Monday, May 3, 2021

When it comes to garlic mustard, doing less is more

Until recently, ignoring problems in hopes they’ll go away hasn’t served me well. However, a decade-long study done by Cornell University researchers has clearly shown that avoidance is the best way to manage garlic mustard (Allaria petiolata), a pernicious exotic plant. Evidently I’ve been doing a great job in the fight against this aggressive and troublesome invader.

Native to most of Europe and parts of western Asia and northwestern Africa, garlic mustard is in the cabbage and broccoli family (Brassicaceae), and indeed was imported to North America as a culinary herb in the early 1800s. It’s not entirely evil, as it has the spicy tang of mustard with a hint of garlic, and can be used as a base for pesto and sauces, and to flavor salads, soups and other dishes. Unfortunately, eating it has not worked well as a control strategy.

Garlic mustard is a biennial that begins as an inconspicuous first-year plant (rosette). At a glance, its rosettes look similar to wild violets, having triangular, somewhat heart-shaped leaves that have coarsely toothed margins and wrinkled leaf surfaces.  In the second year it sends up a tall flower spike, the four-petal white flowers developing into slender pods (siliques) bursting with tiny round seeds. This is one of garlic mustard’s unpleasant features, as it loads the soil with seeds that remain viable for ten or more years.

Like all invasive plants, garlic mustard is not browsed by herbivores (if you don’t count vegetarian humans), and has no effective insect pests or diseases to keep it in check. As mentioned, it gets high marks for reproduction, and can form thick monocultures in forest environments. Its roots exude compounds that alter the soil chemistry to favor its survival at the expense of other species. Known as allelopathy, this mechanism also harms mycorrhizae, symbiotic root fungi which contribute greatly to tree health. When dense armies of these plants compete for water, nutrients and sunlight, natural forest regeneration is curtailed and native ground cover is stressed.

Sounds like we should gather a posse and rise up against this intruder; pitchforks, torches, and pikes at the ready. Well, yes and no. If garlic mustard has just appeared at a location in the past one or two years and their numbers are low, yes – yanking them out by the roots is the thing to do.

But according to Dr. Berndt Blossey, a Cornell University conservation biologist who specializes in invasive plants, pulling up large swaths of garlic mustard is not only futile, it is worse than leaving it alone. It bears echoing: When well-intentioned people rip out this stuff, it actually prolongs the infestation period because the plant self-limits (more on that below) if undisturbed.  Also, these mass garlic mustard-ectomy events do more damage to the ecosystem than the target species itself does.

There are cases where research seems pointless because cause and effect are so obvious:  maple sap flows up from the roots during the day; goldenrod causes allergy symptoms; and garlic mustard wipes out native wildflowers and adversely affects salamanders. These assumptions make sense, given the “evidence,” but upon close examination, all of the above statements are false.

Dr. Blossey has long contended that deer abundance and non-native earthworms are the drivers of garlic mustard infestation. Garlic mustard only establishes after earthworms have invaded a site for some years, he says, and although how deer spread earthworms is not yet known, they apparently do, as exclusion plots show. I first heard Berndt’s idea that well-established garlic mustard should be left alone in 2014 at a talk he gave at Cornell. I was surprised, and admittedly rather skeptical. But he and his team have now done enough controlled trials and amassed enough evidence to back up his assertions.

It turns out that while garlic mustard competes with native species, it does not displace them where deer are excluded or drastically reduced in number. And it is earthworms, not our maligned invasive plant, which make a neighborhood less attractive to salamanders. Furthermore, garlic mustard dwindles in biomass, plant vigor, and site prevalence over time. Within ten to 12 years it becomes scarce as a species, the remaining plants greatly stunted.

Side-by-side controlled trials showed that where garlic mustard is “managed,” the plants are considerably larger, and cover a much higher percentage of a site (at times by an order of magnitude) than the sections where nothing has been done. Not only that, but biomass on the managed sites tended to be roughly stable over the ten-year time frame studied, whereas it declined year after year in the unmanaged plots.

Pulling garlic mustard where it is abundant prolongs its run. It also robs a great deal of nitrogen, macro- and micronutrients, and organic matter from the ecosystem. Mass-removal also results in the site being trampled, and runs the risk that soil and native plants might be inadvertently removed.

