A Chinese shrub spreading in urban and heavily disturbed sites in western Europe and riparian habitats in New Zealand. It forms open to dense stands in sites where no competitors are present and, where suitable seed sources are available, facilitates succession.
B. davidii is a multi-stemmed and spineless shrub reaching a height of 2 to 4 m. The plant is deciduous to semi-deciduous when mature, but seedlings tend to retain their leaves. The species is short-lived with individuals up to the age of 37 years having been recorded. It is not reported as being a nitrogen fixing species. It has been found as growing well in soils with pH values between 5.5 and 8.5.
As a well-known and popular garden shrub it has been developed for the commercial market in a variety of flower colours ranging from white to deep purple. The most common flower colour for naturalised individuals is lilac. Several individuals with variegated leaves have been collected from sites in Belfast (Northern Ireland).
Hundreds of honey-scented flowers are borne in panicles (10-30 cm long) in early Summer. The small (3 mm) and hermaphrodite flowers produce nectar and are frequently visited by butterflies giving rise to itís common name of 'Butterfly bush'. Bees and other insects also frequent it. The seeds are contained in capsules and in Ireland these only release the seeds during dry spells in January-February (mid-winter). B. davidii starts flowering and fruiting after 1 year, although some panicles may be present within the first year. It has a large seed output at approximately 3 million seeds per 'average' plant. The small and light seeds (50-100 per fruit and 315,000 seeds/kg) are wind-dispersed and occasionally by cars, and are easily carried to great distances. They have deep dormancy and may remain in the soil for many years.
When cut back the shrub sprouts vigorously attaining heights of up to 2m in the season after severe pruning. It does not compete well with other trees/shrubs for light. Does not tolerate burning.
B. davidii is a drought tolerator and can survive growing in walls and on rock faces and still attain a large size within such limited environs. It favours heavily disturbed areas such as those found in quarries and railway sidings and is a frequent member of plant communities in urban wastelands. It prefers open areas but will tolerate some light shade.
It is a commercially valuable plant and is much promoted due to its robustness in the garden and reliable flowering. In Britain it is used in many wildlife-enhancing schemes such as those used in schools and local childrenís groups.
Native to the highlands of south-western China (from Tibet to Hubei). The range extends up to 2,600 m a.s.l. It forms low (1-1.5 m high) thickets.
It tolerates a wide annual range of temperature and rainfall.
Restricted to mineral soils and scree slopes.
B. davidii is cultivated throughout the temperate regions, Bergen in Norway being its northern limit. It was introduced to Britain from China in the 1890s. It is a highly successful coloniser, and first came to be recognised as such in England after the Second World War in bombed devastated areas of many cities. It is now widespread throughout the British Isles but the largest populations are found from Glasgow southwards. It is also frequent in many European cities. The forms open to closed, and often monotypic, stands in highly disturbed sites (quarries, railway sidings, derelict building sites, felled forestry plantations). It is widespread in disturbed riparian habitats in New Zealand. In SE Australia it can spread here and there in open woodlands.
Within a year or two of disturbance B. davidii colonises urban or quarry sites wherever a viable seed source exists. Usually, a first widely spaced cohort becomes established and in subsequent years most of the gaps, if suitable for seedling establishment, are colonized mainly from seeds from the newly maturing individuals.
In Europe, B. davidii is found on all soil types but in lowland areas it favours drier mineral soils. When planted it can and will tolerate almost any soil type but does not appear to be able to grow in wet areas. It tolerates a wide spectrum of climatic conditions including oceanic, continental and Mediterranean climate types.
B. davidii is spreading in many floristic regions. With the exception of some open woodlands all spread occurs in highly disturbed sites which are usually free of any vegetation.
In Northern Ireland B. davidii is a facilitator of species successions by providing a positive environment in which other species can establish. This is especially true of scree slopes and mineral spoil tips where a harsh environment exists. Similar findings have been reported from New Zealand where B. davidii, compared to native species, speeds up secondary successions. During the flowering season it is the favourite source of nectar for almost all native butterflies and in Britain it attracts more species than any native plants. The shrubs being highly attractive to insects, it encourages insectivorous birds to visit the sites to forage and in doing so they may inadvertently deliver seeds of other species in their faeces.
The impact of this species on human activities is minimal. It has a high landscape appeal in many waste areas due to its highly colourful flowers. In New Zealand B. davidii is a serious weed in conifer plantations where it competes with, and even out-competes, newly planted trees.
Control can be carried out by hand weeding young plants or by using a glyphosate herbicide. However, due to the high seed output and vigorous growth any disturbance caused by removal of individuals inevitably results in the emergence of a new cohort. If total removal from a site is wished without resulting to chemical control of newly germinated seedlings, then it is best to ensure a rapid ground cover of another species, which will prevent B. davidii from re-establishing. In New Zealand a biological control programme has been initiated to control its spread in forestry plantations.
In the British Isles B. davidii spreads into areas usually colonised by Betula spp. Although both species are wind-dispersed and become established on bare ground, some of their life-history traits are different as the Betula spp. are single-stemmed and medium-sized trees and are wind-pollinated. Other differences include the ability of B. davidii to thrive on soils with low nitrogen content and its seedlings also tolerates higher water stress. Another difference is the apparent greater dispersal ability of B. davidii.
There are some marked differences in climatic requirements between native and invaded ranges with a greater spectrum in the invaded range including oceanic conditions. In China the species is reaches a much greater altitude than in any of the invaded regions.
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James P.H. Paterson