There are about 100 species of Buddleja, which occur in tropical, subtropical and warm-temperate America, Asia, and eastern and southern Africa. Buddleja species are shrubs or rarely trees in which the young stems, leaves and inflorescences are usually covered in a dense white tomentum, composed of branching, stellate and/or glandular hairs. The bark is fibrous, furrowed, greyish brown to black. Branchlets are terete to four-angled when young. The leaves are evergreen or deciduous, decussate (rarely alternate), stipulate, petiolate to sessile, simple, serrate-dentate (rarely entire or lobed). Inflorescences are terminal or axillary; roughly a thyrse, with one to four orders of branching. The flowers are actinomorphic, usually fragrant, 4-merous, hermaphrodite or functionally dioecious; the calyx tomentose outside, tubular or campanulate; the corolla tubular to campanulate, white, yellow, orange, pink or purple with an orange throat; the stamens inserted. The fruit is a dehiscent (rarely indehiscent) capsule (section Buddleja) or berry (section Nicodemia); the seeds often have wings (Norman 2000).
Buddleja was very nearly excluded from New Trees, as the vast majority of its members – in cultivation, at least – are indubitably shrubby, branching freely from the base. Although some, including the ever-popular though weedy B. davidii, can form a trunk with age, it would be stretching the definition to call them ‘trees’.
As can be seen from the cross-references below, a number of well-known varieties of Buddleja davidii are not recognised by Flora of China (Li & Leeuwenberg 2008), but they continue to be useful in a horticultural context.