Invasive plants

Issue date January 2017


Invasive plants

Many foreign plants were introduced into Britain in the 19th century, mainly for ornamental reasons, and a few have become aggressively dominant creating serious problems. Non-native invasive species are organisms that have been introduced by humans either purposely or by accident and that have become serious environmental pests. One reason for their success as pests is that they are typically introduced without the array of associated natural controls (herbivores, parasites, pathogens, predators) that occur in their native range. In addition to the great loss of biodiversity, habitat degradation and other ecological consequences, invasive species cause huge economic damages valued in millions of pounds annually and some pose a human health threat.

Invasive alien plants threaten native species and habitats by competing for critical and often limited resources like sunlight, water, nutrients, soil and space. They succeed through vigorous growth, prolific reproductive capabilities and by causing changes that favour their growth and spread. Invasive plant species displace alternative plant communities, impede forest regeneration and natural succession, change  soil chemistry, alter hydrologic conditions, cause genetic changes in native plant relatives through hybridization and some serve as agents for the transmission of harmful plant pathogens.

These invasive plants include:

Japanese Knotweed
Japanese Knotweed

Regarded as the most invasive plant in Britain. Can colonise most habitats such as river banks, woodlands, grassland and coastal areas. Able to grow through walls, tarmac and concrete. In the UK, although seeds are produced by Japanese Knotweed plants, these are rarely viable. Therefore spread is by vegetative means, either by rhizome (root) fragments, or by crown (base of the stem) and stem segments.

Giant Hogweed
Giant Hogweed

Giant Hogweed A perennial plant which can grow up to 5 metres tall with leaves up to 1 metre across. It can take 4 years for the plant to flower after which it dies. Each plant can produce more than 50,000 seeds per year which can remain viable for 15 years. Seeds are dispersed by wind, water and in soil carried by footwear and vehicles.

Himalayan Balsam
Himalayan Balsam

The tallest annual plant found in the British Isles, growing to more than 2-3 metres in height. Each plant can produce up to 800 seeds which are released explosively from the seed pods, travelling distances of up to 7 metres. The seeds, which are also transported by water, can remain viable for up to 2 years.


Ragwort is usually considered to be a biennial, over- wintering either as seeds or as rosettes. It is also capable of becoming a short-lived perennial if the flower stem is cut (eg. in the lawn), but usually it dies after producing seeds. The leaves are light to dark green and deeply lobed. The lower leaves form a rosette which is present from autumn to late June and dies back when the main stem develops before flowering starts in late summer of the second year. The upper part of the stem is branched and bears yellow flower-heads in large dense flat-topped terminal clusters, nearly always rayed, and daisy-like. A single plant can produce over 150,000 white downy seeds, which are carried away by the wind, and which can remain viable in the soil for up to 15 years.

Injurious Weeds

The weeds act 1959 consolidates earlier legislation concerned with commercial food production and applies throughout Great Britain. The Act empowers the respective Agriculture Ministers to serve notice requiring an occupier of land on which certain specified weeds are growing to take action to prevent the weeds from spreading.

The Act applies to five injurious weeds, which are considered to be a potentially serious threat to agricultural production. These are:

    • Spear thistle (Cirsium vulgare)
     • Creeping or field thistle (Cirsium arvense)
     • Curled dock (Rumex crispus)
     • Broad-leaved dock (Rumex obtusifolius)
     • Common ragwort (Senecio jacobaea)

Identification, treatment and removal

Guidance on the identification, treatment and removal of the above plants is contained in a booklet issued by the Environment Agency - Guidance for the control of invasive plants near watercourses.

The spread of such plants is primarily as a result of human activities which aid their dispersal. Their establishment on a site can result in a displacement of native plants, reduce wildlife interest and in the case of Japanese Knotweed cause damage to buildings and pavings.

Currently the Wildlife and Countryside Act 1981 makes it an offence to plant or cause Japanese Knotweed or Giant Hogweed to grow in the wild.

The prevention and control of ragwort is covered by the Ragwort Control Act 2003.


  • Giant Hogweed also represents a serious health hazard. The stems, edges and underside of the leaves bear small hairs which are coated with poisonous sap and even the slightest touch can cause painful blistering and severe irritation. This reaction can occur up to 48 hours after contact and in some cases results in recurrent dermatitis. As the hairs can penetrate light fabrics, protective clothing should be worn to safeguard against contact with the sap.
  • Chemical control usually takes a minimum of three years to totally eradicate Japanese Knotweed.

The type of invasive plants mentioned above have been discovered on several county council sites to date and it has proved costly and time consuming to deal with. It is therefore essential that any such plants on council sites are managed correctly.

OCC Corporate Landlord (OCCCL) will undertake the following to help with the management process:

  1. Establish a "register" to record details of all county council sites where invasive plants are discovered.
  2. Prior to the completion of any agreement to purchase or lease sites for use by the council (including sites acquired under 106 agreements) ensure that a survey is carried out to identify any invasive plants that may be present.

If the service provider (SP) is aware of the presence of invasive plants at any county council property they must inform OCCCL so that investigations can be implemented.

The SP must arrange with OCCCL for surveys to be carried out where it is suspected that invasive plants such as Japanese Knotweed, Giant Hogweed, Himalayan Balsam and Ragwort may be present prior to detailed project appraisal or project estimate so that the cost of dealing with any invasive plants discovered can be included within the submission.

Any information from the surveys must be forwarded to OCCCL so that the details can be included within the "Invasive Plant Register".

Any work affecting invasive plants may require:

  1. Notification to the Environment Agency.
  2. A method statement to be agreed with the Environment Agency to cover the proposed work.
  3. The designation of all waste produced by the work as "controlled waste" under the Environmental Protection Act 1990.
  4. Any chemical treatment is in accordance with the Control of Pesticides Regulations 1986 (as amended).