Practical techniques: Plants – invasive
UK and Ireland
An invasive non-native species is one that has been transported outside of its natural range and that threatens environmental, agricultural or economic resources. Non-native invasive species have the tendency to upset the balance of the ecosystem as they may be bigger, faster growing or more aggressive than the native species. They can rapidly overrun the native flora of a range of different habitats, including ponds, ditches, streams, woodlands, grasslands and road verges. The native species are often unable to compete and fairly quickly the invasive species takes over.
The number of new invasive non-native species arriving in Europe is increasing. It is estimated that such species cost the UK and Ireland several billion pounds annually in direct economic impacts or through effects on ecosystem services. By way of example, a 2005 audit recorded 2,721 non-native species in England, of which 188 had a negative economic impact and 122 had a negative environmental impact. Fifty-one non-native species were recorded in marine waters around Britain in 1997. In 2007, the Environment Agency indicated that invasive non-native species were among the most significant water management problems in nine out of eleven river basins in England and Wales.
Protection and its implications
The Wildlife and Countryside Act 1981 (as amended) provides the primary controls on the release of non-native species into the wild in Great Britain. In England and Wales, it is an offence under section 14(2) of the Act to ‘plant or otherwise cause to grow in the wild’ any plant listed in Schedule 9, Part II, in which 38 new species were added in 2010. This includes notorious species such as Japanese knotweed and giant hogweed, but also less familiar, although increasingly prevalent, weeds like cotoneaster, Virginia creeper, montbretia and Rhododendron ponticum.
In Northern Ireland, the Wildlife (NI) Order (as amended) makes it on offence for anyone to plant or otherwise cause to grow in the wild any species of plant listed on Schedule 9 Part II or the Order, whereas in Scotland, the offence relates to any non-native species, rather than one listed on Schedule 9. Similarly in Ireland, the Wildlife Acts 1976 – 2000 provide the principal provisions for controlling the detrimental effects of invasive alien species on native biodiversity. Under Section 52 (7), it is an offence to’ plant or otherwise cause to grow in a wild state in any place in the State any [exotic] species or flora, or the flowers, roots seeds or spores of flora’.
Additionally in the UK, the Weeds Act 1959 provides for the control of five specified weeds: common ragwort, spear thistle, creeping or field thistle, broad-leaved dock and curled dock. This legislation is directed at clearing weeds that threaten agricultural production. Under the Act the Secretary of State may serve an enforcement notice on the occupier of land on which injurious weeds are growing, requiring the occupier to take action to prevent the spread of injurious weeds.
In 2008 the UK Government launched ‘The Invasive Non-Native Species Framework Strategy for Great Britain’. The strategy provides a framework for a more co-ordinated approach to invasive species management. In addition, government agencies in the UK can now issue species control orders to force a land owner to manage invasive plant species on their land.
Mitigation techniques
Invasive plants can often be very time consuming and expensive to eradicate, so it is vital that they are identified as early as possible. Commonly encountered invasive species on development sites include Japanese knotweed, giant hogweed, Himalayan balsam and Rhododendron ponticum. These species have different ways of propagating so the approach taken to containment and eradication must be tailored to the species, location, and what the land is to be used for subsequently.
Typical approaches include isolating stands of invasive plants before spraying repeatedly with herbicide. This can be time consuming. The Environment Agency recommends allowing 3-5 years for the eradication of Japanese knotweed. It is also important to note that the rhizomes of species like Japanese knotweed may remain dormant in the ground for at least 1-3 years so surface clearance may not show successful eradication.
Plant material from invasive species may also be buried on site, or removed from a site completely. This approach requires not only plant material but the surrounding substrate to be removed so can involve significant earthworks. Some plant waste, including giant hogweed and Japanese knotweed vegetation is classed as controlled waste and hence movement and disposal is regulated. The mass of plant material and associated soil usually makes this approach an expensive option, and a last resort. In addition, landfill fees and landfill tax are rapidly increasing and are major elements of the cost.
Timing of works
The programme of works for invasive species depending on the plant species and specialist advice should be sought. Generally herbicide treatment is most effective during the summer growth period (Mar-Sept), and excavation may be undertaken at any time of year providing plant distribution has been mapped thoroughly.
Further reading
DEFRA (2008). The Invasive Non-Native Species Framework Strategy for Great Britain.
Environment Agency (2010) Managing invasive non-native plants in or near fresh water.
Environment Agency (undated) Managing Japanese knotweed on development sites: The knotweed code of practice. Environment Agency, Bristol.
Stokes, K., O’Neill, K. & McDonald, R.A. (2004) Invasive species in Ireland. Unpublished report to Environment & Heritage Service and National Parks & Wildlife Service. Quercus, Queens University Belfast, Belfast.