Excavating and Containing Japanese Knotweed: the Cell, the Cap and the Gate

The dig line is dug for, not scaled off

Where the canes stand is not where the plant is, and the gap between those two statements has moved a long way in a decade. The Environment Agency's knotweed code of practice, version 3 of 2013, put rhizome extension at up to seven metres laterally and three metres deep, and that pair of figures is still the basis of a great many method statements written today — even though the document was withdrawn in 2016. Work published in PeerJ in 2018 by Fennell, Wade and Bacon found typical rhizome extension very much shorter than the seven-metre planning figure, and the RICS professional standard of January 2022, Japanese knotweed and residential property, dropped the seven-metre rule in favour of a categorised management assessment made on evidence. The practical consequence is that the extent is now argued rather than assumed, and what you argue from is a grid of holes.

The survey is a machine job, not a walk. Inspection pits are dug outward and downward from the last visible cane on a grid, logged with position and depth, and continued until pits come back clean on consecutive lines. The sequence matters as much as the spacing: work from clean ground inward rather than from the stand outward, so the bucket is never carrying rhizome into virgin soil, and backfill each pit with its own arisings rather than tidying them into a heap. A survey done the wrong way round is itself a spreading event, and the pits are usually the first time anyone puts a machine in that corner.

For the corner above, say the pits come back clean 2.4 metres beyond the last cane on the site side and find rhizome down to 1.5 metres in a band that follows an old field drain. The agreed excavation plan comes out at 11 metres by 5.5, taken to 2.2 metres — deeper than the deepest find, because the last 700 millimetres is what you inspect on the way out. That is 133 cubic metres in the ground, and it is worth noticing that the footing on plot three only ever needed 1.1 metres. The plant set the depth, not the structure.

Now put the bank figure through the swell. Loose, that material is around 166 cubic metres, and on this job that number is not about lorries — it is about whether there is anywhere on the plot capable of receiving it. A cell void, a lined and bunded treatment stockpile, or a vehicle body: those are the three destinations, and all three are sized in loose volume. The recompacted figure the calculator returns alongside it is worth reading too, because material placed back into a cell in thin lifts finishes denser than it left the ground, so the void the cell needs is not simply the bank volume with a percentage on it.

The cell is a lined box and the lid is the part that fails

A burial cell is not a hole with a sheet in it. It is an envelope: a base and four battered faces in one continuous liner, the arisings placed inside it, a lid closed onto the upstands with a made seam, and clean imported fill capping the lot. Everything about it is decided before the first bucket, because there is no version of this that can be adjusted afterwards.

Siting is the hard part on a developed plot. The cell has to sit clear of every foundation and every drain, clear of the ducts and service runs that do not exist yet but are already on a drawing, above the water table, inside your own boundary, and outside the reach of the next machine that comes here. It also has to be dug with faces that stand — battered to a profile the ground will hold, or supported, which is ground the trench and deep-excavation guides on this site cover properly and which a 2.7 metre face absolutely requires. Battering has a second effect people forget when they order membrane: it makes the wetted area of the box very much larger than its plan area.

On the membrane itself, the number to compare across products is not the thickness in microns that appears in the headline. It is the mechanical performance on the data sheet — static puncture resistance to EN ISO 12236, wide-width tensile to EN ISO 10319, dynamic perforation to EN ISO 13433 — because a liner fails where a bucket tooth, an angular stone in the formation or a rock in the cap finds it, not where it is thin in the abstract. Seams are made by the method that product's own installation instruction specifies, welded or taped; they are not made by lapping generously and hoping. And no service, no marker post and no root of anything penetrates the envelope, ever.

