Buddleja davidii (spelling variant Buddleia davidii), also called summer lilac, butterfly-bush, or orange eye, is a species of flowering plant in the family Scrophulariaceae, native to Sichuan and Hubei provinces in central China, and also Japan. It is widely used as an ornamental plant, and many named varieties are in cultivation. B. davidii, named for the Basque missionary and explorer in China, Father Armand David, who first noticed the shrub, was found near Ichang by Dr Augustine Henry about 1887 and sent to St Petersburg. Another botanist-missionary in China, Jean-André Soulié, sent seed to the French nursery Vilmorin, and B. davidii entered commerce in the 1890s.
B. davidii is a vigorous shrub with an arching habit, growing to 5 m (16 ft) in height. The pale brown bark becomes deeply fissured with age. The branches are quadrangular in section, the younger shoots covered in a dense indumentum. The opposite lanceolate leaves are 7–13 cm long, tomentose beneath when young. The honey-scented lilac to purple inflorescences are terminal panicles, < 20 cm long. Flowers are perfect (having both male and female parts), hence are hermaphrodite rather than monoecious (separate male and female flowers on the same plant) as is often incorrectly stated. 2n = 76. 
Buddleja davidii, after Leeuwenberg
In his 1979 revision of the taxonomy of the African and Asiatic species of Buddleja, the Dutch botanist Anthonius Leeuwenberg sank the six varieties of the species as synonyms of the type, considering them to be within the natural variation of a species, and unworthy of varietal recognition.  It was Leeuwenberg's taxonomy which was adopted in the Flora of China published in 1996. However, as the distinctions of the former varieties are still widely recognized in horticulture, they are treated separately here:
Buddleja davidii cultivars are much appreciated worldwide as ornamentals, but perhaps more so for the value of their flowers as a nectar source for many species of butterfly. However, this species is not able to survive the harsh winters of northern or montane climates, being killed by temperatures below about −15 to −20 °C (5 to −4 °F). Even if not killed to the ground, the shrub is usually hard-pruned to just above the lowest pair of leaf buds, i.e. to within 15–25 cm of the ground, in spring once frosts have finished, as the younger wood is more floriferous. Deadheading is also of great value, in extending a flowering season otherwise limited to about 6 weeks, although the flowers of the second and third flushes are invariably smaller; the practice also reduces the nuisance of self-seeding.
There are approximately 180 davidii cultivars as well as numerous hybrids with B. globosa and B. fallowiana grown in gardens. Many of these cultivars are of a dwarf habit growing to no more than 1.5 metres (4.9 ft). The following davidii cultivars held the RHS Award of Garden Merit in 2012:-
Buddleja davidii is naturalized in Australia and most cities of central and southern Europe, where it can spread on wastelands and in gardens. It has been classified as an invasive species in many countries in temperate regions, including the United Kingdom and New Zealand. Within the United States, it is widely established as an escape from cultivation, and classified as a noxious weed by the states of Oregon and Washington.
- Phillips, R. and Martin Rix, Shrubs, Macmillan, 1994, p210
- Alice M. Coats, Garden Shrubs and Their Histories (1964) 1992, s.v. "Buddleia"
- Stuart, D. (2006). Buddlejas. pp 30–34. RHS Plant Collector Series, Timber Press, Oregon. ISBN 978-0-88192-688-0
- 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 B. V., Wageningen, Nederland.
- Li, P-T. & Leeuwenberg, A. J. M. (1996). Loganiaceae, in Wu, Z. & Raven, P. (eds) Flora of China, Vol. 15. Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis, USA. ISBN 978-0915279371 online at www.efloras.org
- Stuart, D. D. (2006). Buddlejas. pp. 119 – 120. RHS Plant Guide. Timber Press, Oregon. ISBN 978-0-88192-688-0
- "Buddleja davidii". Australian Plant Name Index (APNI), IBIS database. Centre for Plant Biodiversity Research, Australian Government, Canberra. Retrieved 18 March 2012.
- Van Laere K (2008) Interspecific hybridisation in woody ornamentals. PhD. Thesis, Faculty of Bioscience Engineering, Ghent University
- Christenhusz, M.J.M. (2009). Typification of ornamental plants: Buddleja davidii (Scrophulariaceae). Phytotaxa 2: 55-55.
- Franchet, M.A. (1887). Plantae Davidianae ex sinarum imperio, part 2 "Plantes du Thibet Oriental (Province de Moupine)". (Nouvelles Archives du Museum d'Histoire Naturelle Paris), ser. 2, 10: 33-198.
- Li, P. T. & Leeuwenberg, A. J. M. (1996). Loganiaceae, in Wu, Z. & Raven, P. (eds) Flora of China, Vol. 15. Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis, USA. ISBN 978-0915279371 online at www.efloras.org
- Index: Buddleja — for Buddleja davidii cultivars.
|Wikimedia Commons has media related to Buddleja davidii.|
- Floridata: Buddleja davidii
- BBC Gardening: Buddleja davidii
- Pacific Island Ecosystems at Risk (PIER): Buddleja davidii
- USDA PLANTS Profile: Buddleja davidii
- RHS Buddleja Trials Report