Buddleja davidii

Buddleja 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 (3–5 inches) long, tomentose beneath when young. The honey-scented lilac to purple inflorescences are terminal panicles, < 20 cm (8 inches) long.[5] 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. Ploidy 2n = 76 (tetraploid).[6]

Buddleja davidii cultivars are much appreciated worldwide as ornamentals and for the value of their flowers as a nectar source for many species of butterfly. However, the plant does not provide food for butterfly larvae, and buddlejas might out-compete the host plants that caterpillars require.[9][10]

The species and its cultivars are 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).

Younger wood is more floriferous, so even if frosts do not kill the previous year's growth, the shrub is usually hard-pruned in spring once frosts have finished, to encourage new growth. The removal of spent flower panicles may be undertaken to reduce the nuisance of self-seeding and encourage further flower production; this extends the flowering season which is otherwise limited to about six weeks, although the flowers of the second and third flushes are invariably smaller.

Hardiness: USDA zones 5–9.[11]

There are approximately 180 davidii cultivars, as well as numerous hybrids with B. globosa and B. fallowiana grown in gardens. Many cultivars are of a dwarf habit, growing to no more than 1.5 m (5 feet).

A plant-evaluation manager at the Chicago Botanic Garden in Glencoe, Illinois (USDA Hardiness zone 5b) rated nearly 50 Buddlejia varieties and cultivars during a six-year trial period, summarizing in 2015 the characteristics of each and the study's findings.[12] University studies have suggested that nectaring butterflies have greater preferences for some Buddleja cultivators than for others, with Lo & Behold 'Blue Chip' and 'Pink Delight' heading a list of eleven.[13]

Other notable cultivars and hybrids include 'Golden Glow' and 'Silver Frost'.[14][15]

Budsleja davidii self-sown along a railroad right-of-way at Düsseldorf, Germany (2016)

Buddleja davidii has been designated as an invasive species or a "noxious weed" in a number of countries in temperate regions, including the United Kingdom, the Republic of Ireland and New Zealand.[16] It is naturalized in Australia[17] and in many cities of central and southern Europe, where it can spread on open lands and in gardens.

B. davidii was first documented as an invasive species in the United Kingdom during 1922. It is now often seen there along railway lines and on the sites of derelict factories and other buildings.[18] The plant frequently grew on urban bomb sites during the aftermath of World War II, earning it the nickname of "the bomb site plant".[19]

B. davidii is widely marketed throughout the United States, where it has reportedly become invasive in some, but not all, areas within which it has been planted.[20][21][22] Although its flowers feed many native butterflies and other pollinators, plantings of the species are now controversial.[21][23][24] To prevent seeding and to promote further flowering, its blossoms need to be removed ("deadheaded") as soon as they are spent.[20]

A number of Buddleja cultivars have become available that have a variety of sizes and blossom colors and that are either sterile or produce less than 2% viable seed.[20][23][25][26] The northwestern U.S. state of Oregon, which designated B. davidii as a "noxious weed" and initially prohibited entry, transport, purchase, sale or propagation of all of its varieties, amended its quarantine in 2009 to permit those cultivars when approved or when proven to be interspecific hybrids.[20][23][25][27] The adjacent state of Washington has taken actions that are similar to those of Oregon to bring parity to nursery sales between the two states.[28] Monarch Watch recommends planting only male-sterile "Flutterby" cultivars.[29]

• Index: Buddleja — for Buddleja davidii cultivars.

• 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. doi:10.11646/phytotaxa.2.1.11.
• 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

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