Plants that have been introduced to a place where they do not naturally occur are known as non-native species. Many of these live happily in the UK without causing a problem, but a few become what's called invasive; a foreign invader.
Invasive species upset the balance of the ecosystem as they may be bigger, faster growing or more aggressive than the native species. They may also have fewer natural predators to control numbers. The native species are often unable to compete and fairly quickly the invasive species take over and can displace native plants, reduce wildlife interests and could cause damage to roads, footpaths and structures, such as building foundations.
Japanese knotweed grows pretty much anywhere and can block footpaths, damage concrete, tarmac, flood defences and the stability of river banks.
Japanese knotweed is probably the most invasive plant in Britain and is scheduled under the 1981 Wildlife and Countryside Act, so that it is an offence to plant or cause it to grow in the wild. In addition under the Environment Protection Act (1990) Japanese knotweed is classified as ‘controlled waste’ and must be disposed of at a licensed landfill site in accordance with the Environment Protection Act (Duty of Care) Regulations 1991. If it spreads onto a neighbour's land, you could be taken to court.
More details on knotweed and its code of practice can be found on the Environment Agency website.
Giant hogweed is a species with both occupational health and environmental problems. When giant hogweed sap comes into contact with skin, it reacts with sunlight and causes chemical skin burns. Giant hogweed sap becomes more toxic as the year progresses and the plant is exposed to more sunlight.
Himalayan balsam or Indian balsam grows in dense stands that suppress the growth of native grasses and other plants.
Injurious weeds are native species, which cause problems for farming. They are harmful to livestock and must not be allowed to spread to agricultural land.
Common ragwort is a weed of wasteland and pasture. The natural habitat is sand dunes but it is prevalent on light, low fertility soils and on grassland that is overgrazed. It frequently infests horse pastures and is often seen along roadsides, railways and on rubbish tips. Toxic alkaloids are present in all parts of the plant and can cause harm to animals.
Managing Japanese knotweed is the responsibility of the owner/occupier of the site. While there is no statutory requirement to control/eradicate this invasive, nor is it necessary to report its presence (it is not listed in the Weeds Act 1959), it is prudent to take action to control its spread quickly.