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.

https://grasslandrestorationnetwork.org/2018/11/08/julianne-mason-on-basal-bark-applications-using-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??

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6 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: http://pvcblog.blogspot.com/2017/11/buckthorn-eradication-20-year-story.html

    Also these powerpoint slides:
    https://pleasantvalleyconservancy.org/pdf/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:
    http://pvcblog.blogspot.com/2010/07/invasive-plants-and-herbicide-leaf.html

  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). https://www.techlinenews.com/herbicides/2014/answers-to-frequently-asked-questions-about-woody-plant-control-in-prairies-using-garlon-4-ultra-and-garlon-3a

    “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.

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