Out of the hundred-plus species of Buddleja, Buddleja davidiihas become the most familiar species in the genus, both as a selectively-bred garden ornamental and as a feral non-native, and potentially invasive, shrub in many temperate regions. So much so, this species is synonymous with the name of he genus, overlooking all other species. The species was introduced into the UK at the end of the nineteenth century, more than a century after Buddleja globosa and decades later than Buddleja colvilei. It soon become a very popular plant and, with its highly mobile and copious seeds, inevitably escaped from gardens; it became established in the wild, conspicuous with its large plumes of bright flowers. The shrub is robust, hardy and.grows rapidly. In the garden, it repays a little attention with a stunning display of flowers in the summer months of July and August. The nectar rich flowers attract butterflies, gaining it the alternate name of Butterfly Bush.

 The species is native to many parts of south and central China; possibly its native range includes Japan, although here it may have been introduced. In its native habitat B. davidii is a plant of rocky slopes and forest edges, and a coloniser of disturbed ground.

 The species can be a small sub-shrub through to a large spreading shrub, or even occasionally a small tree with a trunk diameter of up to 30 cm. The bark is brown and deeply fissured, and peeling on mature wood. The foliage varies from large, elliptic dark green leaves to small narrowly ovate and glaucous leaves, most types having a white tomentose underside. The species is usually semi-evergreen in temperate regions, with the level of foliage retained over the winter being dependent on both the conditions and the cultivar. Generally, the large leaves are lost in autumn and replaced by smaller, more pubescent and grey leaves, which may be retained all winter.

 In its native range flower colour is varied, with flowers ranging from purple to pale lilac, as well as there being a naturally occurring white-form. The plants vary by region, with each having localised characteristics (Chen et al. 2014). The flowers are perfect or hermaphrodite (both male and female parts), but are not self-pollinating, so at least two plants are required to reproduce successfully. B. davidii is tetraploid, with 76 chromosomes, double the number found in diploid species like B. crispa and B. lindleyana.

This example of Buddleja davidii (H&M 1402) is from Sichuan in Western China.

 Material of Buddleja davidii from Tibet was first sent to Europe in 1869 by the French-Basque missionary Père Armand David, for whom the species was named in France as B. davidii FRANCHET in 1887-1888. Hemsley used material collected by Irish plant hunter Dr Augustine Henry from Hupeh province, and also at Kew from Sichuan, to publish the species a year or two later as B. variabilis HEMSLEY, reflecting its diversity of growth habit, foliage, and flower form and colour. Hemsley published this name, probably unaware the species had already been published and named in France. B. variabilis persisted as the name for the species in the Anglophone world for some time. However, the rules of nomenclature mean the first published name is usually adopted and by the end of WW2 Buddleja davidii became the internationally accepted name.

There is a page with the early references for the species here, including several botanical illustrations.

History of Cultivation

 The first specimens were grown from seed obtained from Russian traders, but these plants were not considered of horticultural merit. It was another botanist-missionary in China, Abbé Jean-André Soulié, who sent seed to the Jardin des plantes (Paris) and the French nursery of Vilmorin et fils (Nancy, France), from which superior forms were raised with the species entering commerce in the 1890s. The Jardin de plantes then sent plants to Kew Gardens in London.

 At the beginning of the 20th century, the plant hunter E.H. Wilson collected seed in Hubei and Sichuan provinces (China) and sent the collections to the Arnold Arboretum (Havard, Mass., U.S.A.), the Veitch nursery in Chelsea and Aldenham House Gardens in Hertfordshire. One of the first introductions to the trade from these collection was B. variabilis var. veitchiana, soon followed by B. variabilis var. magnifica, both released by the Veitch nursery. A whole slew of varieties were published, but today none are recognised as subspecies (Leeuwenberg 1979). It would seem at that time there was a conflation between the idea of a botanical variety (= subspecies) and a garden variety (= 'Cultivar'). So, more correctly, we might now write B. davidii 'Veitchiana', treating this plant as a cultivar. Many of today's garden varieties and hybrids are descended from Wilson's collections made during these years (Tallent-Halsell & Watt 2009).

An illustration from Revue Horticole Annèe 1898:pp131.

The Development of
New Cultivars

 The shrub, in the form of these varieties, became much prized for both its large, highly scented blooms and as a nectar source for butterflies, and became widespread as a garden ornamental. Ex situ selective breeding of new Buddleja cultivars inevitably followed once the plants raised from wild collected seeds had become established commercially. It's possible some of the plants of Lemoine et fils (Nancy, France) and from Aldenham House (Hertfordshire) originated from their own seeds, but one of the first published cultivars to be selectively bred was 'Île de France' (1930), originating from Auguste Nonin's nursery (Châtillon-sous-Bagneux, Île de France), a seedling of B. davidii var. magnifica; this cultivar is still quite common, unlike most of the varieties that preceded it.

