By Nicolas Seal – Environet UK Ltd
 
Published March 2018
 
An Information Paper from data obtained by Environet UK Ltd 
 
Japanese knotweed companies are often asked what it costs to treat or remove Japanese knotweed. The answer is invariably “it depends” so we thought we’d share some relevant information based on Environet’s own experience.

The scatter chart below shows the costs for varying quantities of knotweed infested land, (corrected for 2017) colour coded by treatment or removal method across the UK from data we have obtained on quoted projects.
 
The variance in prices is notable but the following conclusions can be drawn. 
 
As expected, cost correlates with quantity. Knotweed spreads year on year, so if it is left the costs are likely to only move in one direction, upwards.
 
Herbicide treatment is the cheapest method, but is generally recognised only as “control” due to the risk of dormancy and regrowth, particularly on land that is disturbed. 
 
At the other end of the cost spectrum is excavation and disposal at landfill, (Dig & Dump), at approximately ten times the cost of herbicide treatment. 
 
The suitability of other more cost effective methods such as on-site sifting or screening, stockpile and treat and on-site burial will depend upon the site conditions, but in almost all cases will provide cost savings of at least 50% compared to Dig & Dump.
 
Sifting/screening methods are dependent upon the ability to screen the soils, some soils being easier to screen than others, which has an effect on cost.
 
Stockpile & Treat relies upon a sizeable area of land being available for the stockpile. Even after successful treatment the material cannot be classified as “knotweed-free”, so it’s not suitable if future off-site disposal is planned. 
 
Likewise cell burial relies upon the buried infested material being contained by membranes - it should not be built over. It’s considered only as “control” since considerable quantities of viable rhizome remain in the ground – any imperfection in the membrane will allow knotweed to escape.
 
There is a lot of variation in prices from the trendlines for a given quantity, which can be explained by variances in: -

Knotweed maturity
Rhizome depth and lateral spread
Site conditions
Access
Guarantee requirements
Ground type and conditions
Geographical location
 
Where a price is outside of the expected range further investigation would be prudent. 
 
 
Low prices for Dig & Dump might, for example, be due to: - 
 
Incorrect estimation of infested soil volume – always check quotes are on a like-for-like basis
Disposal of knotweed at illegal sites – always check your knotweed contractor is fully complying with the “duty of care” requirements. If they’re not, you too are at risk of prosecution under the Environmental Protection Act 1990, check and check again. 
Application of incorrect landfill tax rates – don’t be tempted to consign topsoil to landfill at the lower rate of tax, it’s illegal.
Incomplete excavation leaving viable rhizome in the ground – further excavation or treatment will be required.
The scattergram provides a good check for budget costing purposes. It’s 

recommended however that one gets a fixed-price cost from a reputable knotweed specialist as prices can vary significantly. 

Treatment/removal cost is of course an important consideration, but be aware the added costs if the knotweed is not properly treated or removed. 
 
For a developer, construction programme is key, and any delays in resolving the knotweed issue is likely to have a knock-on effect on following trades. Working closely with your knotweed specialist can resolve most programme issues but remember the rule of thumb, the quicker you want the knotweed removed, the more it is going to cost.
 
Strength of any guarantee is paramount when one comes to sell. A solution that does not provide the required guarantee that enables the land or developed properties to be sold will prove to be a very expensive solution. Nearly all banks and mortgage companies will require a treatment/removal programme with an insurance backed guarantee (IBG) for up to 10 years. 
 
For a residential owner the risk and hence cost of legal claims brought by an affected neighbour could easily be ten, if not hundred times the cost of initial treatment. 
 
In a recent court case in Cornwall a judge awarded costs to the claimant for knotweed encroaching from the defendant’s land, not only to cover the cost of removal, but also to include the diminution in the property’s value.
Furthermore, a property affected by knotweed can expect a 5-15% reduction in value when the time comes to sell, so in the example above, giving the defendant a double hit with the diminution in value of both his and his neighbour’s property. 
 
These illustrations show the importance of using knowledgeable advisors to help find the right solution for your site or property.
 
 
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