The genus Buddleja is being revised in full for Trees and Shrubs Online. This new genus article is the first element to be published, in January 2021, along with the African and American species. The Asian species will follow later this year. In the eighth edition of Bean this genus was treated under the alternative spelling of Buddleia, and all the eighth edition articles will remain available to view under that genus name until this revision is fully published.
Shrubs, less often trees, lianas or suffrutescent herbs, 0.25–30 m high. Bark fibrous, longitudinally fissured. Branchlets subterete, terete or quadrangular and sometimes narrowly winged, often with white or pale-grey indumentum. Leaves opposite in equal pairs, very rarely alternate, petiolate, sessile or sometimes connate-perfoliate; stipules usually leafy, suborbicular, auriculate or reduced to a transverse line; petiole often short; leaf blade variable, generally lanceolate, ovate or cordate; margin entire, crenate, or dentate. Inflorescences terminal and/or axillary, usually many-flowered, thyrsoid, paniculate or reduced to a globose head; bracts mostly leaflike; sepal-like bracteoles . Flowers 4-merous, very occasonally 5-merous, hermaphrodite or unisexual in some New World species. Calyx campanulate or nearly so, less often cup-shaped or obconical, tube usually longer than lobes. Corolla campanulate, cup-shaped, or funnel-shaped, white, yellow, orange, or various shades of lilac, muave or purple; tube cylindrical or widening towards the mouth, often golden-yellow to deep-orange in the throat, straight to curved, usually longer than lobes; lobes imbricate, elliptic or suborbicular. Flowers perfect or unisexual flowers with reduced and sterile reproductive organs of opposite gender. Stamens inserted on corolla tube from bottom to the mouth, usually included, alternating with corolla lobes; filaments mostly short, and long only in species with exserted stamens; anthers, 2-locular, often oblong with deeply cordate base. Ovary 2-locular, or 4-locular in bacciferous species, with several to many ovules per locule. Style short to long; stigma often large relative to flower size, clavate, capitate, or less often 2-lobed. Fruit a septicidally 2-valved capsule or berry, many-seeded. Seeds small, pale brown, often winged; endosperm fleshy; embryo straight. (Leeuwenberg 1979; Li & Leeuwenberg 1996; Norman 2000).
Introduction and History of the Genus
The genus Buddleja consists of approximately 110 species, most of which are woody shrubs of warm-temperate to sub-tropical montane habitats. Of these, roughly 30 are regularly grown in temperate gardens, with another 10 or so often encountered as greenhouse specimen shrubs, most of which will be considered here. In horticulture, the genus is primarily represented by the ubiquitous B. davidii and it hybrids, although several other species are available in the horticultural trade. The most frequently encountered of these are probably B. globosa, the first species to enter cultivation, noted for its distinctive orange ball flowers, and B. alternifolia with its willow-like habit and long flower-covered branches.
Linnæus (1753) described the genus Buddleja based on a single species, B. americana, named in honour of the Reverend Adam Buddle, the vicar of Farmbridge (Essex, UK) and an amateur botanist. Eight variant spellings have been used historically – Buddlea, Buddleia, Buddleja, Buddleya, Budlæa, Budlea, Budleia, and Budleja. Buddleja, adopted by Linnæus, is cited in the International Code of Botanical Nomenclature (Turland et al. 2018), and is therefore the botanically correct spelling, however the common name is often spelled Buddleia, as here.
Geographical Distribution, Evolution and Ecology
Buddleja are native to Africa, Asia and the Americas, with no species indigenous to Europe or Australasia. In Asia, Buddleja are particularly associated with the Sino-Himalayan region, with the greatest diversity in the highlands of south-central China (Hengduan and Gaoligong Mountains). Species in the Americas range from the southern United States south to Patagonia, with a diversity hotspot in the borderlands of Ecuador and Peru. Most species in Africa are found to the south of the continent and on the island of Madagascar. Several species have become naturalised outside their native range, usually as a result of garden escapes. B.davidii is naturalised in many temperate regions of Europe and North America and considered an invasive alien in some areas, for example New Zealand (Tallent-Halsell & Watt 2009), and the tropical species B. madagascariensis is considered an invasive in Hawaii (Rojas-Sandoval 2019).
