I have attended the annual symposiums of the California Invasive Plant Council (Cal-IPC) for 5 years. I have always learned something new and the most recent symposium in November 2022 was no exception. This year there was a lot of important information about herbicides that are widely used to eradicate non-native plants.
Several presentations reviewed the California laws that regulate pesticide use in California. (Slides for one of those presentations are available HERE.) The laws are designed to reduce risks of exposure to both applicators and the public.
The presentations emphasized the importance of legally mandated personal protective equipment (PPE) for applicators. The minimum PPE required by California law is protective eyewear and chemically resistant gloves:
Source: 2022 Cal-IPC Symposium
The toxicity of pesticides is rated by federal law as “Caution,” “Warning,” or “Danger,” with “Danger” indicating the most toxic and “Caution” the least toxic. These ratings are defined as signal words. Signal words of “Warning” or “Danger” require the applicator to also wear protective coveralls, in addition to protective eyewear and gloves.
Other types of PPE may be required by the product label, shown in this picture:
Source: 2022 Cal-IPC Symposium
Comparing the toxicity of organic and synthetic herbicides
Signal words can be used to compare the acute toxicity of different products. For example, the signal word on glyphosate products is “Caution,” indicating that it is considered less acutely toxic than other herbicides with higher toxicity ratings of “Warning” or “Danger.” Signal words are not a measure of long-term health damage of pesticides, such as cancer or kidney damage. Epidemiological studies of long-term health effects of pesticides are hotly disputed and are usually dismissed by the manufacturers of pesticides.
When glyphosate products were rated as a “probable human carcinogen” by the World Health Organization and tens of thousands of product liability lawsuits were filed by users of glyphosate products with cancer, there was a public backlash against the use of glyphosate partly because it is the most widely used herbicide on the market. Glyphosate is found in most of our food and in the urine of most people. The health damage done by glyphosate is the result of 40 years of widespread use by agriculture. Glyphosate’s “Caution” signal word does not reflect the long-term effects of its use.
Consequently, glyphosate has been banned in many places all over the world. Los Angeles County has banned glyphosate. Locally, it is no longer used by East Bay Regional Park District (EBRPD) in developed park areas such as picnic areas, parking lots, and playgrounds. Although EBRPD made an exception for “invasive” plants outside developed areas, they have significantly reduced their use of products containing glyphosate. They are using more “organic” herbicides. Marin County banned the use of glyphosate. They are using exclusively organic herbicides.
What is the difference between synthetic and organic pesticides? In general, organic products are derived strictly from sources in nature with little or no chemical alteration. Synthetic pesticides are products that are produced from chemical alteration.
Are organic pesticides less toxic than synthetic pesticides? The general public tends to assume that organic pesticides are less toxic than synthetic pesticides, such as glyphosate. Based on the signal words the EPA assigns to pesticides to evaluate toxicity, organic pesticides are not necessarily less toxic than some synthetic pesticides. Remember the signal words are “Danger” (the most toxic), “Warning,” and “Caution” (the least toxic.)
Several presentations at the Cal-IPC conference compared the toxicity of organic and synthetic pesticides, using signal words as a proxy for toxicity. This is a slide from one of the presentations:
I also compared the signal words of the organic products used by Marin County and East Bay Regional Park District. Although they are using some organic products not evaluated by the presentation at the Cal-IPC Symposium, many of the organic products they are using have a “Warning” signal word, which means the EPA considers them more toxic than glyphosate.
Clearly organic herbicides are not necessarily less toxic than synthetic herbicides and many organic herbicides are more toxic than glyphosate.
Comparing the efficacy of organic and synthetic herbicides
Are organic herbicides as effective as synthetic herbicides? One of the presentations made at the Cal-IPC Symposium reported the results of a field study comparing the effectiveness of three organic herbicides with three synthetic herbicides, all with “Caution” signal words:
Here’s a description of the field trial:
Here are the results of the field trial (one organic herbicide was removed from the field trial when glyphosate was reported as an undisclosed ingredient in the product):
WeedZap and Fireworxx are the organic herbicides used in the field trial. The organic herbicides used in the field trial were found to be less effective than synthetic herbicides considered equally toxic.
This finding was corroborated by a publication of the UC Nursery and Floriculture Alliance, entitled “Organic Herbicides –Do they Work?” The short answer to that question is, not very well:
- “Organic herbicides kill weeds that have emerged but have no residual activity on those emerging subsequently. Further, while these herbicides can burn back the tops of perennial weeds, perennial weeds recover quickly.”
- “These organic products are effective in controlling weeds when the weeds are small but are less effective on older plants.” The organic herbicides were significantly less effective when weeds were more than 12 days old.
- “…broadleaf weeds were easier to control [with organic herbicides] than grassy weeds.”
Comparing the cost of organic and synthetic herbicides
The field study comparing organic and synthetic herbicides also compared the costs of these different product types:
In other words, organic herbicides are considerably more expensive than synthetic herbicides.
