Japanese Knotweed (Fallopia japonica) is a tall herbaceous perennial with bamboo like stems. It often grows into dense thickets. The leaves and stems are very distinctive and characteristic, also the persistence of last year’s dead canes and distinctive rhizome (underground root-like stems) enables year round identification.

Introduced in the early 19th century as an ornamental plant, it is now common and widespread across the UK. Japanese Knotweed spreads rapidly in the wild by natural means and as a result of spread by humans. Its spread is solely by vegetative means, either fragments of the rhizome or stem. Japanese Knotweed does not produce seed in the UK.

Negative impacts include outcompeting native flora, contributing to river bank erosion and increasing the likelihood of flooding. Can also cause significant delays and cost to development as well as structural damage (it can grow through asphalt and some other surfaces).

Avoid pulling stems. New plants can grow from stems, nodes, or fragments of either. If a cut stem is dried until it is crisp and brown it can be burnt or disposed of as an inert waste. If stems have been pulled up, they will have pieces of knotweed crown still attached at their base. This is highly regenerative and will re-grow, even after the stem has dried.

Non-chemical control

Cutting

Knotweed should be cut with a single clean cut near the base of the stem. Use a simple scythe method of cutting to prevent stem fragmentation. Flail mowing, or similar methods, should not be undertaken.  Cutting will have to be performed every 2-4 weeks during the growing season if it is the sole method of management. Alternatively, treat re-growth with herbicide. Burn cut stems on site or remove to landfill (licence required).

Digging

This is rarely an option that is appropriate to riparian situations. If digging is undertaken, refer to the code of practice and ensure that no knotweed material is allowed to enter the watercourse.

Biological

Grazing by horses, donkeys, sheep and goats may keep the plant in check, provided previous dead growth is removed.

The psyllid bug Aphalara itadori  has been released in 2010 under experimental conditions and it is hoped that in time the bug will reduce the vigour of Japanese knotweed in the UK.

Chemical control

Spraying

Spraying both top and underside of leaves improves control. Chemical treatment is most effective when it is applied in Aug-Sept. A stem injection method can be used to avoid damage to sensitive areas.

When near water, Glyphosate is more effective when applied to mature canes in Aug-Sept. If access or the risk of drift is a problem, either cut or spray the stems earlier in the season to restrict regrowth. A 1 in 10 dilution can be used for stem injection.

In general      

Herbicides can be applied using tractor-mounted, knapsack long-lance or Controlled Droplet Applicator. Control is easier if dead winter stems are removed before grown commences. Be careful to avoid spreading knotweed crowns when clearing dead canes. Application in sensitive areas is best achieved by stem injection or weed wiper.

Control should be undertaken on a catchment basis, working from the upstream end to prevent seed spread.

For more information on how to identify and control Japanese Knotweed please visit the GB Non-Native Species Scretariat webpage here.