Buddleja occur in tropical, sub-tropical or temperate montane habitats, from several hundred to several thousand metres altitude. Usually they form part of the scrub vegetation, occurring at forest edges, along riverbanks, in gullies in the mountains and on disturbed ground. Some species have adapted to xeric environments, growing in high deserts or on the slopes of dry valleys. Most are small to medium shrubs, although a few species will form small trees. B. cordata is the largest species, with occasional examples in Mexico making substantial trees of 30 m or more (Grimshaw & Bayton 2009). They are almost never the dominating species in any habitat and many species are extremely localised, recorded from only a single location. B. longiflora for example, is indigenous to only a single mountain in Brazil. A few do have extensive ranges: B. asiatica is the most widely distributed species in Asia, known across tropical southeastern Asia from India to Vietnam and south to New Guinea, while B. salviifolia is found across a large area of southern Africa, from Kenya south to Cape Town; B. americana is distributed across central America and into north-western South America (Norman 2000; Leeuwenberg 1979).
Recent research (Chau 2017) suggests Buddleja most likely originated in the early Miocene (~20 Ma) in southern Africa and that single transitions from Africa to the New World, to Asia, and to Madagascar occurred in the mid to late Miocene. In the New World, Buddleja probably arrived first in North America before expanding into South America on several separate occasions. Long-distance dispersal and movement through migration corridors in high-latitude pathways are the most likely mechanisms for the expansion of Buddleja from Africa to the New World, Asia, and Madagascar, as the breakup of Gondwana into today’s continents (180 Ma) significantly predates the expansion of the genus.
Pollination syndromes are floral traits, such a scent and flower-morphology, that are predicted to have evolved to adapt to the requirements of specific pollinators. One study of four representative species of Buddleja from different continents used the volatile compounds of the floral scents to predict the pollinators (Chen et al. 2012). The South American B.tubiflora is typical of hummingbird-pollinated flowers which reflect more red light than most insect-pollinated flowers, have a long corolla tube and are virtually scentless. The flowers of the Asian B. lindleyana and the African B. loricata emit compounds attractive to moths and butterflies; however, the two species were not convergent in floral scent composition, nor are they similar morphologically. The Central American B. cordata appears to be most attractive to bees and is anecdotally known to also attract small flies (Diptera). Pollination syndrome suggests many Asian Buddleja, especially those with long corolla tubes, mostly interact with long-tongued Lepidoptera as vectors for their pollen. However, at least one subsequent study (Gong et al. 2015) has shown that several species of Chinese Buddleja are almost exclusively pollinated by bumblebees and honeybees.
Taxonomy and Phylogeny
In 1830 Bartling and others originally placed the tribe Buddlejeae in the Scrophulariaceae, later Bentham (1876) moved the tribe to the Loganiaceae, where it would remain until the 21st century. Buddlejeae has been raised to the rank of family, the Buddlejaceae, by at least one author (see Leeuwenberg 1979). Modern phylogeny confirmed the placement of the tribe in the Scrophulariaceae (Olmstead et al. 2001) and redefined the tribe Buddlejeae as monogeneric (Chau et al. 2017).
