Advice
RHS Help & Advice
Japanese knotweed
Japanese
knotweed (Fallopia japonica syn. Polygonum
cuspidatum) was introduced from Japan in 1825 as an ornamental
plant. The plant is not unattractive but grows to 2.1m (7ft)
in one season and spreads relentlessly, overwhelming other
garden plants. Where established as a wayside weed, native
plants are also aggressively over-run.
Although it does not produce seeds, it can sprout from very small sections of rhizomes and, under the provisions made within the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild. Much of its spread is probably via topsoil or construction traffic.
Cultural control
Digging out is possible, but due to the depth that the rhizomatous root can penetrate re-growth usually occurs. This method also creates problems over disposal as Japanese knotweed is classed as 'controlled waste' under the Environmental Protection Act 1990. This requires disposal at licensed landfill sites. On no account should it be included with normal household waste.
Chemical control
Dichlobenil and sodium chlorate: Soil-acting residual herbicides, such as dichlobenil (Casoron G4 Weed Barrier) and sodium chlorate, provide a moderate check in growth, but because of the extremely persistent rhizomatous root system they are unlikely to eradicate the weed.
Glyphosate: This weedkiller is applied to the foliage and is passed within the plant to the underground parts.
It is useful to cut away old stems during the previous winter to allow good access. The best time for spraying with glyphosate is at the flowering stage in late summer. However, it is difficult to spray at this stage, when the weed is 2.1m (7ft) or more high.
A more practical approach is to allow Japanese knotweed to grow to about 90cm (3ft), which will usually be reached in May, and spray then. There will be re-growth and consequently a second application in mid-summer is useful. Check during September and if it has grown once more, spray again before growth begins to die down in the autumn. Check again the following spring. Avoid spray coming into contact with garden plants. Glyphosate-treated knotweed will often produce small-leaved, bushy re-growth between 50-90cm (20-36in) in height the following spring. This is very different in appearance to the normal plant and it is essential that this re-growth is treated.
Roundup Tree Stump & Rootkiller has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes.
It usually takes at least three to four seasons to eradicate using glyphosate. Professional contractors however will have access to more powerful weedkillers that may reduce this period by half.