Japanese Knotweed Update – Spring 2018

 Pesticide use and long term strategy for remediation and biodiversity improvement

Author: Andrew James, Basis Adviser, PCA surveyor, Manor Estates Groundcare/The Invasive Weed Group


The impact of a long, harsh winter and a late spring on Japanese Knotweed

The winter of 2017-18 and the late arrival of spring has caused a knock-on effect on both the growth of Knotweed and in turn, our treatment approach.

We are currently observing:

  • Growth that is approximately one month behind schedule – at this time of year we would normally expect to observe more leaf and higher stems on invasive weeds
  • Vigorous and a faster than normal rate of growth – with Knotweed late to come out of dormancy, there is a considerable amount of energy stored within its rhizome system, speeding up the usual rate of growth
  • A rhubarb style growth with unfurling primary leaves – where we would normally expect to see more leaf and stem division
  • Knotweed is outcompeting surrounding flora – non-target species such as brambles or nettles have also been subjected to the late spring conditions, but are being surpassed by Knotweed

As a result of these observations, our treatment schedule has been set back by a few weeks – we now plan to send out our treatment teams in mid-May. Furthermore, as we undertake our first treatments, we are finding that the Knotweed still has last year’s growth standing. It is important that these canes are reduced, not only to enable access, but also to reduce the amount of product needed to cover the foliage. Without reducing the canes first, the extra product required would be soaked up by the dead stems, not metabolised and will stay persistent within the winter debris for longer. There is also a risk that it will leach into the environment.

Our advice for utilising an early season stem injection approach

In and around the South East of England we have observed that the cane growth is thicker than usual – another likely impact of the long, hard winter and late spring. While regional ecology and conditions might differ, this is a good year for early season stem injection.

If stem injection is used, careful calibration of the delivery system should be adhered to. By adding a neat solution to the canes, it can be easy to overdose the area, resulting in a bottleneck of product, with the denser crown material that will not be fully translocated throughout the rhizome. Subsequently, the centre of the plant can be killed, with much of the outer rhizomes left in dormancy. This can provide a false sense of success during the treatment process, only for the outer rhizones to regenerate over the next few years – and is why some operators believe stem injection to be ineffective.

During my 15 years of experience of using stem injection systems, I believe there are more factors that should be considered for this method of treatment for it to be successful I believe that in order to bring out the best of this methodology, more research into dilution rates, translocation pathways and timings is needed.

The argument that an integrated vegetation management strategy can create more biodiversity

As we continue to focus on improving our Knotweed control and plan for an eventual eradiation strategy, we need to look forward and take into account the future regeneration and biodiversity within the area currently invested with Japanese Knotweed.

One of the specie’s greatest punitive effect on the landscape is that it creates a mono-environment, which does not support as high a species count as surrounding areas. However, by making a careful selection of herbicides, using a range of different treatment methods and an integrated vegetation management (IVM) strategy, it is possible to create more biodiversity than was originally there.

It’s possible to argue that using pesticides within an IVM not only controls the invasive species, but actually results in making the countryside under your specific stewardship a better place for wildlife.

This makes a strong and compelling counter argument for your clients who might face accusations of “poisoning and destroying the landscape”.  Remember to manage your client’s expectations. Complete eradication of Japanese Knotweed is very difficult, and requires constant monitoring to reduce the energy within the rhizome by treating any regeneration.

An example of how treating Knotweed can improve a micro-environment

In a situation where Knotweed is growing within brambles, one approach would be to initially treat it with glyphosate, followed up in the future with selective herbicides which encourage grasses and beneficial dicotyledons. This will create a clearing where reptiles and insects, normally at risk from exposure to predators and human contact, can bask in the sun within the safety of an impenetrable wall of bramble and associated brush. The food chain will also be boosted in such a micro environment as birds, insects and small mammals foraging for food would normally have to travel to the margins of the bramble/Knotweed and potentially be disturbed by traffic such as dogs and their owners.

What should your long-term strategy include?

The treatment of invasive species and general application of herbicides has come a long way over the past decade. It just isn’t acceptable to just “spray” any more. My advice for the development of a long-term strategy, would be to include the following within your approach:

  • Plan a targeted and specific spot spraying approach, rather than a traditional covering

Repopulation of desirable native weeds can then be encouraged.

  • Avoid the development of resistance in non-target species by avoiding the sole use of a total herbicide with a singular active ingredient such as glyphosate.

When the foliage of the non-target species receives a partial application coating as a result of product drift or run-off from the intended target, it wouldn’t kill the plant. And as evolution has taught us, this leads to resistance.

  • Plan to avoid bank erosion

Total herbicides strip all foliage from an area and leave gradients unstable. Infestations on a steep bank should be sown with grasses that are not susceptible to selective herbicides as soon as possible. This will provide a dense sward which can establish and mitigate the destruction of the Knotweed, and other plants root systems that had historically held the bank together.

Your overall IVM strategy should incorporate the following as best practice:

  • Know your habitat and have a plan that encompasses at least 3 years
  • Within your strategy and environmental risk assessment, you should document the active ingredients in your mode of treatment and understand where it can end up, its biodegradation pathways and half-life.
  • Following best practice for documentation and record keeping can also mitigate against future repercussions from alleged injured parties, should the need arise.

The importance of training

With the continued uptake of BASIS courses (such as invasive weeds, hard surfaces, shrubs, borders and container plants) and organisations such as Lantra, City & Guilds and PCA providing excellent courses, just having a PA1/PA6 should be just part of the legal minimum.

Toolbox talks and in-house discussions should be seen as standard in order to raise the bar with operators understanding. We have had members of the public approach our operatives and ask questions. Operatives should be able to answer these on the spot without a phone call back to the BASIS advisers. This will clearly demonstrate that the people undertaking these works know what they are applying and what impact it has.

Remember the role we play within the wider industry

By following management plans that are best practice within the Japanese Knotweed industry, we have an opportunity to promote to the wider amenity industry how our understanding and expertise is being applied to areas that have historically been left as a herbicide wasteland for years. The general public and particularly the internet activist community – often looking for a reason to create an uprising against glyphosate and associated pesticides – can also be influenced by our approach. I’d gladly take the big stick (environmental regulation) that is used to thwart our work, and use it to our advantage which is possible when we educate our clients and stakeholders with science and common sense.

An integrated pesticide management strategy which includes controlled and targeted pesticide use, in conjunction with a mechanical use, can be, and in some cases is certainly less invasive and damaging to local flora and fauna than a purely mechanical approach. Documentation and the completion of a solid environmental risk assessment can protect everyone involved from any repercussions and improve our industry’s relationship with the general public and system as a whole.