Knotweed on Exmoor

Japanese Knotweed and Himalayan Knotweed are one of Britain's most invasive weeds. It is widespread in Devon and other parts of the South West and has a major impact on nature conservation, archaeology, access, flood defence and the economic value of the land it contaminates. Exmoor is also infested with Japanese Knotweed and areas such as River Lyn,  Tarr Steps, Watersmeet and the Heddon Valley are seriously affected. It is also to be seen along roadsides and verges.

Organisations such as the Exmoor National Park Authority, the National Trust, The Environment Agency and Natural England are working with local landowners to control the knotweed found on Exmoor. This involves spraying, in late Summer or early Autumn after the end of the growing season, with a herbicide which is approved by the Environment Agency and is harmless to humans.

Japanese Knotweed

There are other non-native invasive knotweeds including Himalayan Knotweed. The co-ordinated spraying programme began in 1996 on the River Barle and 2001 on the Exmoor sites. The weed is so resilient that checks will have to be made every few years to ensure it does not start to re-grow, Over 600 sites have been recorded on a database held by is the National Park Authority. If you see any knotweed when you are out enjoying Exmoor, check out whether the site has already been recorded by looking on the interactive map at: http://www.exmoor-nationalpark.gov.uk/index/looking_after/exmoor_knotweed_control_project.htm

Please report new sitings to 01398 322288

Japanese knotweed has recently been classified as one of the top ten threats to our native terrestrial flora. As you may know there is a big problem with this highly invasive species within the Exmoor National Park. For a number of years the Exmoor Knotweed Control Project (a partnership between the Environment Agency, Exmoor National Park Authority, Natural England and the National Trust) has been tackling the problem. Many of the sites which were first treated through the project 7 or 8 years ago are now much reduced; in fact at some of these sites it is now hard to find any evidence of these pests.

Japanese is the most abundant knotweed within the National Park, but Himalayan and giant knotweed are also invasive aliens targeted through the project. The successful eradication of these alien species depends on us knowing where all occurrences of knotweed are located. And for this we continue to need your help. If you visit any of the National Park Centres or Porlock Tourist Information Centre you will be able to view a map of the area showing known knotweed sites. If you know of a new one simply stick a pin in the map!

All your records are incredibly valuable to us, so if in doubt please, please get in touch. We look forward to hearing from you.

Green dock beetles had been spotted munching their way through Japanese knotweed on a farm in Cornwall. The normal hosts for the green dock beetle are docks and sorrels, particularly broadleaved dock. They do attack other plants but these are restricted to a very few plant families and in some cases, when the green dock beetles feed on other plants and lay their eggs, the larvae do not fully develop. So no next generation of green dock beetles. Japanese knotweed and docks are in the same plant family and green dock beetles do feed on Japanese knotweed, but only when beetle numbers are high and all docks and sorrels have been eaten. It’s not clear if green dock beetles lay their eggs on Japanese knotweed or, if they do, whether the larvae fully develop.

Given that the green dock beetle prefers broadleaved dock as a host and will attack other plants, they could not be considered a biological control agent for Japanese knotweed. However, if they are attacking the plants, they’ll certainly be doing more harm than good.
In 2010, following a rigorous six year laboratory-based research programme, Defra licensed the release (to carefully selected sites in England and Wales) of Japanese Knotweed psyllid, a tiny, sap-sucking jumping plant louse, which attacks Japanese knotweed in its native area (Japan and other areas in South East Asia). The important point is that it is host specific and does not attack any other plants. The extensive research was required to identify potential control agents and make sure they did not pose any threat to our native plants and wildlife. The psyllid was identified as fulfilling all the criteria. There was a further release at additional sites in 2011 and the psyllid is expected to spread to other areas naturally. It will not eradicate Japanese knotweed but could render it less aggressively invasive, therefore reducing the problem.

Contributed by: James Robertson