Shrub or small tree, 1–6 m tall. Branchlets 4-angled to 4-winged, stellate tomentose when young, often becoming glabrescent. Leaves opposite, sessile or subsessile; leaf blade narrowly to very narrowly elliptic, 4–45 × 1–15 cm, densely stellate tomentose below, upper surafce stellate tomentose but glabrescent, base cuneate to decurrent, margin crenate-serrate, apex acuminate. Inflorescences terminal, spicate, dense or less often interrupted, 5–20 × 2.5–4 cm. Calyx campanulate, 3–6 mm, outside stellate tomentose and often with a few glandular hairs, inside glabrous; lobes triangular, 1.2–2.5 × 1–1.8 mm. Corolla mauve, purple, pinkish, or lilac, with an orange to red throat, 0.9–1.5 cm; tube cylindrical, 8–11 × 2–3.2 mm, outside stellate tomentose and often with a few glandular hairs, inside villous but basally glabrous; lobes suborbicular, 2–4 × 2–4 mm. Stamens inserted slightly below corolla mouth, filaments very short; anthers oblong to narrowly triangular, 2–2.5 mm. Ovary ovoid, 2–4 × 1.5–2.5 mm, upper half stellate tomentose and often with glandular hairs. Style 0.5–3 mm, basally stellate tomentose and with glandular hairs like ovary; stigma clavate. Capsules ellipsoid to narrowly ovoid, 7–10 × 3–4 mm, stellate tomentose. Seeds pale brown, 3–3.5 mm, with long wings at both ends. (Leeuwenberg 1979; Li & Leeuwenberg 1996).
Distribution Bangladesh Bhutan Myanmar China Guizhou, Sichuan, Xizang, Yunnan India Aranchal Pradesh, Assam, Meghalaya, Manipur, Mizoram, Sikkim Thailand Vietnam
Habitat Scrub on mountain slopes, river banks in forests, forest edges; 900–3200 m asl.
USDA Hardiness Zone 8-9
RHS Hardiness Rating H4
Conservation status Not evaluated (NE)
Buddleja macrostachya is a highly variable species with the widest range of all the Asian Buddleias. It is found from north Vietnam and Guizhou Province in China west to Sikkim in India. Originally described in 1835 (Leeuwenberg 1979), there have been numerous collections since and several have entered botanical gardens and the wider horticultural trade. Leeuwenberg (1979) reduced a number of species to synonyms of B. macrostachya based on common flower morphology.
Some B. macrostachya plants may closely resemble B. forrestii, and the two are easily confused. They can be distinguished from one another by close examination of the flowers, the flower-parts being more stellate-tomentose in B. macrostachya than in B. forrestii; other specimens of B. macrostachya are obviously identified by the conspicuous tomentum on the branchlets and leaf petioles as well. The holotype’s branchlets are four-angled and winged in a fashion similar to B. forrestii, but in other types the stems may be subterete and unwinged. Inflorescences are similarly variable, and may be continuous (such as in the specimen labelled B. cylindrostachya) or interrupted (e.g. in a specimen labelled B. hookeri). The flower colour can range from white to pink, through to a deep burgundy, with a yellow or golden-orange eye.
Commensurate with its extensive range, there is considerable variation in hardiness, growth habit and phenology. Mature examples of the species are successfully grown outside in many parts of the UK, proving hardy down to at least –10°C, including several collections from the southern part of the species’ range (pers. obs.). Many examples of B. macrostachya are large vigorous shrubs reaching 5 or more metres in height and spread, whilst others may be somewhat more restrained, perhaps only 2–3 m in height and spread. Broadly speaking, plants of this species may be either summer-flowering (July-August) or autumn-winter flowering (November-December), the latter type being less likely to successfully flower in cooler regions. SBEC 360, collected in Yunnan, is an example of summer-flowering phenology; PAB 4198 (collected by Paul Barney in Meghalaya, India) on the other hand is an autumn-winter bloomer.
In common with most large-stature Himalayan Buddleia species, B. macrostachya requires a sunny sheltered position and free-draining soil.