Buddleja japonica is a deciduous shrub native to Honshu and Shikoku, Japan, where it grows on mountain slopes amid scrub. The shrub was named and described by Hemsley in 1889, and introduced to Western cultivation in 1896.
B. japonica grows to < 1.5 m in height in the wild, open in habit and sparsely branched. The branches are tetragonous and winged. The leaves are narrowly lanceolate, < 20 cm long by < 5 cm wide, the upper surface dark green and glabrous, the underside tawny felted. The flowers form dense, drooping terminal panicles < 20 cm long, usually pale lilac in colour, from July to October. Overall, the species is considered of little horticultural merit and is also comparatively short-lived. Ploidy: 2n = 38.
- B. japonica var. insignis (Carr.) E. H. Wilson. A plant with a denser habit and more brightly coloured flowers. In the absence of any living specimens or preserved material, the plant was considered probably a hybrid of B. japonica and B. lindleyana by Leeuwenberg 
- Phillips, R., & Rix, M. (1989). Shrubs. p. 211. Pan Books Ltd., London. ISBN 0-330-30258-2
- Stuart, D. (2006). Buddlejas. RHS Plant Collector Guide. Timber Press, Oregon, USA. ISBN 978-0-88192-688-0
- Bean, W. J. (1914). Trees and shrubs hardy in the British Isles, Vol. 1. p. 323. 7th Ed. 1950. Murray, London.
- Chen, G, Sun, W-B, & Sun, H. (2007). Ploidy variation in Buddleja L. (Buddlejaceae) in the Sino - Himalayan region and its biogeographical implications. Botanical Journal of the Linnean Society. 2007, 154, 305 – 312. The Linnean Society of London.
- Leeuwenberg, A. J. M. (1979). The Loganiaceae of Africa XVIII Buddleja L. II, Revision of the African & Asiatic species. H. Veenman & Zonen, Wageningen, Nederland.