With Japanese knotweed control, Dichlobenil (Casoron G4 Weed Barrier) and sodium chlorate, which are soil-acting residual herbicides, provide a moderate check in Japanese knotweed growth, but because of its extremely persistent rhizomatous root system, those products are very unlikely to effect complete Japanese knotweed eradication.
Also used in Japanese knotweed control is Glyphosate which is a weed killer applied to the foliage of the invasive plant and passed within plant to the underground parts of the weed.
During Japanese knotweed control, it is useful to cut away old stems during the previous winter to allow good access for Japanese knotweed eradication. The best time for spraying with glyphosate is at the flowering stage of the invasive plant in late summer. However, it is difficult to spray it as a means of Japanese knotweed control at this stage, when the plant is around two meters (7 feet) or more high.
With Japanese knotweed control, a more practical approach is to allow the plant to grow to about ninety centimeters (3ft). This will usually be obtained in May which is the best time to spray the chemicals on the knotweed to obtain more effective eradication. There will be a re-growth and consequently a second application for Japanese knotweed control in mid-summer is useful to get rid of as much as possible.
During Japanese knotweed control is it advisable also to check during September and if the plant has grown once more, spray Japanese knotweed again before the growth begins to die down in the autumn. To eradicate Japanese knotweed effectively, check again the following spring. When using chemicals for Japanese knotweed control, avoid spray coming into contact with other garden plants as it will most certainly kill them very quickly. Glyphosate-treated Japanese 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 Japanese knotweed plant and it is essential that this Japanese knotweed re-growth is treated quickly and properly.
Japanese knotweed control by chemicals includes Roundup Tree Stump and Rootkiller. The label has a recommendation for the control of the weed, instructing it to be applied to the cut Japanese knotweed canes.
Japanese knotweed eradication usually takes at least three to four seasons using glyphosate. Professional contractors however will have access to more powerful weedkillers to control the plant and eradicate it that may reduce this period by half.
