By: Julianne Mason, Restoration Program Coordinator, Forest Preserve District of Will County
Five years ago, my coworker and I put in some test plots to compare the effectiveness of spraying reed canary grass with glyphosate, a non-selective herbicide, compared to treating it with clethodim, a grass-specific herbicide. Plots were treated in the late fall, early spring, or late spring in 2016-2018. The area is a remnant sedge meadow/ wet prairie in Joliet, Illinois. In the portion of the wetland that was sprayed in the late fall and early spring, reed canary grass had been scattered to moderately heavy, but native species were still present throughout.
Here’s my first article about those test plots: https://grasslandrestorationnetwork.org/2020/04/30/herbicide-treatments-to-reed-canary-grass-clethodim-vs-glyphosate/
When we put in the test plots, we marked treated individuals with pin flags. Since that time, we have not done any invasive species treatments there. The area was burned this past spring. After the burn, we were able to find many of the pin flags and evaluate the status of the treated reed canary grass plants five years later.
To my surprise, there is still a visible difference between the plots that had been treated with clethodim during the fall, compared to the glyphosate plots. Reed canary grass in the plots had been initially sprayed on October 21, 2016. In all plots, there is less reed canary grass now than there had been in 2016. However, there is noticeably less reed canary grass in the clethodim plots than in the ones treated with glyphosate.
In addition to having less reed canary grass in general in the clethodim plots, none of the marked locations from the 2016 treatment have alive reed canary grass now. However, in the glyphosate plots, more than half of the marked locations from the 2016 treatment have alive reed canary grass now.
This is interesting because roughly half of the clethodim-treated individuals had been recorded as alive but stunted in 2017 and 2019, while the other half had been recorded as dead. In contrast, nearly all of the glyphosate-treated individuals had been recorded as dead in 2017 and 2019. This suggests that the initial glyphosate treatment killed the reed canary grass. Since it is a non-selective herbicide, it also killed the sedges and other native species in the immediate vicinity, leaving visible “holes” in the vegetation. These holes probably provided space for reed canary grass to germinate and re-establish in the sprayed locations. In contrast, since clethodim is a grass-specific herbicide, it did not affect sedges and native forbs, and those plants were able to expand into the former reed canary grass locations.
The plots that were treated in the spring of 2017 were a different story. Roughly half of the marked locations where reed canary grass had been sprayed on April 21, 2017, have live reed canary grass now. There was not a noticeable difference between the two types of herbicide used.
Based on this case study, it seems that late fall treatment of reed canary grass with clethodim herbicide is quite effective! For the fall treatments, we used 0.75% Volunteer herbicide (clethodim product), with 1.5% Powerhouse which has ammonium sulfate (AMS) incorporated in the surfactant. Of course, reed canary grass has been re-invading into the plots, but I was completely surprised that the specific locations treated with clethodim herbicide have resisted re-invasion by reed canary grass over the span of the past five years!
This makes me double-down on my conclusion in the initial article: “In many of our natural areas, and especially immediately under invasive plants, the seed bank may not be our friend. It may have seeds of more invasive plants instead of native species. Creating ‘holes’ in the vegetation with non-selective chemical use can just make space for more invasive plant recruitment: spray and repeat, spray and repeat. To break out of that cycle, an end game is to facilitate renewed dominance of the native matrix, which seems to happen better and more quickly when a grass-specific herbicide is used to treat the reed canary grass.”
To my past self – yes, I totally agree! And now I add: late fall is a great time to spray reed canary grass with clethodim herbicide.
grassland restoration network 
Great follow up.
Additional treatment in year 2 and 3 May be needed where graminoids or forbs are scarce. It’s been reported that Cleth is active only on the meristem of growing tips of grasses. Dormant or latent buds of rhizomes are not affected. That has certainly been my experience with Ky Bluegrass and Hungarian Brome as well.
Henry Eilers
Thanks for sharing these findings Juli! When spraying RCG in the Fall, do you spray plants that are waist high and already seeded out for the year or are you spraying basal leaves from plants that may have been cut earlier in the year? Also, do you have any anecdotal observations on the abundance of tall goldenrod in the different treatment plots?
Thanks!
This is the kind of work that really moves us all forward. Fantastic work, and thanks for doing it!
Thank you for continuing to follow up on this study and sharing with us!
Julianne, I have been following your posts very closely, as I do alot of RCG control and I use clethodim whenever possible. The label seems pretty clear that clethodim works when plants are actively growing; however, it appears that you have had success in late fall when plants are not (ordinarily) actively growing. Was the RCG mowed or cut earlier in the year that may have led to a period of active growth in the fall before you treated it with clethodim? Thanks for any clarification, and thanks for the great reporting!
Hi Pete, When we sprayed the reed canary grass in the fall for this study, we had not mowed or cut it earlier in the season. It was mostly waist-high RCG plants that had seeded out earlier in the year, although there were some smaller RCG plants intermixed that were sprayed too. The RCG had some greenish leaves on it still. Most of the other vegetation had fairly well senesced, although the sedges were still yellowish on some leaves. Hope that helps! – Juli Mason
Awesome, thanks Juli!
