Author: Mike Clough

Date Posted: Wednesday 29th April 2015

A little bit like the young child in the back of the car on a long journey repeating the phrase ‘are we there yet, are we there yet?’…many people dealing with the long term management of Japanese Knotweed will hear their clients saying ’is it dead yet?’

How do you answer this question? There have been efforts in the past few years to produce ‘viability tests’ for Japanese Knotweed rhizome – but these are only ever going to prove the viability of the visible piece of knotweed removed from the ground.

Unfortunately the ‘visible’ bits are not the pieces we are worried about!

The pieces of rhizome invisible to the eye, buried deep beneath moved material are the pieces that we worry about; these are the sections of the plant that could cause problems in the future. So really, viability testing of small sections of plant is a bit of a waste of time. I’m not saying there isn’t a place for viability testing, but i am saying the results need to be looked at with a critical view as to what they are actually proving?

It is the fragmented nature of the Japanese Knotweed plant that makes it such a difficult plant to fully eradicate. Many of the mechanical regimes used to remove the growth can actually increase the number of sections of viable plant and introduce whole new areas of growth into new areas when the contaminated material is spread.

One of the less obvious problems with dealing with eradication of plants in general is the seasonality of their growth. Most of the invasive non-native group of problem plants are either deciduous or perennial and die back anyway over winter. Whilst this may be obvious to any decent quality environmentally based company dealing with these plants, some of the more recent additions to the companies offering services in this sector are less well qualified and completely ignorant as to the vagaries of plant growth.

So how do you tell if a plant has actually been eradicated over winter? If it’s not actively growing then this is going to be tricky? During spring or summer, any fragments of Japanese Knotweed untreated will rapidly grow back and produce new growth. Even areas successfully treated will often produce a final flurry of ‘bonsai’ growth with small fine cut leaves and low ground hugging growth.

Sites treated towards senescence (cellular die back during late autumn) will look successfully eradicated even if sprayed with nothing but water! The leaves will colour and fall and the stems will soon be clear of leaves and looking to the inexperienced eye …dead… the plant was going to do this anyway, in preparation for the winter months ahead.

I have even heard less experienced contractors call over wintering plants ‘dead’….

Q. So what can you do when late in the season your client buys a piece of land, full of Japanese Knotweed that he wants to develop immediately?

Well the first thing i would do is book them in for a training seminar…! Anybody involved in land purchase should as a minimum be able to spot problem plants, even if they can’t actually identify individual species they should be able to spot the warning signs and know when to call in an expert.

The majority of the ‘problem’ plants are highly visible even to the untrained eye. Your client needs to be told that a serious infestation of Japanese Knotweed on a newly purchased site can add not just thousands on to your remediation costs but tens of thousands …removal to landfill can often make a site a non-viable prospect.

Once the sale has gone through, providing the vendor wasn’t hiding information to increase the value of the land – it’s too late to argue for the costs of remediation…it’s a case of ‘buyer beware’…

Q. Back to the original question, your client has bought this piece of land, he needs to develop quickly to show a return on the money he has invested, what should he do?

Chemical treatment as a ‘one stop answer’ is out of the question, the plant will not fully absorb the poison.

Your thoughts must turn to landfill…? Its quick, it’s clean …but it’s bloody expensive….

One square metre of surface growth of Japanese Knotweed requires an excavation two to three metres deep and seven metres in all direction from surface growth, this will result in a pile of muck approximately 450m3

450m3 of soil is the equivalent of 810 tons (1.8t per m3)

810 tons taken off site to landfill classed as contaminated with JK will be charged at a landfill tax rate of £72 per ton giving you a tax bill alone of £58320.00…

There are ways to reduce this figure by classing the material in different ways, reducing the % of knotweed and classing as less than 5% will massively reduce the tax liability – however HMRC are rumoured to be changing the legislation to remove this loophole.

There are some new strategies available in the market with companies offering to screen or sift the site arisings and remove the rhizome material – separating it from the clean soil. In the past the screening systems were huge cumbersome pieces of kit designed for large scale site remediation and were unsuitable for the smaller site operations. There are now mini-screeners available for small sites and domestic projects – when used by experienced operatives this can prove hugely beneficial being cheaper than removal to landfill and obviously quicker than any chemical control strategy currently available.

Burial on site may be an option but depends very much on the current development plan with one eye on potential future uses of the site. Ideal opportunities exist on sites where there is land allocated for car parking or perhaps public open space. This technique is based upon digging a large pit in which to bury the contaminated material, the pit is lined with ‘landfill grade’ carpet of material which is then over lapped around the material to be encapsulated and heat sealed or glued to give a sealed pocket within the pit. The pit must be first approved by the local Environment Agency office then covered with a minimum of two metres of cover – the position of the pit must be recorded on drawings which should then be submitted to the EA.

The downside to any mechanical treatment of Japanese Knotweed is that you will always run the risk of fragmenting the plant and leaving small sections of rhizome within the ground. It is impossible to categorically state – ‘we have removed every trace of Japanese Knotweed from your site’ – anybody that says this is lying. You can say – ‘we have removed every visible piece of Japanese Knotweed from your site’ – this is what we would say at Japanese Knotweed Solutions (JKSL). We would also state that our recommendation would be to have an ‘aftercare policy’ in place. We always offer a belt and braces approach at JKSL, for a small fee we will visit your site on a regular basis and ensure that nothing untoward is happening and that any fragments of plant left on site are rapidly dealt with before they can become a problem.

All of the invasive non-native plants that trouble us within the UK are going to be problematic, it’s what they do. If they weren’t awkward and difficult to eradicate there wouldn’t be the legislation in place to control them. It should be noted that many newly set up companies are offering instant strategies to manage these plants, this just isn’t possible. The control of these species is always based on a long term approach and putting strategies in place to visit and return to site to check and monitor and changes within your site.

There is no quick fix solution.

So, in conclusion, the answer to your client’s question ‘is it dead yet?’…probably should be … ‘it’s a long journey and we’ve only just set off…’

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Mike Clough

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