This is an excerpt from a zine, “The Troubles of ‘Invasive’ Plants.” Check out the whole project here.
By Nicole Patrice Hill & Kollibri terre Sonnenblume
“Invasive” plant species are removed using a variety of methods that can be classified into three broad categories: mechanical, biological and chemical. All means in these categories have their own varying rates of success and drawbacks. As practiced, few are effective at removing their targets without killing non-targets, and only then at very small scales. As the size of a particular project grows, so does the likelihood of unintended consequences and collateral damage.
Mechanical means include mowing, tilling, weed-whacking, smothering (with organic materials like mulch or synthetic ones like nylon fabric), soil solarization (covering the ground with plastic to kill plants and seeds), flooding (or alternately draining water if the target species is aquatic), prescribed burning, and simply pulling, hoeing or chopping by hand. Each of these processes varies in its precision (as measured by how many non-target species are also affected). Some, such as flooding, burning or smothering, affect all plants in the area of application. The particularity of others depends on the tools or materials used and on the operator’s skills, attention and concern. Unfortunately, it’s too often the case that operators lack those characteristics or are not properly equipped. The result is damage to non-”invasive” plants, the ones whose well-being is ostensibly of such strong interest.
In some cases, entire landscapes are scraped of all vegetative life. In the example of a project to remove European Beach Grass on a beach in Oregon, before and after photos document a process whose “success” resembles a moonscape. One is reminded of the adage, “We Had to Destroy the Village to Save It.”
Further, non-target species are not limited to plants. Animals can also be displaced, injured or killed by all of the above methods. Burrowing mammals and reptiles can be chopped up, buried, asphyxiated, drowned or have their homes excavated (like the time I accidentally cut a Skink in half with a shovel blade while weeding a garden). Insects are harmed in their various life-stages, during some of which—caterpillars in cocoons, for example—they are unable to attempt escape. Fish and other aquatic creatures might lose an entire generation if their eggs are nestled in plants exposed by lowered water or buried in muddy lake floors that get covered by a “benthic barrier.” (Benthic barriers are sheets made of plastic, nylon, or burlap that are used to smother weeds underwater and which reduce or eliminate sunlight, deplete oxygen, and lead to gas production from decaying matter.)
The time taken to recover from the disturbance made by mechanical means differs depending on method, climate, season, etc. A quick bounce-back could be expected in the case of a careful individual digging up of blackberry canes in the US Pacific Northwest in springtime, for example. By contrast, a much longer time is needed when a Pinyon-Juniper woodland is “chained,” a process in which a very large chain is dragged between two tractors, uprooting everything in its path. Not only do trees need multiple decades to grow back, but the time required for certain soil-borne mosses in these ecosystems to completely regenerate might be well over two centuries.
Additionally, mechanical means can actually encourage the reproduction of particular plants that thrive on certain disturbances. For example, Bindweed (the common name of several vining species in the Morning Glory family, most commonly in the genera Convolvulus or Calystegia) is very effectively spread by getting dug up or tilled under. A severed root fragment of as little as half an inch in length can produce an entirely new plant. This characteristic of Bindweed is well known in agricultural circles and we made the discovery for ourselves during our farming years.
Compared to chemical methods, mechanical ones can more easily be limited to target species because of their hands-on nature. However, they are usually more expensive than chemical methods due to equipment needs and labor hours, and so are often eschewed for that reason. Too often, saving a buck is more important than doing the best job.
Biological methods entail introducing additional non-native species that will consume the target plant. Most commonly, the new species are insects native to the target plant’s original habitat who were predators of the plant there. Ideally, the new species consumes only the target plant (i.e., has “host-specificity”), but it hasn’t always turned out this way. Warns the U.S. Fish and Wildlife Service (FWS): “Classical biocontrol is irreversible and therefore it is essential that all potential consequences are adequately considered beforehand” [our emphasis]. Of course, it is impossible to foresee “all potential consequences.” That means trouble of some kind.
FWS lists a few ways in which introducing new species as a biocontrol can backfire. These are not hypothetical. Each one has been documented.
- Non-target Attacks and Host-Shifting : Despite prior research, new predators can expand their diet to include non-target plants after they have been introduced. Such plants might be native or agricultural, so the damage can be ecological or economic.
- Accidental Introductions : Despite care with collection and transport, other species can accompany the intended one. Cites the FWS: “For example, the pathogen Nosema was accidentally introduced as a contaminant of a weevil (Trichosirocalus horridus) introduced to control musk thistle.” Nosema affects Honey Bees and is a possible cause of colony-collapse disorder.
- Food Web Interactions: An introduced species can throw off the balance in an ecosystem. FWS relates the case of a gall fly introduced to control a non-native plant that ended up becoming “superabundant” itself. This led to a two to three increase in the population of Deer Mice, which raised concerns that they might over eat native plants.
Cornell University’s College of Agriculture and Life Sciences lists over forty species of insects currently being used for biological control of “weeds.” One example is the so-called, “Klamathweed Beetle” (Chrysolina quadrigemina), which was released into the wild in California in the late-1940’s to control St. Johnswort (Hypericum perforatum), a plant that Europeans imported in part for its medicinal uses. St. Johnny, as some herbalists call it, has become widespread throughout disturbed areas such as agricultural zones and along logging roads. Despite the voracious appetite of the beetle—which we witnessed ourselves in herb gardens where we were tending H. perforatum for harvesting—the plant remains common.
