The reason Japanese knotweed is so hard to remove lies underground, in its rhizome. The rhizome is a woody underground stem system, dark brown outside and orange within, that stores the plant's reserves, survives winter dormant, and sends up fresh canes each spring. It can extend several metres from the visible stand and a small fragment can grow a new plant, which is why our treatment methods target the rhizome rather than the canes you see above ground.
What is the knotweed rhizome?
The rhizome is the plant's underground stem system, not a true root, made up of woody, knobbly lengths that are dark brown on the outside and orange-yellow inside when snapped. It carries the crown at the centre of the stand and stores the carbohydrate reserves the plant draws on to regrow. Fine feeder roots grow from the rhizome to take up water and nutrients, but it is the rhizome itself, and its stored energy, that makes knotweed persistent and that any effective treatment has to reach.
How far does the rhizome spread?
The rhizome can extend several metres laterally from the visible canes and a few metres down, though most of the mass sits in the top layers of soil where it can be surveyed and treated. Historic guidance placed a precautionary zone around a stand for this reason, and the current RICS 2022 standard uses a 3 metre zone for assessment. The lateral reach is why fresh shoots sometimes appear a distance from the main clump, and why a survey maps the likely extent rather than just the canes.
Why does the rhizome make removal so hard?
Removal is hard because the rhizome regrows from its stored reserves and from any fragment left in the ground, so cutting or pulling the canes achieves nothing lasting. A piece of rhizome the size of a fingernail can produce a new stand, which is why excavated soil is treated as controlled waste and why disturbing a stand without a plan can spread it. Herbicide has to be drawn down into the rhizome over several seasons, or the affected soil physically removed, to deal with it properly.
How is the rhizome dealt with?
The rhizome is dealt with either by a herbicide programme that the plant carries down into it over 3 to 5 seasons, or by excavation that physically removes the rhizome-bearing soil, or by on-site methods such as cell burial or a root barrier. The right choice depends on the site, the timescale and whether the land is to be built on, and each is set out in our treatment methods. Monitoring afterwards confirms the rhizome is no longer sending up viable growth.
Common questions
- How deep does the rhizome go?
- Most rhizome sits within the top layers of soil, but it can reach a few metres down in favourable ground. Depth affects whether excavation is practical and how much soil must be removed, which is confirmed at survey.
- Can a rhizome fragment really regrow?
- Yes. A fragment as small as a fingernail can produce a new plant, which is why moving soil or garden waste from an affected area spreads knotweed and why disposal is legally controlled.
- Does the rhizome die back in winter?
- No. The canes die back but the rhizome stays alive and dormant below ground through winter, sending up fresh shoots in spring. A dormant rhizome is why a winter stand is not a dead stand.