The Case of the Undead Buckthorn (Foliar Treatments with Triclopyr Herbicide)

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

We have had a recurring issue over the past decade with common buckthorn (Rhamnus cathartica) not truly dying after being foliar treated with triclopyr 3A herbicide in dolomite prairie habitats.  The buckthorns are 2-5’ tall, multi-stemmed re-sprouts that have been repeatedly top-killed by fire over the past several decades.  The buckthorns appear to die properly at first.  After the foliar herbicide treatment, the leaves yellow, then turn brown and fall off prematurely.  However, lots of the shrubs re-sprout vigorously the following year.  This has happened with foliar treatments done in the early summer, late summer, and fall.  Apparently buckthorn is a calciphile in its native habitat, so perhaps it is just particularly hard to kill in calcareous habitats here?  Has anyone else observed buckthorn or other invasive shrubs appearing to die after foliar herbicide treatments, only to rebound vigorously the following year?

Photo 1 caption:  Undead buckthorn.  Many of the buckthorns that had been foliar treated with triclopyr herbicide re-sprouted vigorously the following year.  Photo taken a year after the herbicide treatment.

Buckthorn does seem to die reliably from cut stump and basal bark treatments of triclopyr 4 herbicide in bark oil, in the same dolomite prairie habitats.  Therefore, I hypothesized that perhaps the ester formulation of triclopyr might be more effective than the amine formulation.  To test out this theory, I marked buckthorns that were foliar treated with triclopyr 3A or triclopyr 4 at concentrations of 2%, 5%, or 10% during September 2018 at Lockport Prairie.  All treatments included 1% MSO and 0.4% PenATrate II surfactants.  Around 50-100 shrubs were included in each treatment.  We put color coded flagging on each treated buckthorn to keep track of its treatment type.

All of the treated shrubs appeared to die after treatment; their leaves turned brown and fell off prematurely last fall.  However, many of them rebounded vigorously the following spring.  Contrary to my expectation, I didn’t see any significant difference in mortality rates between the two different formulations of triclopyr (3A or 4), as evaluated 1 year after treatment (YAT).  However, there was greater mortality using the 10% concentration of triclopyr herbicide compared to lower rates.

2% Concentration.  Despite the promising results immediately after treatment,  less than 10% of the buckthorns that had been foliar treated with 2% triclopyr were dead this summer (1 YAT); nearly half of them had re-sprouted vigorously from the base while the rest of them fully leafed out from the top.

5% Concentration.  Less than 25% of the buckthorns that had been foliar treated with 5% triclopyr were dead this summer (1 YAT).  Around one quarter of them fully leafed out from the top, while around half of them resprouted vigorously from the base. 

10% Concentration.  Around 70% of the buckthorns that had been foliar treated with 10% triclopyr were dead this summer (1 YAT).  Conversely, nearly one-third of the treated buckthorns had resprouted and were still alive.

Based on these results, I would recommend using 10% triclopyr as a foliar treatment and be sure to follow up on re-sprouting individuals the next year.  Or, I might try basal bark/base spraying them with triclopyr 4 in an oil-water emulsion.

Bottom Line:  Beware of invasive shrubs appearing to die right after foliar herbicide treatments, only to re-sprout the following year.  Has anyone else experienced a similar thing with buckthorn or other invasive shrubs?  I find it hard to believe that our buckthorns are truly unique.  It is a shame to spend time and money on treatments that are not effective.  Plus, herbicide treatments cause collateral damage to other plants.  It is a double shame to kill off-target species and not actually achieve the goal of addressing the invasive species population.  Mark some of your foliar treated shrubs and check them next year to make sure that the treatment actually worked.  Do you have undead invasive shrubs too??

About Grassland Restoration Network blog published by Bill Kleiman.