A much better use of our time and energy, Dr. Blossey advises, is to scout sites that aren’t known to have garlic mustard yet, and also to kill as many deer as possible. Especially the latter.

An interesting side note is that if deer were managed to 5-7 per square mile, not only would it drastically reduce the rate of garlic mustard spread, Lyme disease would cease to be a human-health threat (this from Dr. Paul Curtis, the NY State Extension Wildlife Specialist at Cornell University). I say amen to that!

Professor Blossey’s February 26, 2021 talk “When Doing Nothing is the Best Invasive Plant Management Tool” can be found at https://www.youtube.com/watch?v=vRQal0Hq5nM

A former Cornell Cooperative Extension Educator, Paul Hetzler is often in a recliner, helping to fight garlic mustard.

Photo courtesy of New York Invasive Species Information: http://nyis.info/invasive_species/garlic-mustard/

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Paul Hetzler

Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.

You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World




15 Responses

  1. John Jongen says:

    Paul Hetzler, I respectfully disagree with a do-nothing strategy of exotic pernicious weed control for Garlic Mustard. Having successfully de-weeded my half-acre gardens from Garlic Mustard by aggressively pulling and mowing the second-year plants before they produce viable seeds, gradually expending all of the remaining seeds in the soil. That strategy also works for Jewelweed, Bitter Cress, Celandine Poppy, Bindweed, Swallowwort, and other unwanted invasives. And it gradually reduces the interest of roaming deer herds.

    • Paul Hetzler says:

      Hello John,

      Thank you for sharing your experience — I am pleased to hear that you were able to eradicate GM from your half-acre plot.

      I think Dr. Blossey’s message is that hand-pulling can in fact work for smaller infestations, as you demonstrated. What he takes exception to is the well-intentioned groups who haul pickup-loads of GM out of large, firmly established, multi-acre infestations.

      Although I was reporting on his team’s findings, I do think large infestations could potentially be conquered through annual pulling by a very dedicated and energetic group over many years. But I have to wonder what the ecosystem cost would be from removing large amounts of N, P, K, Ca, Mg, and other nutrients from the site, as well as from the disturbance. I don’t know the answer, but it’s an important consideration.

      • Melissa s. Listman says:

        I have been working on several acre plots to remove honeysuckle, oriental bittersweet and garlic mustard. I weed garlic mustard all year long if soil is thawed and compost it on site. I try and weed the garlic mustard before its flowering stage, but do have to remove it also when it flowers. In really sensitive wooded areas in which I have been working on for several years, I bag it and remove it if its flowering but in other areas like sunny edges and field, I leave it piled on site. In the sunnier areas, the asters, pokeweed and goldenrods seem to compete well with the garlic mustard and hopefully overtime the garlic mustard will decline.
        I have observed in woodlands where I have reduced the population of garlic mustard, there are native woodland flowering plants, grasses, and sedges returning to the area. My property is hunted every year, but I still have too many deer.
        I am really interested in the earthworm part of the story, since I have been buying and planting potted native trees and shrubs in several areas and therefore are likely adding more earthworms. I have also been adding some woodland plants, but usually do these by seed or bareroot. Do you know where I can get more information on the earthworm issue?

  2. Michael Welles says:

    Land Conservation Practitioners here in Massachusetts are both aware and not in support of this study. Our experience is somewhat different. You might want to further explore the pros/cons of the study. thank you for your great reporting.

    • Tim-Brunswick says:

      Apparently you presume that the approach by Massachusetts is spot on??…, how about you explore alternative approaches by other States other than Massachusetts??

  3. Martha says:

    Wow. This is good news, indeed, despite the fact that I’ve been pulling this weed for year. My theory is it spread in my woods after a town bridge replacement next to us.

  4. Boreas says:

    In what habitat(s) does this invasive grow? I don’t believe I have seen it, yet I constantly have deer on my property. But my soil is very sandy and only really have earthworms in my garden beds that have more organic matter.

