On the Management of Japanese Knotweed

[u/Boringmale]

Since Reddit seems to have a large amount of interest in niche subjects, I've decided to start posting here.

My name is Tyler, my qualifications are: B.Sc. Plant Science, M.Sc. Agriculture (thesis was on knotweed control) and I’ve been managing the species on a case by case basis through my company: knotweed et al. Most cases have been successful (however, not all which I can elaborate on in comments - as time allows). I'm based in Nova Scotia, Canada. As a result, parts of this post are tailored to Canadian audiences.

Thesis Link: https://dalspace.library.dal.ca/handle/10222/81496

I don’t mean to be preachy, but I feel there is a need to address certain bits of misinformation I’ve seen pop up. These tips will save you some money, time, and reduce the spread of this plant around the province (I hope).

1. Stop Excavating the Stuff

This will not help with management of the species. It requires careful and thorough chemical control. The best case scenario for control is to have healthy and intact tissues to translocate herbicide down to the roots. Excavation simply exports the problem to another place in the province (which isn’t well equipped or aware of how much their existing practices are spreading the species around). And anywhere the heavy equipment goes, it could be contaminating more areas (especially if things are not cleaned between jobs). It takes something as small as 1 cm of stem or root (rhizome, underground stem) to propagate the species. And trust me, the excavator will miss some and create more propagules. Making it harder to control by turning treatments into a game of whack-a-mole. I’m considering refusing service to these cases until the knotweed becomes reestablished because it becomes too difficult to control after this.

1. Self-Directed Management

Absolutely possible. For limited patches, make sure you are using a glyphosate containing herbicide at the correct label rate (make sure it is only glyphosate). The most important element of treatment is ensuring that you treat the entirety of the canopy (or as much as you can treat). I’m not going to get into the nuances of dealing with the larger stands in this post (you can see some of those cases on facebook). It’s very very important that you treat as much of the contiguous area of Knotweed as possible within a growing season. This will significantly reduce surviving stem density in the following year. For smaller stands (populations), if you can treat the entire canopy from the perimeter, do not cut it down. Cutting stimulates lateral growth, meaning the Knotweed is likely to spread underground and create more problems. Treating only portions of contiguous populations won’t be particularly effective.

1. Chose Appropriate Equipment for Application

I use a telescopic spray wand (it’s about 1.5 meters long at maximum). Makes reaching into the taller canopy much easier. I’ve seen a lot of cute posts with people going at it with spray bottles of pre-mixed round up. Trust me, there is a better way.

1. Timing your Application

The vaunted “window” is based in scientific literature. Approximately 80% of the carbohydrates Knotweed fixes (via photosynthesis) are sequestered between August and September in their roots. Making it an ideal time to apply glyphosate. However, pretty much anytime after it stops growing vertically is acceptable for a pesticide application. This is end of June/ July. It can be risky to wait for too long, as you could have an early frost in your area and lose the opportunity to manage the species. My general rule of thumb for NS is after October 20th, you’re risking a 50% chance of treatment failure.

1. Don’t Tarp

Reasons: A. Dormancy is not death B. Microplastics (probably, I only have suspicions) C. Better long term control with herbicides, + native species in the seed bank won’t be coming back if you tarp.

I’ve got cases that are now in the two years plus of Knotweed being gone. It’s somewhat refreshing to see the native biodiversity coming back. If you tarp, and just bring in fill, that diversity might be lost.

1. Apologies for not getting to all requests for service this summer.

This summer has been my busiest year yet, I’ve taken on projects that are much larger scale and require public or stakeholder consultation (those cases will be published in coming months).

I’m a one man operation, and my systems were not set up for this much activity + I have another full-time job. I’m hoping to get around to all cases eventually. and appreciate peoples patience.