Hi Julianne,
I am currently conducting a study looking at the effect of fall spraying of clethodim and mowing on reed canary grass. I just finished analyzing the data from my two field seasons and am seeing very little effect from the clethodim. Is there an email address that I could contact you at? I’m interested in hearing more about your trials to see if I can pinpoint the reason for the lack of control I’m seeing.
Hi Juli and Hannah,
I have some positive news regarding fall herbiciding of RCG with clethodim! Last fall I sprayed a narrow band of RCG that rims a small wetland depression at Deer Grove FP. The wetland is dominated by Carex lacustris. Dry conditions allowed a window for me to experiment with a fall treatment of 1% clethodim. I sprayed approximately one half of the wetland edge. This week I went to assess the result, and was pleasantly surprised to see almost NO RCG along the wetland edge where I sprayed, but there was a fair amount of RCG on the edge left unsprayed. This result is very exciting!
I would love to continue spraying clethodim on the rest of the wetland this spring, however with the rainfall the wetland is holding ~4″ of water, so I will have to limit application to higher ground around the edge where it is dry. Thanks again Juli for this strategy!
Hi Pete, good to hear! Thanks for sharing your result.
So really, I just need to spray Clethodim in the fall and wait 5 years?
Hi Eric, As you can see from the photos, there is still reed canary grass in the treatment area – probably both ones that I missed in the initial treatment and new ones that have established in the years since then. So, I would definitely recommend follow-up treatments. But, it was a huge shock to me too to find the pin flags that marked treated individuals, and to see no live RCG in those locations five years later. I think it’s also worth noting that my treatment area had suppressed sedges and other natives under and amongst the RCG. I think a complete RCG monoculture that lacks natives may not respond as well, if it lacks natives to fill in the space when the RCG is hit.
Hi Juli – I’m working on RCG treatment in western Washington and have been experimenting with clethodim. My late April treatment of clethodim in a scirpus microcarpus wetland has seemed quite promising, with the bulrush becoming the dominant feature on the landscape a month after treatment, in comparison to non-treatment areas where the bulrush is still relatively healthy, but dominated by RCG by the late spring. I’ll be curious to see if the RCG bounces back next year or if the treatment really knocks it back.
I’m curious if you have any thoughts on efficacy of spring vs. late fall treatments of clethodim? Late fall can be tricky for spraying in wetlands here because we can get very wet, very quickly in the fall, closing the window for non-aquatic certified herbicides.
Thanks for all your work!
Hello, we have had seemingly mixed success with spring treatments to RCG using clethodim. Sometimes it’s nearly magical, kills the RCG, and sedges/rushes/forbs are untouched. Other times, the RCG is just stunted and comes back the following year. I am currently marking treated individuals in a number of different clethodim treatments this spring and early summer to see if there are weather or phenology patterns that correlate with the results, or maybe some areas are just getting a follow-up treatment and others areas are not, or maybe there’s a weird delayed effect going on… Anyway, let us know how the RCG in your treatment responds next spring, and (fingers crossed) maybe I’ll have results to report too.
Thanks for your response! I’m definitely recording as much as I can around my treatments to see what I can learn. I’ll try to write something up and send it over as I get data.
One more question: When do you start spraying clethodim in fall? Is there a certain life stage of RCG that you are looking for so that you don’t spray too early?
Hi Awaldrop, for the fall application in my experiment, I was aiming for the window after the first good frost, when the RCG and sedges are still green but other vegetation has started to senesce. It’s a lot easier to spot the RCG after other vegetation has started to brown, but I haven’t tested if earlier fall treatments would work as well. – Juli Mason
This would be the first good frost but not a hard freeze, correct? A hard freeze will make everything go dormant.
Yes, after the first good frost. The reed canary grass and sedges still had a bit of green in the leaves, but most other vegetation had started to senesce.
I am still a little perplexed. You wrote “This is interesting because roughly half of the clethodim-treated individuals had been recorded as alive but stunted in 2017 and 2019…….”. When you saw these stunted reed canary grass plants you did not spray them again with another shot of Clethodim?
And over a 5 year period all your treated and marked plants sprayed with Clethodim were dead. But also some new plants had sprung up in the plots, but fewer in the Clethodim plot vs the Gylphosate plots.
Thanks for helping me understand. Eric
Hi Eric, that’s all correct. We didn’t do any follow-up herbicide treatments in that area. I was shocked when we found the pin flags that had marked the locations of the treated reed canary grass plants five years later and nearly all of the plants treated with clethodim were gone. And, on a qualitative basis, the plots that had been treated with clethodim in the fall looked much better (less reed canary grass, more natives) than the glyphosate plots, and better than they had looked five years previously before the initial treatment. I wasn’t expecting this result, especially after the reed canary plants just looked suppressed during the first year or two after treatment – I had expected them to rebound fully and for the plots to look much worse. However, this was one treatment, small sample size, etc. I’m curious to see the results if I and others mark treated individuals and track them through time across more treatments.
Pingback: Clethodim Treatment Tracking – Reed Canary Grass | grassland restoration network