Two monographs are considered the definitive revisions of the genus, but they are notable for employing quite different species concepts: Norman (2000) describes 65 New World (North and South American) species including two species of the genus Emorya; and Leeuwenberg (1979) describes 37 Old World (African and Asian) species, controversially reducing a number of species to synonyms or, in other words, “lumping” the Asian Buddleja into a fewer species than previous authorities. The reduced number of recognised extant Asian species, and total exclusion of subspecies, by Leeuwenberg represents a different methodology to Norman’s monograph, the latter recognising very similar plants as separate species (e.g. Norman treats B. araucana as separate to B. globosa) and further divides some species into several subspecies (e.g. subdividing B. cordata into subspecies cordata, tomentella, and ovandensis). There are subsequent surveys of the genus in China (Li & Leeuwenberg 1996) and Brazil (Coelho & Miotto 2018). Several new species have been described since the publication of the two monographs: in the Americas B. bordignonii (Coelho & Miotto 2017) and B. filibracteolata (González & Morales 2007); and in China B. subcapitata (Liu & Peng 2004), B. microstachya (Liu & Peng 2006), and B. jinsixiaensis (Zhu et al. 2013).
Marquand (1930) originally divided the Old World species of Buddleja into four series – Gynandrae, Alternifoliae, Curviflorae and Rectiflorae. According to Leeuwenberg (1979), series Gynandrae was based only on an anomalous specimen of B. paniculata (syn: B. gynandra) with stamens free from the corolla tube, the Curviflorae species were considered insufficiently distinct from the Rectoflorae to justify separation into a separate series, and Alternifoliae (represented only by a single species: B. alternifolia) differed only in its alternate leaves; he therefore rejected Marquand’s series.
Leeuwenberg (1979) divided the genus Buddleja (as it stood at the time) into four sections: Buddleja, B. americana and three additional species from the Americas; Neemda, all the Sino-Himalayan species plus many of those from the Americas and several from Africa; Chilianthus, consisting of those African species with exerted stamens once considered as a separate genus, namely B. glomerata, B. dysophylla and B. saligna; and Nicodemia, also once treated as separate genus and mostly native to Madagascar, which produce berries rather than dehiscent capsules. Norman (2000) later amalgamated sections Buddleja and Neemda, thereby incorporating all the New World species into the new expanded section Buddleja, and subdivided those in the Americas into 12 series based on inflorescence type.
The latest research employing DNA-technology and modern phylogeny (Chau et al. 2017) demonstrated Buddleja species cluster very closely according to geographical origin; this has somewhat superseded the traditional taxonomic revisions. The genera Emorya and Gomphostigma were reduced in this study to synonymy with Buddleja, making the tribe monogeneric. Several species were renamed accordingly, for example Emorya suaveolens Torrey becomes Buddleja normaniae J.H. Chau, and Gomphostigma virgatum (L.f.) Baill. returns to its original binomial classification as Buddleja virgata L.f.. The authors suggested seven sections for the new enlarged Buddleja genus: Salviifoliae; Gomphostigma; Chilianthus; Pulchellae; Nicodemia; Alternifoliae; and Buddleja. Additional data from a later study suggests a further reorganisation may be necessary (Chau, Rahfeldt & Olmstead 2018).
The genera Polypremum (Tetrachondraceae) and Peltanthera (Peltantheraceae), historically considered as members of the tribe, are now known to be unrelated to Buddleja.
Students of Buddleja may be understandably frustrated by the apparent dichotomy that exists between the two monographs of the genus, the result of Leeuwenberg being a ‘lumper’ and Norman a ‘splitter’ (albeit mostly at the subspecies level) and this is reflected in their treatments of the Old World and New World species respectively. Nevertheless, their two works (Leeuwenberg (1979) and Norman (2000)) remain the definitive accounts of the genus thus far, and any academic writing on the subject has to reflect this. Readers are advised to note well any commentary in the ‘taxonomic note’ field of the individual species articles.
Although B. davidii and its hybrids are by far the most widely garden-grown Buddleja, the first species to be introduced into gardens was B. globosa, available from the Vineyard Nursery In Hammersmith (London, UK) as early as 1774. Before this, B. globosa was already cultivated in gardens in its native Chile (Norman 2000). B. davidii was originally introduced into Europe from Tibet in 1869 by the French missionary and plant collector Père David, for whom the species is named. Subsequently, the species was collected several more times and horticulturally superior forms introduced into gardens. With its large number of light and winged seeds and a robust nature, it wasn’t long before the species escaped from gardens. Feral populations of B. davidii are now well-established across much of the UK and Ireland, and in many other countries where the species has been introduced as a garden ornamental (Tallent-Halsell & Watt 2009; Bean 1976).
