Japanese knotweed is one of the most invasive weeds in Britain. If left unchecked it can take over gardens and force its way through floors. So it is a matter of urgency to prevent its spread. It is illegal to grow it on your land and all landowners are obliged to be rid of it. Unfortunately, this is not easy, as it can rejuvenate from one tiny piece, and it spreads by underground rhizomes, horizontal stems that take it underground for long distances. It then kills all plants in its neighbourhood by drawing all nourishment from the soil. I have even seen it kill a small elder tree.
Some owners bring in Japanese knotweed specialists. These people use excavators to get down to the crown and dig it up. Then they put a geotextile over the site and cover it with earth. This prevents the knotweed from sprouting again. Another method being tried by scientists is to introduce a psyllid, a small insect from Japan that eats it. This has been tried in three selected areas, whose location is secret, but the results are not available yet.
But what can ordinary gardeners do? Firstly, get it early. When I spotted some on in a corner of the car park on my allotment, I attacked and kept on spraying with weed killer until it did not come back. Weedkiller is essential for this plant. Exclusively organic techniques do not work. Constant vigilance afterwards is necessary. A good idea is to use a weedkiller containing glyphosate. This is non-residual in soil and works by travelling down the plants xylem to the root, which it destroys. A good idea is to use the hollow stems to advantage. cut of the top and pour glyphosate down the stem, so that it gets to the roots quicker.
A relative of mine found some in a neighbouring garden of an empty house. He dug six feet down, which is the depth of the crown, and destroyed it, but even then occasional sprouts were coming up for a few years, which he then destroyed by weedkiller.
Keeping ground well mulched with plastic covers or organic mulches impedes all weed growth. A thick mulch of leaves or leaf mould from a relaible source may slow the growth of the weed, weaken it and strengthen your attack on it.
It is also important to take wood chip from reliable sources, as sometimes rogue/cowboy woodchippers include knotweed in what they illegally dump, and it is from this practice that it can spread
The aim is to keep on attacking the plant until the crown from which it grows is weakened to death, so ensure that ground is kept weed free
It is legally obligatory to burn any remains of Japanese knotweed. It may not be composted with ordinary garden debris. You may not remove any stem from your allotment or garden.