Japanese knotweed
What is a knotweed rhizome, and why can a survey not measure it?
The rhizome is the underground stem the plant grows from. The surveyor sees canes, leaves or dead stems above ground. They do not excavate the rhizome, and they cannot tell you how far it runs under a patio or a foundation. A management category is based on what is seen and how it affects the property, not on a dug-up map.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
The 2022 journal moved the off-site reporting distance from the old seven-metre habit to three metres for category D, because seven metres had been an overcautious measure of distance. That is still a reporting rule for plants that are seen, not a licence to say the rhizome stops at three metres. A specialist who needs the extent may dig or test. That is their instruction, not the home survey.
GOV.UK’s concern is spread: moving soil moves the plant. The survey’s concern is to identify and report. When you brief a specialist, ask them to distinguish visible growth from ground they have not opened. Do not let a quote assume the rhizome is under the house because a cane is in the border.
What should you ask your surveyor?
- What growth did you see above ground, and where?
- Are you making any claim about how far the rhizome extends?
- What would a specialist need to open to map it, if mapping is even necessary?
What are the typical next steps?
- Do not dig exploratory holes along the foundation before you own the house.
- Brief the specialist with the surveyor’s location, not with a guessed radius.
- If category D is about a plant next door, remember the rhizome you cannot see is on land you cannot work on without permission.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Does a patio prove the rhizome has been stopped?
No. A patio hides the ground. It is not a barrier the survey has tested.
Is three metres a safe distance to build an extension?
The journal uses three metres as a reporting distance for off-site plants. It is not a foundation design rule. Ask the specialist and the building-control body before you design.