Perhaps because of its tenacity B. davidii has steadily become the most popular species for gardens. Selective breeding and hybridisation has given us a huge number of cultivars and hybrids, now numbering more than 200, and that number only looks likely to increase given the current interest in developing new varieties that are dwarf, seed-sterile, or with more impressive flower panicles. Modern gardening is often on a smaller scale, in town gardens, or utilising raised beds and containers where space is limited. To meet this demand, plant breeders have focused on breeding new varieties of Buddleja, either through intensive breeding or hybridisation, which are reduced in scale. With growing concerns over the invasiveness of B. davidii, one aim has been to breed sterile or near-sterile Buddleja, which can still be grown and sold in regions where the species has been declared a noxious weed, for example the state of Oregon in the US (Tallent-Halsell & Watt 2009).
Lo and Behold® Blue Chip, a complex hybrid bred in the US (Werner & Snelling 2009), was one of the first of the new generation of cultivars to reach the market. It was hailed as the first true dwarf Buddleja, although the diminutive B. davidii ‘White Ball’ from Boskoop in the Netherlands precedes it by several years (Stuart 2006). The Buzz® series of Thompson and Morgan (UK) is commercially successful and the range has steadily expanded to include approximately eight different flower-colours. The result of line-breeding B. davidii, the cultivars are smaller, denser shrubs with larger flowers (C. Valin, pers. comm. 2020), and grow to about one half the size of more traditional cultivars when fully mature.
Following the success of these introductions, the number of cultivars has expanded rapidly as nurseries both in the US and Europe have competed to have a ‘me too’ range of small Buddleja – Monarch®, Humdinger®, Flutterby®, Pugster® (syn. Rocketstar®), Summer Lounge®, Summer Teens® and Summer Kids® to name a few. Many of these plants, particularly those bred in America, are more suited as containerised patio plants. Although they promise much their garden performance can be extremely poor outside the areas for which they have been bred, and they often fail to match up to the claims made in the publicity, including a considerably higher fertility and seed-set than advertised. Problems such as reduced insect and fungus resistance, brittle stems, failure to establish and delayed flowering are prevalent in many of these varieties (pers. obs.).
Other species of horticultural interest include the aforementioned B. globosa, a popular garden shrub notable for its orange flowers, resembling miniature golf balls. Other commonly grown species include, from China, B. alternifolia, a hardy and deciduous shrub or small tree known as the Fountain Buddleja because of the long trailing panicles of lilac-mauve flowers in the spring; and B. lindleyana, with glossy leaves and interesting curved purple flowers. Several of the B. crispa Benth. types are grown for their dramatic grey-felted foliage and early spring blooms. Some of the larger Himalyan species such as B. colvilei, B. forrestii, B. macrostachya and B. nivea are more suited to the larger garden, as they grow vigorously and require both space and shelter to perform at their best. The ‘Kewensis’ cultivar of B. colvilei is particularly prized for its large ruby-red flowers. Of the African species, B. loricata provides year-round interest with its robust evergreen foliage and sweetly scented white flowers in late spring. B. salviifolia, named for its sage-like leaves, is mostly hardy in temperate gardens given some protection and ample summer heat; the flowers, which can be white, lilac or blue, have one of the finest perfumes of any shrub (Stuart 2006).
Compared to the extensive, selective breeding and hybridisation that has taken place with B. davidii, there has been relatively little specific breeding in the rest of the genus. Named cultivars are often selected from wild-collections rather than being ex-situ bred, for example this is the case wth B. globosa ‘Cally Orange’ and B. colvilei ‘Kewensis’. Among the small number of selectively bred cultivars of species other than B. davidii are several more compact and less rangy B. lindleyana selections: ‘Miss Vicie’ (Stuart 2006), ‘Little Treasure’ and the double-flowered ‘Floral Fanfare’ (Moore 2020). Several hardy hybrids involving species other than B. davidii are worth noting, for example ‘Silver Anniversary’ (syn.’Morning Mist’), a hybrid of B. crispa and B. loricata, and ‘Longstock Pride’, a hybrid of B.lindleyana and B. crispa (Moore 2020).
