The botanic name has been the source of some confusion. By modern practice of botanical Latin, the spelling of a generic name made from 'Buddle' would be Buddleia, but Linnaeus in his Species Plantarum of 1753 and 1754 spelled it Buddleja, with the long i between two vowels, common in early modern orthography. The pronunciation of the long i in Buddleja as j is a common modern error. The International Code of Botanical Nomenclature has gradually changed to incorporate stricter rules about orthographic variants, and as of the 2006 edition requires (article 60, particularly 60.5) that Linnaeus' spelling should be followed in this case.
Of the approximately 100 species nearly all are shrubs <5 m (16 ft) tall, but a few qualify as trees, the largest reaching 30 m (98 ft). Both evergreen and deciduous species occur, in tropical and temperate regions resp. The leaves are lanceolate in most species, and arranged in opposite pairs on the stems (alternate in one species, B. alternifolia); they range from 1–30 cm (0.39–11.81 in) long. The flowers of the Asiatic species are mostly produced in terminal panicles 10–50 cm (3.9–19.7 in) long; the American species more commonly as cymes forming small, globose heads. Each individual flower is tubular and divided into four spreading lobes (petals) about 3–4 mm (0.12–0.16 in) across, the corolla length ranging from around 10 mm in the Asiatics to 3–30 mm in the American species, the wider variation in the latter because some South American species have evolved long red flowers to attract hummingbirds, rather than insects, as exclusive pollinators.
The colour of the flowers varies widely, from mostly pastel pinks and blues in Asia, to vibrant yellows and reds in the New World, while many cultivars have deeper tones. The flowers are generally rich in nectar and often strongly honey-scented. The fruit is a small capsule about 1 cm (0.39 in) long and 1–2 mm (0.039–0.079 in) diameter, containing numerous small seeds; in a few species (previously classified in the separate genus Nicodemia) the capsule is soft and fleshy, forming a berry.
The genus is endemic to four continents. Over 60 species are native through the New World from the southern United States south to Chile, while many other species are found in the Old World, in Africa, and parts of Asia, but all are absent as natives from Europe and Australasia. The species are divided into three groups based on their floral type: those in the New World are mostly dioecious (occasionally hermaphrodite or trioecious), while those in the Old World are exclusively hermaphrodite with perfect flowers.
As garden shrubs Buddlejas are essentially 20th-century plants, with the exception of B. globosa which was introduced to Britain from southern Chile in 1774 and disseminated from the nursery of Lee and Kennedy, Hammersmith. Several species are popular garden plants, the species are commonly known as 'butterfly bushes' owing to their attractiveness to butterflies, and have become staples of the modern butterfly garden; they are also attractive to bees and moths.
Some species commonly escape from the garden. B. davidii in particular is a great coloniser of dry open ground; in urban areas in the United Kingdom, it often self-sows on waste ground or old masonry, where it grows into a dense thicket, and is listed as an invasive species in many areas. It is frequently seen beside railway lines, on derelict factory sites and, in the aftermath of World War II, on urban bomb sites. This earned it the popular nickname of 'the bombsite plant' among the war-time generation.
Popular garden cultivars include 'Royal Red' (reddish-purple flowers), 'Black Knight' (very dark purple), 'Sungold' (golden yellow), and 'Pink Delight' (pure pink). In recent years, much breeding work has been undertaken to create small, more compact buddlejas, such as 'Blue Chip' which reach no more than 2–3 ft (0.61–0.91 m) tall, and which are also seed sterile, an important consideration in the USA where B. davidii and its cultivars are banned from many states owing to their invasiveness.
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List of Buddleja species and naturally occurring hybrids
The many species of Buddleja have been the subject of much taxonomic contention. The listing below includes the names, still prevalent in horticulture, of many former Asiatic species sunk by the late Anthonius Leeuwenberg as Buddleja crispa and adopted as such in the definitive Flora of China.