Bill Kleiman's daytime job is manager of Nachusa Grasslands. We are looking for guest authors on various topics of grassland habitat restoration. Contact me with your ideas or drafts.
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16 Responses to The Case of the Undead Buckthorn (Foliar Treatments with Triclopyr Herbicide)

  1. Julianne, you are not alone in experiencing the ability of buckthorn to resprout following treatment. Anecdotally, I have also witnessed late-season foliar treatments with Garlon 3A that did not achieve effective kill of buckthorn sprouts upon review the following year.

    I recommend reading through posts by Tom Brock at Pleasant Valley Conservancy about their experience documenting buckthorn resprouts from dormant buds on the root system, in addition to any seed bank. Here is a good summmary of their work over the past 20 years:

    Also these powerpoint slides:

    Click to access Buckthorn%20Ecology%20and%20Eradication%20for%20Web.pdf

    Tom’s “leaf spritz” method with Garlon 4 in bark oil may be a good approach in your prairie setting to avoid damage to desirable veg:

  2. Julianne Mason says:

    Hi Adam, Thanks for the corroboration and useful links. I will give the leaf spritz method with Garlon 4 in bark oil a try. – Juli Mason

  3. Mike Saxton says:

    Thanks for the thoughtful piece, Juli. Nice work!

    According to the manufacturer, Garlon 4 and water for foliar application generally provides better woody control than Garlon 3A, which will only provide “fair to good control when applied as a foliar treatment.”

    DowDupont was broken up into 3 companies (Dow dedicated to commodity chemical production, DuPont to specialty chemical production, and Corteva dedicated to agricultural chemicals.) Corteva has a publication called Techline News (so…yeah…you have to consider the source).

    But they do have a great breakdown of the differences between triclopyr ester (G4) and triclopyr salt (Vastlan or G3A).

    “Garlon 4 Ultra will provide better woody plant control in general than Vastlan (Garlon 3A) when applied as a foliar, basal cut stump, and basal bark treatment. Vastlan will provide good control as a cut surface application (always mix with water) and will provide fair to good control when applied as a foliar treatment (must add an approved adjuvant also).”

    It’s unfortunate that you’re not seeing this in your field studies.

    Down here in Missouri with our hard water, I’ve taken to adding ammonium sulfate to our herbicides. After speaking with a crop scientist, they recommended 3oz – 6oz added per gallon. Low end during ideal growing conditions and higher end during times of plant stress. I think this has helped to increase our mortality rate for foliar sprayed privet (Ligustrum obtusifolium). This is anecdotal though! I don’t have data to support that.

    • Hi Mike, thanks for the feedback! Like you, I had thought that hard water in the root zone might be neutralizing the herbicide acid and making it less effective. So, during June 2016, I did a set of trials comparing 3A foliar treatments on buckthorn in a dolomite prairie with and without the addition of AMS. I was using 2.3 oz/gal of AMS, which is lower than your recommended range. In my trial, there wasn’t any significant difference in mortality rates between buckthorn foliar treated with 3A with or without the addition of AMS. Not the result that I was hoping for! But, perhaps I should try again with a higher rate.

      It would be great if you could mark some of the plants that you treat with AMS added, and some plants treated without AMS added to see if your anecdotal observations are supported.

  4. James McGee says:

    “Has anyone else observed buckthorn or other invasive shrubs appearing to die after foliar herbicide treatments, only to rebound vigorously the following year?”

    All the time, and not only with foliar application but also with cut stump and basal bark applications too. The concentration of herbicide being applied is important. However, concentration alone will not predict whether an individual buckthorn will be killed. This is because some of those two to five-foot-tall sprouting buckthorns are about 30 years old and some are only a few years old. A two to five-foot-tall buckthorn grub that is 30 years old has a much larger diameter root and a lot more tissue than a buckthorn grub that is only a few years old. It takes a much larger dose of herbicide to kill larger-older sprouting buckthorn (substitute any other woody species) grubs. For foliar spray, a larger dose can be applied by increasing the concentration (as you trialed) or repeatedly applying more herbicide as the plants leaf out or send up new sprouts. I know it seems like time and money is wasted when a treatment does not kill the intended target. However, achieving a high kill rate needs to be balanced with potentially using too much herbicide. The best way to minimize the amount of herbicide used is to apply a dose proportional to the diameter of the trunk, or crown in the case of grubs, and follow up by retreating any survivors.

