Japanese Knotweed

Download our Technical bulletins for details on control with Roundup ProVantage and Roundup ProBio.


  • Japanese Knotweed (Fallopia Japonica) is a fast growing, deep rooted perennial weed.
  • Japanese Knotweed was introduced to the UK in 1825 and widely planted as an exotic garden ornamental before the invasive nature of the plant became clear.
  • Japanese Knotweed is probably the most invasive plant in Britain and is scheduled under the 1981 Wildlife and Countryside Act so that it is an offence to plant or cause it to grow in the wild.
  • In addition under the Environment Protection Act (1990) Japanese Knotweed is classified as ‘Controlled waste’ and must be disposed of at a licensed landfill site in accordance with the Environment Protection Act (Duty of Care) Regulations 1991.

Growth habit

  • Fleshy, red shoots appear in the early spring from an extensive underground network of stems (rhizomes). The shoots can reach 1.5m by May and 3m by June.
  • Long racemes of cream flowers appear in late summer and dieback occurs at the first frost of the autumn, leaving the leafless stems to remain throughout the winter. The rhizomes from one plant can be 2m deep and 7m across.
  • In the UK the plant does not produce viable seed, except in the rare instances of hybridisation with other similar introduced species.
  • Many areas of the country have campaigns to eradicate the alien weed and Roundup Pro Biactive plays an important role by controlling the weed with maximum safety to operators, the public and the environment.

Chemical Control

  • Chemical control in aquatic areas needs Environment Agency approval. You will need to fill in form WQM 1 and submit it to your local EA office.
  • Always clear the previous year’s growth during the winter. This evens up the new stems and makes spraying easier.
  • Treated plants take up to 6 weeks to show symptoms. Only remove plant material when there is no further sprouting. Only re-spray if there are no visible signs of dieback after 6 weeks.
  • As with most broad-leaved perennials, optimum control will be achieved from treatment at flowering in August or September but before die-back.
  • Treatment late in the season is the most effective because the glyphosate is transported deep down into the underground rhizome structure along the natural flow of plant nutrients down for winter storage. Japanese Knotweed is sensitive to frost so late season applications should be made in advance of the first frosts.
  • For established stands it is important to plan an effective management programme over several years as repeat applications may well be necessary, either to control very large plants, with their associated underground mass of rhizomes, or to control those plants which were missed by earlier applications due to shading. Sites should be monitored for at least three years.

Application Techniques

Foliar Spray at Flowering
This is the optimum timing. Use of specialist extending hand lances is recommended where plants are 2-3m tall. Spray the underside as well as the upper surface of the leaves.
Two Foliar Sprays at 1 Metre Stem Height
Spray the plants at 1-1.5m tall, in late May and repeat on any re-growth later in the season once they reach 1.5m again. This technique can be used where stands are particularly thick, as part of an integrated control programme or where long lances are not available.
Weedwiper Technique
Applications using a hand held weed wiper have proved successful.  This method can be useful where treatment of nearby vegetation is to be avoided or for spot treatment of small re-growth. It has very high success rates, but is labour intensive.
Stem Filling Technique
In certain situations such as when the Japanese Knotweed is growing on sites with a ground flora considered of particular value or a Cornish hedge, or in a garden situation where the Knotweed is growing close to other plants, a stem filling technique may prove a suitable alternative. Although requiring a far higher labour input, the herbicide can be more accurately directed. This technique involves cutting the stem and introducing Roundup Pro Biactive in solution with water directly through the top. It has been developed by the National Trust, in conjunction with the Cornwall Japanese Knotweed Forum. Full details of the method are available on request.
JK Injection Tool
This recent introduction from the USA can be used in similar circumstances to the stem filling technique to treat small stands, particularly by water, new invasions and to tidy up escapes from eradication control programmes. The tool uses a needle to inject directly into the stem rather than cutting the stem down. More at http://www.cdae.co.uk/stem_injection_systems.