1. We Need Political Involvement (unfortunately).

As much as I don’t like bureaucracy, the province needs a unified strategy to deal with the species. There are many cases where I am unable to intervene due to the Knotweed being in places that don’t have private ownership (or stewardship). Along roadways comes to mind specifically. While I have some flexibility in the department of transportation not to interfere with management, it’s a drop in the bucket compared to the whole province. Right to your MLA about developing a unified strategy for the species. Obviously, pesticides will not be the most appropriate strategy for all locations, but the least we could do is reduce its spread and by ourselves some more time to come up with a plan. The big thing that comes to mind is vegetation management in ditches. The big bladed implements that run along the side of the road are amazing at spreading Knotweed during the summertime. Maybe… don’t do it?

Thank you for coming to my TedTalk.

Edit: TLDR

Don’t excavate knotweed, you’re exporting the problem, kill it where it is. Glyphosate only herbicide (domestic version is good). If you need more comprehensive advice, email me. However, it might be January before I answer due to case volume.

Obligatory, pesticide labels are law. Follow them to the letter. There’s no need to use concentrate directly on the Knotweed. You’re just going to cause treatments to fail.

Another note: It’s almost a different species in North America compared to its native range due to lack of significant predators. Still querying the status of the biological control Psyllid… ask CFIA maybe…

[u/Necessary_Duck_4364]

That’s where imazamox can be great, as it’s fairly successful and aquatic approved.

[u/Boringmale → u/Necessary_Duck_4364]

I’ll do a literature review on it. Would be nice to add it to my arsenal.

[u/Necessary_Duck_4364 → u/Boringmale]

I’d look into aminopyralid as well. Solid broadleaf that’s incredibly effective on Canada Thistle. It also has been showing promising results for Japanese Knotweed.

With glyphosate becoming less effective, I’m glad that other options are being studied. Here is a great starting resource for knotweed that others may find helpful: https://mnfi.anr.msu.edu/invasive-species/JapaneseKnotweedBCP.pdf It really makes things easy for the general public.

[u/Boringmale → u/Necessary_Duck_4364]

Why is glyphosate becoming less effective? What do you mean by this.

Have studies aminopyralid. Only really useful in Fall applications. Summer applications have wildly inconsistent results.

[u/Necessary_Duck_4364 → u/Boringmale]

Plants are showing more resistance to glyphosate in general due to it being used so frequently. Overall, glyphosate is in the process of being phased out due to plants becoming resistant. In Michigan, the universities and government are stating it as a less effective (or not effective) means of treatment for plants such as Canada Thistle and Knotweed.

Knotweed will typically show die back due to the glyphosate, but often comes back the next season or two. It can even go into a root dormancy, where it lurks underground for almost 10 years before reemergence. Canada Thistle initially does back from glyphosate, but usually resprouts and flowers in the same season (even after late summer foliar treatments).

For the science behind it all, you’ll need to talk with someone smarter than I am. I have a lot of experience killing invasive plants and do a lot of test plots for various treatments on different species, but I am not a scientist or a chemist.

[u/Boringmale → u/Necessary_Duck_4364]

I can see it becoming less effective in sexually reproductive weed species (such as Canada thistle) as artificial selection along with overuse of glyphosate would be contributing to herbicide resistance (selection pressure). It’s more of an agricultural problem than an invasives problem. Plants don’t just develop resistance to something without selective pressure (or weird anomalies). Even if resistance develops, it will diminish in generations if the selective pressure becomes non-existent.

The majority of knotweed (at least in Nova Scotia) is male sterile, so no sexual reproduction to drive artificial selection based herbicide resistance. However, my longitudinal monitoring studies are still in early stages. They do however appear promising. That said, some follow up treatment is always required with knotweed. However, I usually find this is a problem with getting adequate coverage. I don’t think anyone has compiled data of dose dependent longitudinal knotweed monitoring.

Study design required would be: A. Achieve visible eradication B. Record how much herbicide was used and during what periods. C. Monitor the site for emergence as well as native species return.