    I keep track of all my applications. I use this information to constantly adjust my application method. My goal is to have 95 % of treated individuals controlled on the first pass. Interestingly, I am reducing the concentration of herbicide I apply to frills and the amount of herbicide I apply during basal bark treatment to try to get to my goal of 95 %. The exact opposite problem you seem to be experiencing with foliar application.

    • Hi James, I completely agree that invasive shrubs treated with foliar herbicide applications are affected very differently from basal bark applications. Keeps life interesting, right? Thanks for the feedback.

  5. Julianne
    I love your analytical approach to these problems!

    I know I’m late to this buckthorn party but we experienced a similar problem in our 2019 foliar treatments of glossy buckthorn (Frangula alnus) in the Chiwaukee Prairie. We used 5% Garlon 4 Ultra with adjuvants of 1 oz/gal FS MSO plus 1 oz/gal Liberate. We experienced distressingly low mortality rates 1 YAT. I have since learned that several contractors are using Loveland MSO Concentrate with Leci-tech® at 2 oz/gallon with Garlon 4 so that is what we tried this year.

    I don’t know how important adjuvant chemistry and concentration are in these cases but we are hoping for improved results next year.

  6. Nathan Robertson says:

    I finally tabulated the one year after treatment (YAT) results of our glossy buckthorn triclopyr foliar treatments in 9 different trial plot areas on dense buckthorn stands in the Chiwaukee Prairie. All of these buckthorn were old and had been mowed and triclopyr treated more than once a number of years ago with little effect on them. These plots were treated in 2020 and scored YAT (August 2021). We treated with either 5% Garlon 4 Ultra (triclopyr ester) or 5% Vastlan (triclopyr choline) or with a combination 5% G4U plus metsulfuron at 0.02 oz/gallon. We also compared results of Liberate (Loveland) with MSO Concentrate (Loveland) used with 5% Vastlan.

    All looked to have died during the initial year of treatment, but our YAT results were far less than great. Mortality rates ranged from 0% to 95% using the same treatment regimens. The addition of metsulfuron, at the rates used, had little to no effect on mortality. Our average mortality rate was 25% with a range of 0% to 95% mortality. This overall mean result was surprisingly similar to the results Julianne got in her common buckthorn trials. There was little difference seen in the effects of Garlon 4 Ultra compared to Vastlan. I’m baffled by the high variability and low overall mortality of our results.

    The 2 trial areas that achieved 80% to 95% mortality had been treated 2 years in a row with 5% G4U but showed low to no mortality YAT from the initial treatment. The 6 trial groups of buckthorn treated for the first time last year all had low mortality that ranged from 0% to 30%. Anecdotally, I had seen other dense groups of buckthorn result in high mortality rates when treated 2 years in a row even though they didn’t die after the first year’s treatment. I hypothesize that the first year treatment does kill part of the root system, weakening the plant so that when treated a second year, death results. Against this hypothesis, there was one large trial area that had been treated for 2 years in a row but 0% mortality resulted YAT. We will try 10% triclopyr treatment this year and will keep you posted.

    Did anyone try 10% triclopyr treatments last year? We would love to hear of your results whether anecdotal or otherwise!

    • Nathan, There is a lot you say there. How about we make it a full blog. I have some questions to ask you to help me understand your various treatments. Does this seem a good idea to make it a blog to you? If so, email me the text and any photos you might have.

    • usagi2215 says:

      Hi Nathan, Thanks for sharing your results! It’s encouraging that the two years in a row treatment seemed to have more mortality. I did a series of trials with foliar treatments of 2%, 5%, 10%, and 15% triclopyr 3A (and maybe 4) on buckthorn many years ago. I’ll dig out the results when I get back to my office later this week and post them.

