Do Nothing about Invasive Plants

Paul Hetzler, ISA Ceritfied Arborist

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 most 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 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.

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