    Notwithstanding, I am all in on reducing deer numbers – especially in populated areas. DEC hunting laws forbid discharge of firearms close to roads and habitation – and deer are quick to realize this. It is theoretically possible to obtain permits to cull deer in populated areas, but seems to be a rare occurrence. I am lucky if I ever get a response to a call or email that I have sent to DEC for any reason – burn permits for example here in Region 5. Com. Seggos, are you listening? If we have “adequate” DEC staffing/funding, why can’t we get a response? It is either policy to ignore residents, or the staff are too busy.

    • Paul Hetzler says:

      Hello Boreas,

      Same thing with my woods near Colton — no garlic mustard. Fingers crossed it doesn’t find us…

      It prefers hardwood stands, though it can show up in mixed woods. Very seldom found on sites having all conifers.

      Good luck deer hunting this year!

  5. Tammy says:

    “There are cases where research seems pointless because cause and effect are so obvious: maple sap flows up from the roots during the day; goldenrod causes allergy symptoms; and garlic mustard wipes out native wildflowers and adversely affects salamanders.”

    It’s actually ragweed that causes most of those allergies, not goldenrod. Though people blame goldenrod because the two plants flower at the same time.

  6. Allen Whipps says:

    1) “A much better use of our time and energy, Dr. Blossey advises, is to scout sites that aren’t known to have garlic mustard yet”
    My issue: Then what? So, I scout a site it hasn’t hit yet. then I do what?

  7. Allen Whipps says:

    2) Deer. “and also to kill as many deer as possible. Especially the latter.” “while garlic mustard competes with native species, it does not displace them where deer are excluded or drastically reduced in number. ”

    My issue: All ‘leave it’ options seem predicated on reduced deer populations. Excluding or reducing deer numbers is not an option. I’m in a suburb surrounded by woods. There are way too many deer–no one can grow flowers unless you’re perfect at applying deer spray. And, any single individual within several square miles can stop a planned bow hunting deer culling.

  8. Allen Whipps says:

    3) “…while garlic mustard competes with native species,”

    My issue: Then they DO compete with native species. After going after it, last year, I’ve found only a few this year on my property and near a trail where I also pulled them up–not the large number we had, last year–and now there are lots of wildflowers.
    Again, the long-term solution seems to be predicated on an area with a lack of deer and therefore a lack of non-native worms (I know from other reading that those worms leave the surface riled up.) I’ll accept the mysterious deer/non-native worm connection for the sake of argument.

  9. Allen Whipps says:

    I think it could have been more clear about ‘worms.’ I’m not sure, but I think he’s referring to the invasive earthworms, not the beneficial kind. aka, “jumping worms,”

    “It seems to displace its organic matter with a grainy sand like particle that’s basically part of what it produces when it consumes everything through its body,” said Tangora. “In the making of this kind of coffee like texture, it really robs the soil of nitrogen and other nutrients that help our plants survive.”

  10. Mary K. says:

    Getting rid of the deer isn’t easy either. The # of hunters, especially young hunters declines annually, and most of the hunters want to get a big buck and shy away from killing does. The deer pop in WI woodland areas has exploded in recent years as has GM.

    Based on our observations, garlic mustard has not yet invaded our woodlands that have very tall deciduous trees. Lowland valley areas with tall ferns (3-4 feet tall) also seem to deter garlic mustard. Woodland edges seem to be the preferred GM areas.

  11. Elias Putzig says:

    If you find a dense growth of garlic mustard on a hike in the addirondacks, leave it alone. If you find a patch in your neighbourhood, pull with gusto, just make sure to keep coming back for a few years.

    I think this is irresponsible journalism. It looks like the author was going for a catchy title, and putting all the drama he could in the piece. It is being used to argue that pulling garlic mustard, in general, is pointless. The point, that a poorly maintained intervention can be worse than nothing, could have been madewithout causing the confusion, but perhaps it wouldn’t have been as catchy.

    It is always good to pull sparse garlic mustard, when it isn’t yet dominating the area, assuming you don’t spread seeds everywhere. It isn’t too hard to maintain an intervention; you just need to keep the second-year plants from seeding, and occasional weeding will do the trick. It is only bad to take a passerby approach or e.g. a once-a-year approach with an area that is grown over with garlic mustard.

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