For the area, forget the plan footprint. Take the contained portion of a cell — nine by seven metres at the base and 2.7 metres tall, battered one horizontal to two vertical, with the cap dug above it. That is around 63 square metres of base, something like 113 square metres of sloping face and a lid of roughly 120 once the laps are counted: call it 300 square metres of sheet before any allowance for cutting and waste, against a plan footprint of 63. Enter the developed area, and read the area line in the breakdown rather than the roll count, because the roll count is worked against a 300 square foot garden-fabric roll and a root barrier does not come that way. The built-in overlap allowance is a tenth, which is right for a mulch bed and short for a barrier with made seams; add your own lap into the area you enter rather than trusting it.

1. Set the cell out clear of every foundation, drain, duct and future service run, and record the setting-out before a bucket goes in.
2. Batter the faces to a profile the ground will stand at, or support them — 2.7 metres of face is not something anyone works beside untouched.
3. Trim and blind the formation so nothing angular is left bearing on the liner.
4. Lay base and faces as one continuous envelope with the laps running the way the installation instruction says, and turn every edge up well above the finished level of the fill.
5. Place the arisings in thin lifts with light plant, keeping the bucket clear of the liner throughout.
6. Close the lid onto the upstands, make the seam by the specified method, then cap with clean imported fill in compacted lifts and reinstate.

The half you cannot treat is on the other side of the fence

Nothing above helps with the part of the stand growing in someone else's garden, and there is no general right to walk onto their land and treat it. Section 14(2) of the Wildlife and Countryside Act 1981 makes it an offence to plant or otherwise cause Japanese knotweed to grow in the wild — Schedule 9, Part II lists it — but that is not an obligation on your neighbour to do anything about theirs. The civil route exists: in Network Rail Infrastructure Ltd v Williams [2018] EWCA Civ 1514 the Court of Appeal held that encroaching knotweed can be an actionable nuisance through loss of amenity, and the Supreme Court returned to what the recoverable loss actually is in Davies v Bridgend County Borough Council in 2024, where causation defeated the claim. Local authorities have a faster and blunter instrument in the community protection notice regime of the Anti-social Behaviour, Crime and Policing Act 2014. All of these take longer than six weeks.

What you can build on your own land, today, is a vertical root barrier along the boundary. A trench, taken below the depth the survey found rhizome and returned or toed at the base, with the membrane hung against the contained face and the trench backfilled. Along the rear boundary and one return that is 38 metres at 600 millimetres wide, taken to 2.5 metres: about 57 cubic metres of excavation. Run it with the pipe diameter set to its minimum, because there is no pipe in a barrier trench and the calculator will otherwise subtract a void that does not exist.

Then read both ends of the answer, because they go to different places. The headline is the loose granular that has to arrive — around 65 cubic metres of it — and it has to arrive because the arisings from a trench dug through the exclusion zone are themselves knotweed-bearing and cannot simply be shovelled back on the clean side of a membrane you have just installed. Meanwhile the excavation line in the breakdown is 57 cubic metres joining the disposal total. A boundary barrier is not a cheap alternative to a dig; it is a second dig with an import attached, and it is the only one of the two that a neighbour's inaction cannot undo.

Weighed at the gate, described on the ticket

If it leaves, it leaves by weight. The 133 cubic metres from the main dig is about 174 cubic yards, which at the planning density this catalogue publishes for clay is near 261 short tons — roughly 237 tonnes. Add the barrier trench and another 57 cubic metres becomes about 112 short tons, and the corner of the site has produced something like 373 short tons, near 338 tonnes, before anybody has laid a brick. Feed it the bank volume rather than the bulked one, for the reason set out at length in the guide on getting spoil off a small site.

The classification is the part people get wrong in both directions. Soil carrying knotweed is a controlled waste, so the duty of care in section 34 of the Environmental Protection Act 1990 attaches to it exactly as it does to any other, and in the List of Waste established by Commission Decision 2000/532/EC it sits at 17 05 04, soil and stones, where it comes out with the ground; cut green material on its own falls under 20 02 01. It is not hazardous waste, so there is no consignment note under the Hazardous Waste (England and Wales) Regulations 2005 and no assessment under WM3 to do. What controls it is where it may go and how it is handled on the way. The duty-of-care machinery around that — checking the carrier's registration, what a transfer note has to carry, how long the folder is kept — is set out in the spoil guide and is no different here. What is different is the description you write on the note: a clean-soil description on a knotweed load is not an administrative slip, it is how a receiving site ends up spreading it across its own restoration.

