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Updated by Jeff Nevil on Apr 12, 2018
Headline for 5 Invasive Plant Species Causing Damage in the UK
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Jeff Nevil Jeff Nevil
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5 Invasive Plant Species Causing Damage in the UK

There are many non-native species in the UK, but not all of them are invasive. In invasive ecology terms, “any species transported intentionally or accidentally by a human-mediated vector into habitats outside its native range” is defined as an introduced species. The most common of these is Japanese Knotweed, but what about the lesser known invasive species?

Giant Hogweed

Giant Hogweed is a generally biennial plant, living for a few years, that is commonly found along river corridors, wasteland, woodlands and heathlands. From the Caucasus and Central Asia, and a close relative of cow parsley, Giant Hogweed can be tall, reaching up to 4m in height. It can also be quite dangerous. The sap can cause severe skin irritation and painful blisters.

Currently there is no statutory obligation for landowners to eliminate giant hogweed, but local authorities often take action to remove it in public areas, due to the potential hazard to the public.
Giant Hogweed can be treated non-chemically, by chopping off the flower heads before they set seed, or chemically, with herbicides. However, caution is strongly advised when treating giant hogweed. Appropriate protective clothing and equipment is recommended.

Himalayan Balsam

Himalayan Balsam can invade gardens but is more common on wasteland and particularly along stream and riverbanks. It grows fast in dense clusters, impoverishing habitats and preventing native plants from developing. As an annual plant, it dies each winter, which can leave large areas of bare riverbank, making them susceptible to erosion.

Between June and October, it produces clusters of purplish-pink, policeman’s-helmet-shaped flowers. Each plant can produce almost 1,000 seeds, which are propelled up to 7m away from the exploding pods. Next to watercourses the seeds can be readily dispersed long distances downstream. They can then persist in the soil for around two years before germinating.
Considering the often sensitive and diverse ecology of riparian habitat, effective control of Himalayan balsam often requires treatment of the entire river catchment, which is no small undertaking.

In order to prevent plants from reseeding the following season, it is important that management, takes place before plants set seed in late summer. As the plants are annual and shallow rooted, it can often be effective to simply pull them up when the stems are tall enough in early summer. Alternatively, herbicides can be applied, which is less labour-intensive than pulling.

Rhododendron

This species is native to the Mediterranean, particularly in Spain, Portugal and Turkey. It became popular in in the UK in the Victorian era as an ornamental plant but now invades large areas of the British countryside.

Rhododendron forms dense branches that can extend horizontally for many meters, re-rooting where they touch the ground. This allows plants to cover wide areas, casting dense shade under which other plants cannot grow. Additionally, Rhododendron produces a phenol-based toxin, which is concentrated in young tissues and is exuded by buds. This makes the young shoots inedible for herbivores and small invertebrates, which might otherwise keep the plants’ growth in check.

The most efficient methods for controlling Rhododendron are cutting down the plants; herbicide treatment of the leaves is also possible but their waxy cuticles can mean that wetting agents or other additives are needed.

Floating Pennywort

Floating Pennywort was imported in the UK from North America as a tropical plant for aquaria and garden ponds. It has lobed emergent leaves that form large floating mats that can completely cover a pond’s surface. It mostly propagates vegetatively, but can sometimes spread by seed.

Floating pennywort roots in the shallow margins of slow flowing waterbodies and can quickly dominate the habitat, impeding the water flow, preventing other plants from growing and substantially upsetting the ecological balance and the amenity uses of it. Efficiently controlling Floating pennywort can be tricky. The best option is a repeated combination of mechanical removal and intensive hand picking of the fragments left on site.

Buddleia

Originally from China, the Buddleia is a large and fast growing deciduous, woody shrub, with long arching shoots, lance-shaped grey-green leaves and tubular, fragrant flowers. It is very common along railways, building sites, waste land and dilapidated buildings.

It can produce 3 million seeds per annum, which are small and easily distributed by vehicles, wind, animals, water and movement of contaminated soil. Its strong root system can also cause damage to hard surfaces and buildings. Eradication can take many years, particularly as the soil seedbank around the plants can be abundant and long lasting.