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28 Mar 2024 < Back

Glyphosate induced dormancy: Case study

Guest blog from Andrew Ford - Inspectas Land Remediation 

As many PCA members are aware, dealing with Japanese Knotweed through herbicide application can only be considered a control measure, not a complete eradication programme, as there remains the risk of new shoots emerging due to factors that can include glyphosate-induced dormancy. But why is this and what evidence do we have to support the notion? 

First of all let’s take a look at Robert Mitchell’s article on Japanese Knotweed dormancy for a little more of an understanding on this perceived phenomenon. 

So, we can conclude that dormancy can be triggered by the incorrect use of herbicides which in turn can result in stunted growth, or in some cases no regrowth at all.   But should we also add that even with the correct application of glyphosate this can also result in placing an infestation into induced dormancy? 

Let’s discuss …    

Case Study – Japanese Knotweed excavation in Nottingham

As the saying goes, a picture paints a thousand words. So, let’s examine a recent site we have remediated through excavation, discuss what we found, the implications moving forward, and the necessary conversations to have when advising our clients on the available remediation options. 

Site background

For this example, the recent site in question represents a municipal car park that was bought from the council with plans to construct nine new build homes on the site.  The car park had been in operation since the 1970’s and prior to that had a row of terraced houses present that were subsequently demolished to make way for the car park.

So where and when did the Japanese Knotweed come to be present on the site?  Was it present in the 1960’s within the garden or yard of one of the properties?  Were contaminated soils brought to site when constructing the car park?  Or could contaminated soils have been fly tipped to the area when the car park was in operation?

All of the above are viable, but unfortunately it is very difficult to pin-point the source or reason as to why the Knotweed came to be present. 

Site survey

In March 2021, Inspectas Land Remediation received instructions to conduct a survey for two areas of Japanese Knotweed located within the footprint of a planned development for both a new library and residential homes on the site.  One of these areas, situated in the car park, included two large Japanese Knotweed crowns. 

To give some additional background, it was understood that the council had previously applied herbicides to the area over several years in an attempt to deal with the Japanese Knotweed. 

Anyway, back to the survey and we observed dead canes, some reaching over 2.5 meters in height, emerging from a large crown with smaller bonsai-like growth pushing out in front of the dead canes along also some smaller plants puncturing through the tarmac in one of the parking bays. 

Site observations through 2021, 2022 and 2023

Fast forward to May 2021, and we received instructions to excavate the Japanese Knotweed located within the old library site across the road from the car park. During this process, we revisited the car park infestation and observed no substantial regrowth in the area, aside from bonsai-like growth observed in late March. Monitoring visits conducted through 2022 and 2023 revealed no regrowth at all on the car park site.

Japanese Knotweed excavation of the car park site in 2024

In early 2024 however, our client approached us once more letting us know they were ready to deal with the Japanese Knotweed in the car park and needed the area to be excavated so they could commence their development activities.

So, what were we faced with? As with previous monitoring, we saw two large crowns appearing to be dead with no visible knotweed growth or remnants of dead canes from a previous growing season.

All was good and we tweaked our proposal and began the excavation works.  So, what did we uncover during the Knotweed excavation?

Well, for the smaller crown we discovered black, mushy remnants of rhizome concluding they were non-viable and incapable of generating new growth.  However, for the larger crown, we found bright orange rhizomes sitting directly under the crown, that are absolutely capable or generating new above soil growth if given the chance.

Fact – Glyphosate induced dormancy is real! 

As with all management plans we undertake that have a monitoring element baked into them, we can conclude that we saw no new regrowth from an area that has previously had herbicides applied to it for numerous years.  What we did find, as previously noted, were viable rhizomes in the subsoil that have the potential to develop above soil plant growth.  We should note that we had no control over the application of the herbicides, but we do know that this was carried out for numerous years by the council. 

Also, if we dig around the internet we can assume (or at least understand) that Nottinghamshire District Council takes the problems posed by Japanese Knotweed seriously and understands when best to apply herbicides to the plant and do monitor areas post application: 

So where does this lead us? 

Quite simply, that despite witnessing no regrowth, viable rhizomes can persist even after the application of herbicides. And, if provided with an opportunity—whether through soil disturbance or crown destruction—these rhizomes have the potential to generate new growth visible above ground level.

In such a situation the management plan will be viewed as being successful and the client may believe that the Japanese Knotweed has been successfully eradicated versus being in a state of dormancy.

Why is this important and what conversations does this lead to with our clients?

It’s important because for some of our clients, they believe that the application of herbicides is a singular solution that will effectively deal with Japanese Knotweed, resulting in no regrowth, and for all intent and purposes, will believe the issue to have been eradicated.

The truth is very different. 

Our conversations need to address the potential dormancy period even after we have successfully seen no regrowth within an area.  We need to highlight the limitations of implementing a herbicidal management plan and discus what risks there are for new growth to emerge even after we have signed off and issued a completion certificate.  

In conclusion

To conclude, and to borrow from a Johnny Nash song, ‘There are more questions than answers…’ for us to truly understand what the implications and causes of induced dormancy are, and how we can better determine the likelihood of this occurring within a specific setting.



Nic Seal

Andrew, a very good article, and something we have been banging on about for years. I'm pleased you too have brought this subject into the limelight. Whilst herbicide treatment has its place, as an industry we must not hoodwink customers into thinking their problem is solved by simply spraying, or worse injecting, herbicide into this very resilient plant.

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