Japanese knotweed

How difficult is it?

Many people regard ridding themselves of knotweed as virtually impossible and equate the plant as somewhat akin to a Triffid. This is not the case, although the perception has allowed a few unscrupulous contractors to scaremonger in their sales pitches and panic people into employing their services. In truth, knotweed treatment can be more straightforward.

Treatment is perceived as being much more complicated than it because there are certain truths about knotweed treatment that clients do not want to hear:

• 100% eradication of knotweed cannot ever be guaranteed. As knotweed can regenerate from tiny parts of itself and has a long dormancy threshold, it is impossible to say that all of the knotweed is dead on a site just because there hasn't been any new growth for a year or two. Some companies have tried to get around this by offering viability tests on knotweed material removed from the ground. While this can produce some assurance it is far from a 100% guarantee. Viability tests depend on random sampling so there is always the risk that one of the pieces of rhizome that wasn't tested is the one that's still alive. Strong assumptions can be made from lack of regrowth and viability test results but you can never categorically state that knotweed is 100% eradicated. Beware anyone who claims otherwise.
• Knotweed treatment takes time unless you are prepared to spend a substantial amount of money. Everyone in the knotweed industry is chasing the ideal of a 100% eradication method achievable within a single growing season because this is what their clients - particularly the construction industry - want. Many claim to have found it, but always remember the phrase 'buyer beware': knotweed can react against being hit too hard with herbicides in too short a period of time by going into dormancy, which can produce misleading results and lack of new growth for a season or two.

The options

There are a myriad of treatment methods offered in the marketplace. In essence, though, they boil down to variations of 4 methods:

• excavation;
• mechanical disturbance;
• root barrier; and/or
• herbicide control.

Excavation: This involves digging out an extremely large hole in the ground and removing all knotweed material. The size of hole will vary. Any contractor should be able to tailor the dig out to the actual spread of the knotweed by chasing out the rhizomes, rather than excavating a hole to set dimensions and resulting in clean soil being needlessly removed. Once the knotweed has been fully excavated, there are a number of options as to what to do with it. The traditional method has been to dispose of it as landfill (the so-called 'dig and dump' method) but this has been recognised as unsustainable (not to mention expensive) and is regarded by the Environment Agency in their Knotweed Code of Practice as a last resort. On-site treatments of excavated material can include burying it in a cell burial (enveloped in root barrier and with the top of the cell no less than 2 metres below finished ground level) or relocating it to another part of the site where there is more time to treat it.

Mechanical disturbance: This is not an eradication method in itself, but involves weakening the rhizome of the plant by cutting it up into smaller pieces and pulling some of the deeper rhizomes closer to the surface. This makes the knotweed more susceptible to other forms of treatment.

Root barriers: Barriers are used to contain knotweed that, for one reason or another, cannot practically be excavated (e.g. rhizomes that spread across boundaries). You must use specialist root barrier membranes for this as standard plastic sheets are not strong enough to prevent knotweed growth simply ripping straight through them. Use root barriers in combination with other treatment methods, as all they do is contain the knotweed rather than kill it.

Herbicide control: This involves the regular use of herbicides, usually over a few growing seasons, to gradually kill off the plant. Some contractors like to exclusively use residual herbicides, as they are stronger and keep working in the soil for months - even years - after initial application. However, residual herbicides can create a good initial result by killing off the knotweed closer to the surface, while sending the rest of the knotweed into dormancy where it will wait out the treatment and emerge years later. The most commonly used non-residual herbicide is one containing the active ingredient glyphosate. This chemical takes longer to stop the emergence of any new growth (usually 3-4 years) but is more likely to have a lasting result. For chemical advice, consult a BASIS qualified adviser.

The above methods are often used in conjunction with each other. For example, the reason for relocating knotweed to another section of the site will be to allow the stockpile to be treated under a herbicide programme. The shallower the stockpile the more effective the programme will be: a shallow stockpile encourages all of the rhizome to grow rather than allowing some to remain buried and dormant. Mechanical disturbance is also often used in tandem with a herbicide treatment programme to shorten the overall duration. Root barriers are used to encompass a burial cell and they can also be used to horizontally 'cap' a partial excavation (knotweed is excavated to a certain level and the remaining rhizomes are left where they are and 'capped' to prevent them re-emerging). The permutations can be many and varied.

Timescales for treatment

Timescales vary according to the control method chosen. Generally speaking, the more time available for treatment, the more options there are and the less it will cost. For Japanese knotweed, the basic timescales are:

• excavation and removal to landfill - a few weeks at most;
• excavation and burial on site - a few weeks at most;
• excavation and relocation - a few weeks at most;
• herbicidal treatment of relocated knotweed - 2-4 growing seasons;
• cultivation and herbicidal treatment - 2-4 growing seasons; and
• herbicidal treatment programme - 3-4 years (average).

A less mature stand of knotweed will take less time to treat with herbicide than a mature stand and a mature stand that had already been treated but not eradicated a few years previously could end up taking longer than 3-4 years to completely kill off.