Herbicide Treatments to Reed Canary Grass, Clethodim vs. Glyphosate

Subtitle:  We need to focus on winning the war and not just the battle

By:  Julianne Mason, Restoration Program Coordinator, Forest Preserve District of Will County

April 21, 2020

Like all invasive species, reed canary grass is a formidable opponent.  It is aggressive, robust, produces tons of seed, spreads rhizomatously, and can form monocultures in many wetland community types.  Even during these shelter-at-home days, we have been able to do solo work to treat reed canary grass in high priority areas.  Our current work restrictions have highlighted the need to be both strategic and effective in our treatment actions.

Several years ago, I put in some test plots to compare the longer-term effectiveness of treating reed canary grass with glyphosate, a non-selective herbicide, compared to treating it with clethodim, a grass-specific herbicide.  I used 2% AquaNeat (glyphosate product) with 0.4% Pen-A-Trate II, which is our normal nonionic surfactant.  For the grass-specific herbicide treatments, I used 1.5% Volunteer (clethodim product) with 1.5% Powerhouse, which has ammonium sulfate (AMS) incorporated in the surfactant.  Adding AMS, a fertilizer, to the herbicide solution is suggested on the clethodim label.  Unfortunately, clethodim is not approved for use over water; there is no aquatic-approved, grass-specific herbicide.  The clethodim treatments were made when the treatment areas were fairly dry and no standing water was present.  Plots were sprayed in the late fall, early spring, or late spring in 2017 and 2018.

Photo 1 caption: Reed canary grass plants were treated with clethodim or glyphosate herbicide and marked with pin flags which were color coded to the treatment type.  In this plot, the clethodim treatment was pretty effective since no flowering reed canary grass heads are visible in the July photo.

Not surprisingly, the reed canary grass that was treated with glyphosate died quickly.  However, glyphosate is a non-selective chemical, and there were dead zones resulting from overspray during the treatments.  This was especially significant for the late spring treatments. The reed canary grass was big and tall when it was sprayed, resulting in a lot of herbicide spray used and a lot of overspray death.

Photo 2 caption:  Reed canary grass that was sprayed with glyphosate died quickly.  The quick kill was gratifying, but note the dead zones from the non-selective chemical’s overspray.

The dead zones were less obvious but still present during the late fall treatments, which were made after many other species had gone dormant, and during the early spring treatments, which had less overspray because the plants were treated when still small.  The dead zones were mostly colonized by reed canary grass seedlings, which is not surprising. There must be a lot of reed canary grass seed in the seedbank under reed canary grass plants.  Within two years, the reed canary grass stand that was treated with glyphosate during the late spring had returned to a robust reed canary grass stand.  If you had seen the end result, you would not have believed that the reed canary grass had been sprayed with herbicide and killed just two years prior.

Photo 3 caption:  By the second year after treatment, the area treated with glyphosate had returned to a reed canary grass stand.

In contrast, the reed canary grass plants that were treated with clethodim took many weeks to months to turn yellow and senesce.  All of them looked sick and none of them flowered, but they didn’t look especially dead after treatment.  Even a year or two after treatment, many of them had resprouted weakly and were not fully dead, just severely stunted.  However, the other plants in the area that had been previously suppressed by the reed canary grass, like sedges and wetland forbs, were unaffected by the herbicide treatment and had greatly increased in vigor and cover.  Two years after the initial herbicide treatment with clethodim, the treated area had recovered the native cover and diversity of a sedge meadow.

Photo 4 caption:  In the clethodim treatment plots, reed canary grass was killed or significantly reduced.  Sedges and forbs were unaffected by the herbicide treatment and greatly increased in vigor and cover.

Overall, the best results in this study were: 1) treated in the late fall with 2% glyphosate, when the reed canary grass was still slightly green but almost all the other vegetation was dormant, and/or 2) treated with 1.5% clethodim in the fall or early spring, with a follow-up treatment to missed individuals and ones that did not fully die.

Photo 5 caption:  Reed canary grass was treated with clethodim herbicide, and a sedge-dominated community remained two years after the treatment.

Although following up with multiple treatments of clethodim may seem like more work, I think it is worthwhile and actually less work in the end because it allows the native sedges and forbs to flourish.  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.

Photo 6 caption:  Comparison of two adjacent plots where the reed canary grass was treated with clethodim and glyphosate herbicide, viewed two years after the treatment.

NEWS: Due to Covid 19 crisis we are postponing the GRN workshop of this August to next August, 2021 in Barrington, Illinois.

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10 Responses to Herbicide Treatments to Reed Canary Grass, Clethodim vs. Glyphosate

  1. Kevin Scheiwiller says:

    Great article! I have noticed very similar results, but only anecdotally. Thank you for taking the time to compare the two treatments.

    I use clethodim for pretty much all my wetland plantings and will only use glyphosate in the areas that never dry out. It has been producing very similar results!

  2. Ron Cress says:

    Bill and Julianne,
    Thanks so much for this article and study. Bill, as you know, we’re got a pretty signifciant RCG problem in a portion of our little prairie north of Kirkland. The land owner was ready to spray with glyphosate until I suggested (for multiple times :-)) that it look at clethodim. Thankfully he wound up using clethodim.

  3. Bryon Walters says:

    Interesting! I will try Cleth in any high and dry patches of RCG that we find. Thanks for posting.

  4. Juli Mason says:

    Glad that you’re finding similar results

  5. Eric Smith says:

    I wonder if the RCG areas treated with cleth revert back to pre-treatment levels 4-5 years down the road without repeated management

  6. My experience has been the grass herbicides need reapplication every few years. The herbicide sets back the grass, but does not kill it. I have turned reed canary grass meadows to sedge meadows with a few annual applications of grass herbicide. I am pretty sure if I stopped for five years the reed canary grass would be back to dominance.

    The glyphosate/imazapyr will kill the grass, and all else, and the RCG will likely return without consistent follow up. Sigh.

  7. Peter W says:

    For fall applications of Clethodim, what does “late fall” mean? After the first frost? When average temperatures fall below a certain point? I’m getting ready to spot treat pockets of RCG in our prairie restoration (we’re in south central WIsconsin, west of Madison) with Clethodim for the first time. In the past I tried Poast (Sethoxydim), but that resulted in only temporary top-kill. Heavy rains from the past two years led to a near-complete RCG takeover of one section, and as a last resort I wiped out most of it with Glyphosate last fall. Maybe the floods were a mixed blessing.

  8. usagi2215 says:

    Hi Peter, the “late fall” treatments were done on 10/21/2016 and 10/20/2017 in Joliet, Illinois. Both dates were a week or few weeks before the first frost, on relatively warm fall days (high temps of 55 and 80, respectively. The reed canary grass and some sedges still has green on them, but other vegetation was pretty well senesced. Good luck on your treatments!

    • plweil says:

      Thanks @usagi2215, that’s very helpful. For us that would probably mean the last week or so of September, depending on the temps. Just gotta make sure the warm season grasses are done and the RCG is not dormant…

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