Urban nature: invading a new homeland

Plants can spread beyond their natural range, or even native country. This can be a gradual and natural process, but in most cases man plays an important part. Some plants favour the specific habitats created by human activity or in our cities. Others are generalists that will happily grow anywhere or can explore a specific niche. There are also plants that are introduced by humans and makes themselves quite at home, escaping cultivation. There is another category: the truly invasive species that have become established in their new homeland and are taking over forcing out the natural vegetation.

The Butterfly bush (Buddleia) is truly beautiful, and most gardeners wouldn't be without. They are a pioneer species from China that can tolerate drought and quick changes in temperature. Hence is also grows as a pioneer species in our climes, especially on wastelands, derelict sites and along railways. The bushes can overtake such a site completely. The flowers are typically the wild lilac pink colour. I even spotted plants growing in the Antwerp Harbour on "Het Eilandje".

Another plant favoured for its large flowers is the Himalayan Balsam Weed (Impatiens glandulifera) also known as Copper's Helmets. This tall annual originated in the Himalaya and comes in a range of colours. It was first grown as an ornamental, but quickly escaped cultivation and can now be found throughout the northern hemisphere. In the Benelux is mainly grows in moist and wet often shady places. In the shade it isn't very problematic, but in full sun it tends to outcompete native perennial species.

The plant that can put fear into even the calmest of estate agents and project developers is Japanese Knotweed (Fallopia japonica). This plant originates in East Asia and was introduced by the Victorians in to parks and gardens as a tall decorative. It was first collected by Von Siebold in Japan and brought to Leiden. Although resembling a bush it is a perennial that makes arching stems from a rhizome. It has such a bad reputation as it is difficult to get rid of and even a small section of root left behind can regrow a large stand. It is capable of pushing up through concrete and asphalt (shown left). The plant smothers everything so nothing else will grow. Roots go down as far as 3 metres, so spraying with a herbicide often only has temporary results. It also produces large amounts of creamy white flowers in late summer and early autumn. It can reproduces from seed, but is mostly spread by ignorant people moving soil or green waste containing rhizomes. This plant is one of the most invasive species in temperate climates!