Buddleia

The worst invasive characteristics occur in disturbed sites, especially if the disturbance is continued or repeated. Typical areas invaded are quarries, urban wastelands, railways, gravel workings, and building sites. Riparian areas, dam upland forests and plantations are also colonised in the Antipodes and, to a lesser degree, the USA. In the UK it has spread by wind borne seeds, following the low pressure drag created by trains, throughout the rail network past and present. Here the loose surfacings of stone and soil embankments form a happy substitute for native slopes and screes, and the thickets formed can encroach on safety zones and hamper access for maintenance etc. Much damage is also caused to built structures in the railway environment, where any minute crack or softening of mortar, which can admit a seed, is as suitable for germination and growth as any fissure in a rock face. Deeply penetrating and thickening roots and woody stems soon force masonry apart to costly effect. It is, of course, a widely established plant also on waste ground and in many other disturbed habitats. In New Zealand much of the seed is spread by water along upland river banks, leading to most of the native flora being out competed. As with experience in the UK with Japanese Knotweed and Himalayan Balsam, the denudation of the under storey of native plants renders banks susceptible to erosion in times of flood. The mass of shallow rooted seedlings can furthermore be washed into the watercourse, giving rise to blockages and further flooding. Recreational and maintenance access is also impeded. There are also serious problems of lost production and high control costs in New Zealand forestry areas. In Hawaii and the Pacific Islands it is set to become a predominant threat to the economies of these islands and to biodiversity.

Father Jean Pierre Armand David was born in 1826 near Bayonne in south-west France, and died in Paris in 1900. Showing a keen interest in the natural sciences from an early age, he entered the Congregation of the Mission in 1848, and was ordained as a Roman Catholic priest in 1862. Shortly after he was sent to Peking and his long career as a missionary started. Despite the great amount of scientific work he accomplished, he was diligent both in his missionary labours and obedience to every detail of the rules of his order. He found and identified, including many new to science, over 200 species of wild mammal, 807 species of bird, numerous reptiles, amphibians, moths and insects, and a collection of plants of supreme importance, including many tens of unknown Rhododendrons, Primulas and Gentians.

Perhaps best known to us of his discoveries was the Giant Panda and, of course Buddleja davidii, which he collected in 1869. It was not seed from his collection which first was grown in Britain, but some years later, either via another French plant-collector and missionary, Jean Andre Soulie, or perhaps from a Russian source. These proved to be lax plants of poor colour. Better forms were developed by the Vilmorin nursery from introductions made in 1893, but the forerunners of the splendid modern forms were collected by the plantsman GF Wilson in the first ten years of the 20th century.

In these islands it is rightly known as a major insect attractant, providing many species of butterflies, moths, bees and hover-flies with an invaluable source of nectar, rich in energy. More species of insect are attracted to Buddleia flowers than in the case of any native plant in its season. For those species of butterfly which hibernate as adults, such as Commas, Small Tortoiseshells and Peacocks, this late nectar source is an important factor in encouraging survival. Few genuinely native plant species flower at this late season, so indirect competition (in which the seed production of native plant species is reduced because the available pollinating insects are attracted to an alien plant species) is unlikely to occur. Following the swarms of insects come dragonflies and birds of many kinds, such as summer migrant insectivores like Blackcaps, Whitethroats and Willow Warblers, and residents including most species of Tit and Wrens are commonly present. In winter, seed-feeding birds such as Finches can feast, and many invertebrates and small birds find shelter in the seed-heads or amongst the: semi-evergreen ranches. For all these benefits, contrasts occur here as well. Buddleia is alien, and no butterfly or moth caterpillar feeds on its leaves, except, in rare cases, for the Mullein Moth. In the garden setting, a large range of food plants is present, but where Buddleia becomes dominant, as it can in urban wasteland and railway sidings for instance, the suppression of competing plant species reduces food availability for herbivorous larvae and adult insects alike. In some urban situations, especially on wasteland in the south west, Buddleia takes part in the establishment of novel plant associations. These have something in common with the urban pseudo-coppice of Japanese Knotweed on the banks of the river Don in Sheffield and other naturally generated "urban Commons" in other major cities. The Buddleia forms dense shrubberies, intermingled with seed-grown birch and various willows.

Buddleia occurs in hardiness zones where mean winter minima of as low as -29° Celsius are experienced. In UK conditions it seems slightly less hardy, to about -15°. My own experience in the early 1980's suggest that it can be killed by prolonged severe frost. It is a vigorous and fast growing shrub, making at least 2m regrowth in the season following severe pruning. In natural situations it is comparatively short lived, with 37 years having been observed as the maximum lifespan. In a garden situation it is undoubtedly capable of longer life; typical annual pruning and regeneration seeming to confer longevity in the same manner as in the virtual immortality of native hardwoods when subject to a regular coppice cycle.

It is propagated by the large numbers of small, light seeds it produces. That rare organism, the "average plant" is said to yield in its "average" lifespan about 3 million seeds. The majority of these are wind dispersed, though flowing water, vehicles and other human agency play their part. Buddleia has other features frequently common to invasive plants. It is precocious in seed production, often from as early as one year, and the seeds exhibit deep and very variable dormancy, allowing the establishment of a seed bank which can continue to engender fresh plants, following soil disturbance, for many years. It also shows characteristic adaptability to soil type when away from its native ranges. The natural environment in which Buddleia exists is one of mineral slopes and screes, though it shows far greater tolerances in other continents. In Europe it favours drier lowland sites on mineral soils, though other soil conditions seem equally suitable, including low levels of soil nitrogen and severe drought. In Australasia, by contrast, it prefers nutrient-rich watercourses and creeklines.

There seems also to be divergence, either in observation of fact or emphasis, concerning the manner of invasion and spread in suitable areas. Some observers note that given a seed source, plants appear, within one or two years of disturbance, as a widely spaced cohort. The seeding of maturing individuals from this first wave, then fill in all suitable gaps in succeeding years. Other research indicate that the initial populations are the most dense; with several million seedlings establishing per hectare. This cohort gradually self-this over 10 years to about 2500 plants per hectare.

Buddleia is a vigorous, hardy, deciduous to semi-evergreen shrub, from 1-3m (occasionally 5 metres) in height. Branchlets are subquadrangular in section, flexible. Leaves are opposite, lanceolate, finely toothed, acuminate, from 2.5-7.5cm wide and 10-30cm in length with dark green, sometimes shiny upper surface and white felted below. Stipules are leaflike, present on main branches only. Petioles are short, to 5mm. Inflorescence is a large, thick terminal panicle, from 10-30cm (occasionally 75cm) in length; composed of many-flowered cymes. Flowers are small, 10mm x 3mm; fragrant, pale violet to purple, with an orange eye (a far wider colour range is found in plants of garden origin). Fruit is a cylindrical brown capsule, 8mm long, which splits in two on maturity, releasing 50-100 seeds.