Tender species and hybrids may be grown as conservatory and greenhouse specimens at temperate latitudes. B. asiatica is winter-blooming and the delicate white flowers exude a particularly fine scent. B. madagascariensis is a particularly beautiful species, with panicles similar to B. davidii but coloured an intense golden-oranage. B. indica Lam. is occasionally grown as a houseplant in the US for its attractive foliage, where it is known as Indoor Oak and Oak-leaved Buddleja (Stuart 2006). Tender hybrids suitable for a warm greenhouse or conservatory include ‘Orange Sceptre’, a hybrid of B. stachyoides and B. tubiflora (Lindstrom, Dunn & Renfro 2009), and B. × lewisiana ‘Margaret Pike’, a hybrid of B. madagascariensis and B. asiatica (Stuart 2006).
Clearly there is much more to the genus for the gardener than the ubiquitous lilac-flowered B. davidii, often seen in gardens as a volunteer plant blown in on the wind. Recent breeding activity has produced small and compact plants that are very different from the traditional large and unruly cultivars. A large number of Buddleja species are readily available in the horticultural trade, the greatest range from specialist nurseries; some these may be exotic-looking yet undemanding and hardy, whilst others might be suitable as a specimen plant for a large greenhouse.
Horticultural Care and Maintenance
Buddleia have a reputation for being unfussy and robust, although this generally applies only to the tenacious B. davidii. With the exception of B. virgata (syn. Gomphostigma virgatum), which is associated with riparian habitats, Buddleja require a well-drained soil; species from dry habitats require very sharp drainage. They grow in a range of soil types from slightly acid to very alkaline, although slightly limey conditions are generally preferred. The larger Himalayan species, for example B. colvilei, can require copious watering in dry summer weather, as they have adapted to the annual monsoons of south-central China and neighbouring territories, although they will not tolerate winter water-logging.
Hardiness in temperate gardens is very much dependent on the origin of the plant, and is both a function of the latitude and the altitude at which it grows. Several of the species that can be grown in temperate gardens are from tropical latitudes, but with a little shelter can survive relatively cool and even cold conditions in winter due to their evolving at high altitude. Some species are extremely hardy, for example B. alternifolia which is found in locations at high altitude in Xizang (Tibet) and can withstand a harsh winter and temperatures below –20°C. At the other end of the spectrum, many species from the tropics may be killed by a relatively light frost or any exposure to subzero conditions and are therefore unsuited to a temperate garden; such species may require a heated greenhouse to thrive and to eventually flower.
Most species will regenerate vigorously after pruning. The time for pruning, if required, varies widely according to species; as a general rule, long-day (summer) flowering species should be pruned in early spring and short-day (spring) flowering species immediately after the flowers have finished. Buddleja are greedy feeders and perform best if grown in rich soil and fertilised regularly. There are no specific nutrient deficiencies associated with the genus.
For the most part, Buddleja are trouble free. In glasshouses or particularly dry conditions the foliage of non-xeric species may be prone to two-spot spider mite (Tetranychus urticae). Insects associated with other members of the Scrophulariaceæ, such as Figwort Weevils (Cionus and Cleopus sps.) and Mullein Moth caterpillars (Cucullia verbasci), do not present a serious hazard. Occasionally, leaf and bud eel-worm (various nematode species) may infect a plant and cucumber mosaic virus has been reported in Buddleja. Fungal problems are encountered in excessively wet or damp conditions: water-moulds (Pythium and Phytophthora) will cause root-rot in wet soils; excessive damp will often lead to grey-mould (Botrytis) on leaves and stems (Stuart 2006, pers. obs.)