Japanese knotweed is not an easy plant to control as the extensive underground rhizome system sustains the plant even when top growth is removed. Therefore treatment often needs to be repeated for long-term control to be acheived.
For detailed information on control and prevention measures, please see the Environment Agency’s Code of Practice.
Users must be aware of the risks involved when using chemicals to control any plant especially if it grows near water. Consent to use specific herbicides near UK waterways must be sought from the Environment Agency.
If there is no risk of run-off to watercourses and where no sensitive vegetation will be affected, a wider range of herbicides can be used. Those chemicals that are persistent in the soil may delay the planting of replacement species. Herbicides are usually sprayed but can also be applied directly to target plants using a weed-wiper or herbicide glove. Commonly used glyphosate-based herbicides are most effective in late summer however specialist advice for the most appropriate treatment should be sought (see Useful links). Some herbicides can also be injected into the hollow stems of the plant immediately after cutting, however this is time consuming and costly.
Repeated herbicide treatments over several years are normally recommended for complete control of Japanese knotweed. Continued monitoring of the treated areas should also be carried out to ensure that no new shoots appear.
For short term control, Japanese knotweed can be cut, taking care that fragments of splintered stem are not spread. The young shoots of Japanese knotweed can also be eaten by sheep, goats, cattle and horses and grazing may be used in suitable situations to keep the plant under control. However, this will not eradicate Japanese knotweed and the plant will continue to grow once grazing ceases.
Japanese knotweed can regenerate from very small fragments of rhizome (as little as 0.7 gram). It is an offence to cause this plant to grow in the wild in the UK under the 1981 Wildlife and Countryside Act and any waste material from cutting, mowing or excavation, should be disposed of according to the Environmental Protection Act 1990 (Duty of Care) Regulations. Equipment that is likely to result in further spread of Japanese knotweed, such as a flail mower should not be used.
The British Government's Non-Native Species Policy Review gives an estimate of the costs to control knotweed countrywide of £1.56 billion were it to be attempted.
Swansea is one of the worst affected areas in the UK. Here alone, the cost of completely treating the infestation using chemical or manual treatment (£1 and £8 per sq/m respectively) would cost around £9.5 million. With the current rate of treatment standing at 2ha per year the current infestation will take 50 years to treat without accounting for its rapid spread to new areas.
It is the building sector that feels the financial impacts of knotweed the most with removal costs from development sites being very expensive. One 30mx30m site in Wales cost developers an extra £52,785 to deal with the removal of knotweed. The worst case scenario for a 1m2 patch of knotweed on a development site has been estimated to be up to £54,000