with orange-yellow throats. Numerous named cultivars have been introduced over the years, expanding the range of flower colors to include pinks, yellows, whites and reds. Flowers (each to ½” long) are mildly fragrant, and, as the common name suggests, very attractive to butterflies. Flowers are also very attractive to hummingbirds and bees. Flowers give way to two-valved seed capsules that split open when ripe (about 50 seeds per capsule). Finely toothed, elliptic to lanceolate leaves (6-10” long) taper to long points. Leaves are sage green above and white tomentose beneath.
The genus name is frequently listed today as Buddleia. However, Linnaeus named the genus Buddleja (pronounced with a silent “j”) which is still considered to be the proper spelling (first name survives) according to the International Code of Botanical Nomenclature.
Genus name honors the Reverend Adam Buddle (1662-1715), English botanist.
Specific epithet honors Pere Armand David (1826-1900), French missionary and naturalist, who found this species growing in China in 1869/1870 along the border of China and Tibet.
'Black Knight' is a butterfly bush cultivar which features almost black flowers. It is a deciduous shrub with an arching, spreading habit which typically grows 6-8' tall if not cut back in late winter and 4-5' tall if cut back. Features spike-like 4-10" long terminal clusters of very dark purple flowers which bloom from June to September and sometimes to first frost. Flowers are fragrant, and, as the common name suggests, very attractive to butterflies. Willowy gray-green foliage. Popular fresh cut flower.
No serious insect or disease problems. Nematodes can be troublesome in the South.
Provides valuable summer and early fall flowers when few other shrubs are in bloom. Best grown in massed plantings in borders, cottage gardens, rose gardens or butterfly gardens. Usually does not make a good single specimen shrub.