Japanese Knotweed - Control

Control

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. Glyphosate is the best active ingredient in herbicide for use on Japanese knotweed as it is ’systemic’; it penetrates through the whole plant and travels to the roots.

Digging up the rhizomes is a common solution where the land is to be developed, as this is quicker than the use of herbicides, but disposal of the plant material is difficult, governed by law in the UK, where it is classed as controlled waste.

More ecologically friendly means are being tested as an alternative to chemical treatments. Soil steam sterilization involves injecting steam into contaminated soil in order to kill subterranean plant parts. Research has also been carried out on Mycosphaerella leafspot fungus, which devastates knotweed in its native Japan. Research with Mycosphaerella has been relatively slow, due to its complex life cycle.

In the UK, it is an offence under section 14(2) of the Wildlife and Countryside act 1981 to "plant or otherwise cause to grow in the wild" any plant listed in Schedule nine, Part II to the Act, which includes Japanese knotweed. Over £150m is spent annually on Japanese knotweed control, and a decision was taken on 9 March 2010 in the UK to release into the wild a Japanese psyllid insect, Aphalara itadori. Its diet is highly specific to Japanese knotweed and shows good potential for its control.