Not at all incidentally, the primary concern with St. Johnswort is that it causes phototoxicity in sheep. So the issue is economic, not ecological, and is being undertaken in the interest of a species that is itself non-native. Furthermore, though it is often claimed that the plant pushes out native plants in the disturbed areas where it thrives, we could not find any sources that actually demonstrate that allegation. For example, one article that is repeatedly cited to back up that claim merely restates it but offers no data or additional citation. In fact, the article is not even about the growth habits of St. Johnswort at all, but about the use of aphids as a biological control against it in Australia. One might be forgiven for wondering if the numerous people citing this article actually read it.
Domesticated animals are also used to eradicate invasive plants, but their role in these efforts is quite small compared to other methods. More significantly, cattle and sheep have played a major role in the distribution of non-native plant species, and in some areas—such as the arid non-agricultural west of the USA—have been one of the main vectors. Brush goats (Capra aegagrus hircus) have been increasingly popular in recent years—including in urban areas—but they will famously eat virtually anything (including plants that are toxic to them) so care needs to be taken.
Chemical methods for eradicating invasive plants are the most common because they are cheap and effective. Of course, they are also effective at killing non-target plants, and that result is quite common. In fact—and shockingly—less than 1% of a sprayed herbicide application ends up being delivered to the intended target. The remainder—if one can use that word to mean “the vast majority”—is dispersed into the surrounding environment. As a science, it’s quite a far cry from “exact.” A 1% success rate in just about any other endeavor would be considered a dismal failure.
What do the “extra” chemicals do? Let’s look at glyphosate, the most commonly used herbicide in the world, which is manufactured by Monsanto and is the active ingredient in their notorious product, Round-Up. As a “broad spectrum” agent, it kills many kinds of plants, both terrestrial and aquatic, including algae. Sublethal doses are also harmful and lead to higher rates of fungal diseases and lower rates of micronutrient uptake. Additionally, glyphosate destroys beneficial bacteria and microorganisms in the soil, complicating recovery for native plants who no longer have the soil components required for health. As if that wasn’t enough, the bacteria that break down herbicides increase in number, further throwing off soil balance. Soil structure is also detrimentally effected by the way glyphosate binds with soil particles, which can lead to lower crops yields (and defeats the point of using it).
In the Animal Kingdom, glyphosate is also highly problematic. It can “cause genetic damage in fish, and also disrupt their immune systems… can cause genetic damage in insects… [and] can harm amphibians in a variety of ways, including causing genetic damage and disrupting their development.” In humans, “symptoms of exposure to glyphosate include eye irritation, burning eyes, blurred vision, skin rashes, burning or itchy skin, nausea, sore throat, asthma and difficulty breathing, headache, lethargy, nose bleeds, and dizziness” and it has been associated with “increased risks of the cancer non-Hodgkin’s lymphoma, miscarriages, and attention deficit disorder.”
For years, the dangers of glyphosate to humans had been considered a matter of debate, but in a landmark court case in August 2018, a jury ordered glyphosate-manufacturer Monsanto to pay $289 million in damages to a California grounds keeper who was dying of Hodgkin’s lymphoma. With this precedent set, further lawsuits are expected. This is good news.
The use of any herbicide at all that kills non-target, native species reduces the area’s biodiversity, and not just of the plants. Any animals and insects that depend on those plants are also impacted. Furthermore, these holes punched in the ecosystem adversely affect the natural processes of succession that previously existed. Herbicides take the story “off script,” so to speak, and there’s no guarantee the remaining players will be able to improvise themselves out of their conundrum.
Direct exposure to herbicides is not necessary to suffer from them. Through a process known as “biomagnification,” levels of toxins increase in the natural food chain. So, a tainted plant is nibbled by a mouse who is eaten by a snake who is caught by a bird of prey. Not only is the bird poisoned, but the resulting level of accumulation is at a higher concentration than would happen through direct exposure.
The more that herbicides are used, the more that certain target plants can adapt and survive. “Pesticide resistance” has become a real issue, and the solution so far has been to apply more poisons, which of course leads to more “collateral” damage. It’s a vicious cycle that we cannot afford to continue.
But there are definitely “conservationists” and even “environmentalists” out there who are enthusiastic about dumping poisons on living things when the targets are invasive plants. Something about the “invasive” concept works to sweep aside thoughtfulness.
Alien species seem practically designed to excite public concern. Almost by definition they are most abundant, and most visible, in the most highly human-modified habitats, such as towns and cities. Personal encounters with aliens are routine, so everyone has an opinion, and it’s often ‘obvious’ that aliens are actively supplanting natives, even if that isn’t what’s happening at all. It’s equally ‘obvious’ that something must be done, even if it’s not clear what that should be, and even if ill-judged intervention might only make things worse.
It is our stance that herbicides are always an “ill-judged intervention” in restoration. If that seems extreme, that’s only because invasive ideology has pushed the mainstream of discussion to such an extreme place, where the perverse logic of war has been made commonplace.
 Jones, Allison & Catlin, Jim & Vazquez, Emanuel “Mechanical Treatment of Piñon-Juniper and Sagebrush Systems in the Intermountain West: A Review of the Literature” (Wild Utah Project).
 Thompson, Ken. Where Do Camels Belong? The Story and Science of Invasive Species. (London: Profile Books, 2014), epub edition, location: 82.5.