Japanese Knotweed and Other Invasive Non-native Species

Posted by Sara King BSc (Hons) on 24/01/2011


Defra (2010) describes non-native species as ‘an animal or plant introduced outside its natural range (present or past) through the direct or indirect action of man’. Japanese knotweed is the species that normally comes to mind and this species along with other non-native species are listed upon Schedule 9 of the Wildlife and Countryside Act 1981 (as amended), which makes it 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. There are currently 56 non-native invasive plant species and 69 non-native animal species listed on Schedule 9 of this act.

Several non-native invasive species are difficult to control, as the UK does not support any pests or diseases to control them naturally. As a result, non-native species can threaten species native to the UK and dominate habitats. The grey squirrel, currently a schedule 9 species, is a good example of the potential dominance a non-native species can have over native species in the UK. It is estimated by Defra that non-native species cost the UK around £2 billion a year. In 2008 the Invasive Non-native Species Framework Strategy was developed in the UK to meet the challenge posed by invasive non-native species in Great Britain. This framework aims to increase the efficiency of control of these species in the UK.

Japanese Knotweed

Japanese knotweed was brought into Britain in the mid-nineteenth century as an ornamental garden plant and subsequently escaped. The roots of this species can extend to at least 7m wide and at least 3m deep underground, using a rhizome system. This plant can threaten and dominate native species as well as cause structural damage to roads and buildings. The plant can be easily spread, and can affect future development and infrastructure projects. Ecosulis have extensive experience in identifying and controlling Japanese knotweed in the long-term by developing and implementing suitable control strategies and prevent future spread.  Cutting is the most widespread method of control, and involves cutting the base of the stem with a single clean cut. Other methods of control include the use of chemicals for areas near water and grazing of shoots by livestock. Cutting and grazing can be applied at any time of year; however chemicals are more effective in August and September.

Indian (Himalayan) balsam

Indian balsam, also known as Himalayan balsam, is usually found along watercourses, and is usually spread by water. The plant spreads using it seed pods, which explode when touched, scattering seeds up to 7m away. Control methods include chemical treatment, mechanical methods, including cutting, strimming or pulling on a regular basis, and grazing throughout the growing season by livestock. Control measures should be implemented in spring before the plant flowers in June. Ecosulis are currently designing and implementing a control strategy for this species on a watercourse in Swindon, which includes mechanical methods of control and disposal through burning. Chemical treatments cannot be enforced of the watercourse, as water voles have been recorded along the watercourse.

Giant hogweed

Giant hogweed is a perennial plant which spreads rapidly by watercourses and takes up to four years to mature. It forms dense colonies that suppress native plant growth, leaving banks bare and exposed to erosion in winter. Growth begins in March and each plant can reach up to 5m high. Control methods include chemical control in summer, cutting down the stems before flowering and digging out the base rosette of the plant. Giant hogweed contains poisonous sap which can cause severe skin irritation & blistering

Further invasive non-native plant species include water primrose, water fern, parrot’s feather and floating pennywort. Invasive non-native animal species include grey squirrel, signal crayfish, black swan and wild boar.

Ecosulis’ Countryside Management team can provide a range of experience for controlling invasive plant species, such as Japanese knotweed, Indian (Himalayan) balsam and giant hogweed, including survey, develop and implement control strategies. Ecosulis will devise the most cost effective solution and manage its implementation .

The Environment Agency have recently published ‘Managing invasive non-native plants’ which gives further advice on how to manage non-native species.