Not every permitted landfill will take it. Sites that do may require pre-notification and place it deep within their own cell, which is why availability, rather than price, quite often decides how far the wagons go. Get the acceptance in writing with the description you intend to use, before the programme is built around a gate that turns out to be forty miles further away than the one you priced.

North American practice has no federal equivalent to lean on: Japanese knotweed is not on the USDA APHIS federal noxious weed list, and control is state by state — restricted in Wisconsin under chapter NR 40 of the Administrative Code, prohibited in New York under 6 NYCRR Part 575, and elsewhere differently again or not at all. Where the state is silent the receiving facility's acceptance policy does all the work, and the question to ask it is the same one: will you take this, described this way, in writing.

Every movement is a chance to plant it somewhere else

Counting loads matters here for a reason that has nothing to do with payload ceilings. At around 166 cubic metres loose against a twelve cubic metre body, the main dig is fourteen movements. Each movement is a sheeted vehicle, a transfer note, a cleaning cycle and several miles of public highway, and a fragment shed off a tailgate onto a verge is a new stand on somebody else's land with your transfer note pointing at it. That is why loads are not part-filled and topped up with clean soil from elsewhere on the plot, and why the vehicle is cleaned before it goes back on a job where nobody is thinking about this.

The exclusion zone is the other half of the same discipline, and it is fenced to the survey line rather than to the canes — which usually surprises whoever has to move the site cabin. One entrance, signed so a delivery driver understands what the fence means. A dedicated bucket and, ideally, a dedicated machine for the duration. A cleaning point inside the line, so nothing crosses it dirty; the compound guide on this site covers how to build one that does not simply wash the problem into a drain. And the vectors nobody fences against are the small ones: barrow wheels, boot treads, the trackway mats, and the length of scaffold board somebody laid across the mud and then carried to plot one.

1. Fence to the surveyed line rather than to the visible canes, and sign it so it reads as an instruction rather than as decoration.
2. Give the zone a single entrance with the cleaning point inside it, so nothing leaves the line uncleaned.
3. Keep one bucket and one machine in the zone for the duration and clean both before they work anywhere else on the plot.
4. Sheet every load before it moves and check the tailgate seal — the highway is where a fragment becomes somebody else's problem and your liability.
5. Log each load against the excavation plan, so what has left can be reconciled with the block the survey drew before the hole is signed off.

Two prices for the same 338 tonnes

Of the five routes in the table earlier, exactly one resolves into a single figure before anybody starts: the last one. Three hundred and thirty-eight tonnes at whatever the permitted gate quotes is the whole of dig-and-dump, arrived at in one multiplication and final on the day the last wagon leaves. Every other route is a programme, a land take and a set of obligations that outlive the job. So work that one number out first and put it on the table, because until it is there the other four are being argued about in the abstract with a client who only wants the corner cleared by Friday.

Two things to establish before either figure is trusted. Whether landfill tax is inside the quoted rate or added to it, and — more slippery — which rate applies at all: that is decided against the Landfill Tax (Qualifying Material) Order 2011 and HMRC's own guidance on it, and it is a question to put to the operator in writing rather than one to assume from the fact that soil is soil. And whether there is a minimum charge, because on a job that ends with three part loads of inspection-pit arisings, a minimum charge is most of the bill for those three.

Then set the gate figure against the on-site alternative in full: the deeper hole, roughly 300 square metres of membrane, imported cap fill and its compaction, plant standing while the envelope is laid, and a permanent land take disclosed at every future sale. Neither total is a rate this site will print — gate and haulage rates are local, dated and published by somebody else — and both are arithmetic on figures the reader has been quoted. The third column never gets priced at all: what the site costs to hold for the seasons a herbicide programme runs.