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Giant hogweed, Himalayan Balsam and Japanese Knotweed are alien plants. They were brought over to the UK in the 19th century as ornamental plants and have spread rapidly, eliminating some of our native species.

Himalayan balsam and giant hogweed are both spread by seed. Each giant hogweed plant can produce more than 50,000 seeds every year, and these remain viable for up to 15 years. The seeds are spread by wind, water and, unwittingly, in the soil carried on shoes and vehicles.

Japanese knotweed spreads very quickly by rhizomes (underground stems). Any disturbance can activate more growth; fly-tipping has been a particular problem because new plants can grow from even a tiny fragment of rhizome or fresh stem. In Japan it's eaten by native species, but here it has no competitors or predators to keep it under control.

It is thought that the Victorians planted giant hogweed, which originates from the Russian/Turkish border, in the Crickhowell area. Over 100 years on, the plant has become a major problem along the River Usk.

Himalayan Balsam, which was sold in garden centres until recently, is found in dense stands along riverbanks and areas of damp ground all over Wales.

Japanese knotweed can survive and eventually dominate just about anywhere, from railway lines, riverbanks, roads and footpaths to graveyards and derelict sites. In Blaenau Gwent alone it has been found at over 500 sites.