Urban nature: exploiting a new homeland

Some plants that have been introduced by botanists or through horticulture are so at home in their new homeland that they have become part of the indigenous flora. Some do this without much trouble as they fill a particular environmental niche. These plants are mainly associated with human activity and examples include well-known wildflowers like Cornflower and Poppy that became naturalised with the spread of agriculture (and are now in decline as a result of changing agricultural practices). Other examples include Mediterranean plants and rare ferns that spread along railway tracks and grow on walls in our towns and cities.

Feverfew (Tanacetum parthenium - left) was widely grown as a medicinal herb and later became a cottage garden favourite. It is now considered indigenous in the Low Countries despite its natural rage being in the Caucasus. The Black Locust (Robinia pseudoacacia - middle) was introduced from North America in 1601 as an ornamental tree for parks and gardens. It proved very hardy and resilient resprouting easily from the roots and also spreading by seed. The plant has now colonised mainly dry sandy soils, especially on railway sidings and on disturbed ground on brown field sites. Another prised ornamental for the gentleman of taste was the rhododendron. Especially Rhododendron ponticum (right) from Georgia, Turkey and beyond has naturalised, outcompeting all other vegetation. This suckering shrub incidentally grew naturally in Europe before the ice ages, but wasn't able to recolonise naturally. It grows especially well on poor acid soils.

Orange Hawkweed (Pilosella aurantica) was grown extensively as an ornamental garden plant; I remember it well from my grandmothers garden. Originally from the Alps it has become established on poor soils in full sun, especially invading grasslands in the Low Countries. It is not seen as problematic as it hardly replaces native species and serves as a food plant to many insects. Another introduction that feels well at home on acid dry soils is the Red Oak (Quercus rubra) from North America.  First introduced to forestry around 1800 it is now seen as a plague by naturalists as it performs much better than the indigenous Pedunculate oak (Quercus robur). The large tree produces an abundance of small acorns that sprout new trees readily. Avenues of Red oak will even develop beautifully on poor compacted soil. A woodland with the American oak is also more diverse than one with the Common oak, so I don't really understand the hysteria... A plant that has been naturalised for such a long time that it is seen as indigenous, is the Evening primrose (shown: Oenothera biennis). They were introduced to Europe from North America in 1614 by an Italian physician. Now they grow wild as far north as Sweden and the Southern tip of Finland. The plants also prefer dry sandy or rocky soils and have found a niche of their own.