 Mention must be made of the Buddleja × weyeriana hybrids of Mr van de Weyer (Corfe Castle, Dorset), which are F2 hybrids of B. davidii var. magnifica crossed with B. globosa; and 'Eva Dudley', a hybrid of B. davidii and B. asiatica from the Farquhar Nursery of Boston, Mass., U.S.A. (both before 1920), both named and released to commerce prior to 'Île de France'.

 In the latter half of the 1930s, named cultivars started to appear from nurseries in the U.S.A.. 'Hartwegi'. 'Charming' and 'Royal Purple' had all been released some time prior to 1937, and soon after 'Fortune', 'Orchid, Beauty', 'Dubonnet', 'Eleanor', and 'Concord' were released by Schmidt's Flowers of Ohio, promoted as a cut flower. 'Fortune' has the honour of being the first Buddleja to be granted a US Plant Patent (USPP206).

 'Royal Red' and 'Empire Blue', two cultivars that have stood the test of time and both progeny of 'Royal Purple', were bred by Karle A. Lucal, released by the Good and Reese company (Ohio), and patented in 1942 (USPP556 and 557 respectively). However, the company of Vilmorin et fils in France continued to release new plants under varietal epithets based on their characteristics well into the 1930s; alongside these, the old European cultivar 'Pink Pearl' appears in the catalogue of Vilmorin et fils in 1938-9.

An illustration of 'Île de France' from Revue Horticole (Annèe 1931) pp314.

 After WW2, new Buddleja named cultivars continued to be released on both sides of the Atlantic. Perhaps the most notable plant, released in 1959 and from the Netherlands, is 'Black Knight'; one of the darkest flowers of any cultivar and still very popular today. Both 'Black Knight' and 'Royal Red' continue to receive the RHS Award of Garden Merit recognising their excellent garden performance (RHS Trials Office 2010). The number of selectively-bred cultivars and hybrids of B. davidii continued to expand at the tail-end of the 20th century, albeit at a slow and steady rate.

 From the end of the Noughties, there has been an massive increase in breeding efforts on both sides of the Atlantic. Modern cultivars and hybrids have been selected for bigger, brighter and more intensely coloured flowers. Smaller plants, selected for shorter internodes and well-branched habits, have proved popular for smaller gardens and containers, for example the Buzz^® series from Thompson and Morgan UK. The current fascination for miniature Buddleja has led to the development of complex multi-species hybrids with a dwarf habit, often coming with the added advantage of low or zero seed-set, making them suitable for cultivation in jurisdictions where B. davidii is legally defined as an invasive species, such as in New Zealand, and the states of Oregon and Washington in the U.S.A..

 There is now well in excess of two hundred named cultivars and hybrids of B. davidii. However, how many of these new diminutive plants will prove garden worthy is open to debate. Plants released just a couple of years ago already have been withdrawn from sale due to poor performance or difficulties during production for sale. I suspect the majority will not only become rare, but will die out completely as they are short-lived and vulnerable to both the weather and disease. Only a very few are likely to have the longevity of 'Black Knight' and 'Royal Red'.

A comprehensive listing of B. davidii cultivars can be found here

Publicity from the 1940s for 'Royal Red'.

Reputation as an Invasive

 Buddleja davidii has developed a reputation for being a coloniser and even invasive. Once established as a popular garden shrub, it was able to set seed and escape from gardens into the wider landscape. Each plant is able to produce thousands of tiny winged seeds, which are carried on air currents. Therefore the species is able to spread easily, particularly along roads and railway lines. The gravel ballast of railways in particular is an ideal habitat for the seedlings, which favour free drainage. Demolition sites, disturbed ground, old quarries, neglected buildings and forest clearings will play host to this Buddleja species, and it has become a familiar sight in many cities in northern Europe, all the more noticeable for the large bright flowers in summer. Although there are costs associated with the removal of feral Buddleja from buildings or wasteground, it is not usually considered as pernicious in Europe (Tallent-Halsell & Watt 2009).

 In Oregon and Washington states in the US, and in New Zealand, B. davidii is considered an invasive species, and its spread and cultivation legally controlled. Oregon classifies it as B Designated Weed and Washington as a C Noxious Weeds. In New Zealand, B. davidii has proved problematic in both natural habitats and pine plantations.

 In my opinion, the reputation as an invasive weed is somewhat undeserved and in the UK it doesn't out-compete native plants or disrupt ecosystems in the way that, for example, Himalayan Balsam or Rhododendron can. Healthy, undisturbed habitats are not usually colonised. Buddleja does not form thickets like rhododendrons, nor do they cause toxicity in the soil they inhabit. When they do get a hold it is due to a lack of basic maintenance: the Buddlejas that Network Rail have to expensively deal with are often several years old, whereas the young plants are relatively easy to remove. Significantly, Buddleja davidii is not legally defined as an invasive plant in the UK. Schedule 9 of the Wildlife and Countryside Act 1981, as amended in 2010, does not include Buddleja davidii, although many other common garden plants are included .

Feral Buddlejas often have sensational flowers