The Control of Knotweed on Exmoor

Project Aim

The Exmoor Knotweed Control Project is a collaboration between the Environment Agency, English Nature, The National Trust and Exmoor National Park Authority, supported by both Devon and Somerset Highways Agencies.

The overall aim ofthe project which began in 2000is to control and eventually eradicate knotweed (Japanese, Himalayan and giant knotweed, and associated hybrid species) from within the National Park and its associated river catchments.

There are currently over680 known knotweed sites on Exmoor, covering a total area ofover 10,000m². The plant is found in a range of locations and habitats, including riverbanks, hedges, roadside verges, and gardens. The location of the sites can be viewed here

Knotweed

Japanese knotweed (Fallopia japonica) is a highly invasive plant that was introduced into Britain as an ornamental and fodder plant in the early 19th century. It is now widely established in Britain and commonly found across Devon and Somerset.

It is listed in Schedule 9, Part 2 of the Wildlife and Countryside Act 1981, making it an offence under Section 14(2) to plant or cause Japanese knotweed to grow in the wild.

In addition to Japanese knotweed, there are other introduced knotweeds that have become naturalised to some extent; these include Giant knotweed (Fallopia sachalinensis), Himalayan knotweed (Persicaria wallichii) and lesser knotweed (P. campanulata).

The presence of knotweed can have a severely detrimental impact through its ability to invade and out-compete native flora. It spreads via an extensive rhizome system, and small fragments of the plant can regenerate and develop into new plants. Knotweed can cause damage to buildings and other structures and therefore can have economic as well as environmental implications.

Identification (PDF 417 KB)

Site surveying and recording

The details of all known knotweed sites on Exmoor are mapped and recorded on a database. Once a site has been reported, the landowner is contacted to determine if they would like the site to be treated as part of the Knotweed Control Project. The site is surveyed to establish which species is present, and the exact location and size of the stand. Where the knotweed is growing on or near a watercourse, the Environment Agency is contacted on behalf of the landowner to obtain consent to carry out treatment with herbicides.

Within the National Park, over the last few years many sites have been reported by the general public, and the project partners have carried out surveys on targeted areas. Survey work initially concentrated on the Lyn and Heddon river catchments,but over the last few years it has been extended to coverall watercourses within the National Park.Surveying work iscarried out over the summer months when the plant is most vigorous.  

2006 surveys

These concentrated mainly on previously un-surveyed upper reaches of the Washford River, particularly where there are old farms or large houses where knotweed may have been introduced historically. In addition the remainder of the Pennycombe Water (River Barle tributary) was surveyed.

2007 surveys

A number of un-surveyed watercourses in the far eastern part of the National Park were surveyed. These included the Monksilver Stream and the upper headwaters of the River Pill. All watercourses within the National Park have now been surveyed, and the majority of knotweed sites are being treated.

Site treatment and control work

Currently the most effective method of control is repeated spraying with herbicides over a number of years, which gradually reduces the vigour of the plant.

With funding from the project partners, a qualified contractor is employed to carry out a programme of herbicide spraying. This is carried out in early autumn, when the herbicide in thought to have the most impact on the plant. New sites and larger stands may also be sprayed in early summer as well, to stunt the growth before the autumn spraying. Many sites have now greatly reduced in size and vigour but are still treated every year to prevent regrowth or spread.

If you require any further information about the Exmoor Knotweed Control Project or would like to report a new site, please contact

Bea Davis

Conservation Advisor (Farming and Wildlife)

bdavis@exmoor-nationalpark.gov.uk

Tel: 01398 322288