Japanese knotweed (Reynoutria japonica) is one of the most destructive invasive plants in North America and Europe. Not to be confused with Japanese indigo (Persicaria tinctoria), the harmless dye-garden annual in the same knotweed family. It throws up hollow, bamboo-like canes to 8–10 feet in a single season and spreads through a deep, aggressive rhizome network that forces its way through tarmac, drains, retaining walls, and building foundations. It dies back to the ground each winter and re-emerges from the rhizome in early spring. This is not a weed you pull - it is a colony you fight.
What makes it so hard to be rid of is the rhizome. It runs several feet down and many feet outward, and a fragment as small as a fingernail can start a whole new stand. That is why it spreads along rivers and on disturbed ground, and why moving contaminated soil is the main way it travels. Never compost it, never dump the cuttings, and never rotovate a stand - every chopped piece is a new plant. In the UK it is legally controlled, can hold up a mortgage, and knocks value off a property.
Eradication takes years, not a season, and usually a combination of methods. Cutting every cane to the ground every three to four weeks through the whole growing season, repeated for several years, slowly starves the rhizome but rarely finishes it alone. The most reliable chemical control is glyphosate applied in late summer to early autumn, when the plant is drawing reserves down into the roots - stem injection is used where spray drift near water is a concern, and it takes repeat seasons. For full removal, the entire rhizome must be excavated; the spoil is treated as controlled waste, either buried deep under a root barrier membrane or taken to a licensed landfill. Smothering a stand under heavy geotextile weakens it but seldom kills it.
There is now a biological front as well. Aphalara itadori, a sap-sucking psyllid from Japan, was the first insect ever released to control a weed in the EU, with UK field releases beginning in 2010, and it has since been approved for release in North America (Canada in 2020, followed by US sites). The tiny insect feeds on the sap and stunts the canes. It is not a quick fix or a stand-alone cure - the aim is to wear the plant's vigour down over time so the other methods have less to fight.
Root tincture
The root is the part used, and it is dug in late fall after frost has taken the canes down, when the plant has pulled its reserves - including the resveratrol and emodin it is known for - back into the rhizome. Scrub the root well, chop it small while it is still fresh, and fill a clean jar about two-thirds full. Cover completely with 80-proof vodka, keeping at least two inches of alcohol above the root, and seal. Shake daily and keep it out of the light for six weeks, topping up the alcohol if the root ever rises above it. Strain through cheesecloth or a coffee filter, bottle in dark glass, and store somewhere cool and dark.
Knotweed root is a traditional remedy rather than a treatment. It is best known in North America through Stephen Buhner's Lyme protocols, but it is not a substitute for antibiotics or medical care. The emodin in the root is a laxative, and the resveratrol fraction can thin the blood, so it is avoided alongside anticoagulants and in pregnancy. Talk to a clinical herbalist or doctor before taking it.