    • Juli Mason says:

      Hi Nathan, Regarding your question about trying 10% triclopyr as a foliar treatment on buckthorn, yes I tried twice. The first set were sprayed in June 2016, and a different area was sprayed in September 2018. Both areas had been foliar treated in prior years too, but I wasn’t tracking if individual plants were sprayed for consecutive years. Here are the 1 YAT results. The take-home message to me is that there’s too little mortality and too much collateral damage with foliar treatments, but I don’t have an effective alternative for these multi-stemmed buckthorn resprouts that have been repeatedly topkilled by fire and herbicide.

      2016 plots:
      3% triclopyr 3A – 43% dead
      5% triclopyr 3A – 67% dead
      10% triclopyr 3A – 58% dead
      15% triclopyr 3A – 64% dead

      2018 plots:
      2% triclopyr 3A – 8% dead
      2% triclopyr 4 – 2% dead
      5% triclopyr 3A – 15% dead
      5% triclopyr 4 – 23% dead
      10% triclopyr 3A – 71% dead
      10% triclopyr 4 – 67% dead

      • Nathan Robertson says:

        Thank you, Juli!
        You have some very interesting data. Besides the application time of year, are there any other factors that you can think of that may account for the much higher mortality in the 2% and 5% triclopyr in year 2016?

        Do you know if the 2016 group and the 2018 group were of significantly different ages?

        Were each of these treated populations fairly solid stands of buckthorn? How many plants or how big of an area per variable?

        I forgot to mention that I my studies so far, I haven’t seen any significant differences in mortality attributable to adjuvant used.

        I don’t think these plants came from Europe, I think they came from Hades!

  7. Kyle says:

    I worked on a riparian restoration project near Stevens Point, WI and we used chainsaws to cut every buckthorn we could in June, and then made a couple passes with Garlon to treat the stumps and kill the seedlings. It’s just plain hard work getting rid of buckthorn. Interestingly though, this was an “old growth” stand. We cut over a hundred year old Rhammus cathartica from the center of this plot. James McGee raises a good point yhat concentration and amount of application sbould be somewhat limited to avoid putting too much into the environment at once. It’s just nasty stuff, and you should be proud if you ever get rid of it! Good luck!

  8. usagi2215 says:

    Hi Kyle, I have had good results using the cut stump method on large, single-stemmed buckthorn as well. However, the vigorous, multi-stemmed re-sprouts that have been repeatedly top-killed by fire and/or foliar herbicide seem to be another matter entirely.

    Nathan, to answer your questions, the buckthorns that were treated in 2018 were in a dense area where we have top-killed them with fire and/or herbicide every 2-3 years since the 1980s or so. The ones treated in 2016 were in a sparser section that may have had more younger individuals. The relative age and susceptibility to herbicide treatment likely plays a role, but I think your 2-years in a row line of thinking might have some legs. The 2016 treatment area had been pretty thoroughly foliar treated the year before, but the 2018 treatment area had received a rather sketchy treatment in the year before I did the test plots.

    Both treatment areas are mesic dolomite prairie. They were buckthorn thickets several decades ago, but the prairie vegetation recovered after the big buckthorns were removed and the area managed. Unfortunately, back when the buckthorns were initially cleared, they didn’t use herbicide in higher quality natural areas and the buckthorns re-established as very robust, multi-stemmed re-sprouts. So, we have just been suppressing them with fire for a long time and haven’t been able to get them to die with herbicide treatments in more recent decades. Our risk is if we miss a few years of burning, they will be back to a dense thicket very quickly.

    Prairie diversity is still way lower than in areas that had not been invaded with buckthorn, and I would like to get the buckthorn to die properly so that the prairie can recover more fully. But, so far, we have spent a lot of resources (time and herbicide) out there over the years without getting a good management result. Maybe we just haven’t been consistent enough in treating them thoroughly for consecutive years. However, I have been disillusioned about the effectiveness of foliar treatments on invasive woodies, and am moving toward more and more wicking or base spraying with triclopyr 4 (15%) in bark oil.

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