The publication of the UC Nursery and Floriculture Alliance agrees: organic herbicides “are expensive and may not be affordable…Moreover, because these materials lack residual activity, repeat applications will be needed to control perennial weeds or new flushes of weed seedlings.”
Clearly, organic herbicides are not a substitute for synthetic herbicides because they are not less toxic, not as effective, and are very expensive. Cal-IPC considers that assessment of organic herbicides a justification for continued use of synthetic herbicides. I consider it an argument for declaring a truce in the war on “invasive” species. We have waged that war for over 30 years. We have not won that war. In fact, we lose ground every year. We have done more damage to the environment with our chemicals than the “invasive” species did. We have reached a dead end.
Herbicides and Climate Change
The most valuable lesson I learned at the Cal-IPC Symposium was that climate change is making herbicides less effective. Higher temperatures and higher levels of CO₂ are reducing the effectiveness of herbicides. This revelation was mentioned only briefly in a presentation by Regional Invasive Species and Climate Change (RISCC) Management Networks. A search of the scientific literature substantiated that revelation:
- ”Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate:” “Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.”
- “High CO₂ and Temperature Increase Resistance to Cyhalofob-Butyl in Multiple-Resistant Echinochloa colona:” “…herbicide efficacy was reduced on Multiple-Resistant plants grown under high CO₂ or high temperature about 50% compared to Multiple-Resistant plants at ambient conditions.”
- ”Herbicide effectiveness in controlling invasive plants under elevated CO2: Sufficient evidence to rethink weeds management:” “We found that responses of the weed species to herbicide under elevated CO2 were species-specific… However, the C3 [cool season] grasses tended to be the most sensitive to herbicide application followed by the herbs and C4 [warm/hot season] grasses while shrubs and vines demonstrated the highest resistance. Our results highlight the need for broader testing to determine the species most likely to exhibit increased tolerance to herbicide in the future in order to improve management options beforehand and thus offset a future liability.”
These studies are just a small selection of the studies that respond to a search for “impact of heat and CO₂ levels on herbicide efficacy.” They all point to yet another reason why the chemical crusade on introduced plants is a dead end.
Climate change is a reality and it is here to stay. Climate change has changed the ranges of where native plants can survive and it has made it impossible to destroy the non-native plants that are capable of surviving in the changed climate. Switching from one poison to another will not overcome the forces of evolution, which dictate that vegetation changes when the climate changes.
Conservation Sense and Nonsense 
I respect your skepticism. But your conclusions seem simplistic. Invasive plant species are being introduced and allowed to invade because humans have been spreading them around without a care for at least 500 years as our technology and colonization has allowed, not just due to climate change which likely has not had a significant effect until this past century, invasive species were a problem them but we cared less.
Webmaster: I did not intend to imply that introduced plants are solely the consequence of climate change. In fact, climate change has only recently become a factor in the shifting ranges of native plants, but it is expected to become a major factor in the future.
In fact, the movement of species around the globe has occurred since life as we know it evolved over 500 million years ago. Most movement was natural in the sense that it was caused by wind, storms, ocean currents, etc. Movement by animals has always been a significant source of movement of species. Migrating birds carry seeds and plants on their feathers and feet, as well in their bellies to new locations. These were the natural forces that populated islands that emerged from the sea without any plant life.
Yes, movement of plant and animal species accelerated when human civilization began their global exploration about 500 years ago. However, even then, most of the plants and animals were carried by humans unintentionally, such as in the ballast in their ships or as diseases to which they were immune but the indigenous inhabitants were not. Global trade accelerated the pace of introductions in the 19th and 20th centuries.
We still don’t care that much as a society, I will give you that. It is the rate of invasive species spread and introduction that is a concern. WE can care more about the green things that all terrestrial life depend on and the ecosystems indigenous to the land we occupy. Invasive species have consistently been shown to be inferior habitats for most native wildlife species.
Webmaster: In fact, invasive species have not “consistently been shown to be inferior habitats for most native wildlife species.” Here are three recent meta-analyses done by academic scientists of studies of the impact of introduced species that say otherwise:
• Many scientists, “consider that geographic origin is of minor importance; like many indigenous species, most introduced organisms have negative impacts on some natives, positive on others, and mostly neutral impacts overall.” (“Invasion Biology: Evidence, Assumptions, and Conservationism,” Nancy M. Correa, et.al., National Academy of Science of Argentina, 2021.)
• Dov Sax has published a study that asks the community of academic ecologists to re-evaluate their negative opinion of introduced species in light of decades of evidence that many are beneficial and most have a neutral impact on ecosystems. https://www.economist.com/science-and-technology/2022/11/02/alien-plants-and-animals-are-not-all-bad?fbclid=IwAR2aUqNPwgTlEtBU4ltFeO3EyHwJ7ol5Q9zE21mNWBNQMafnUq6p6R76Hug
• “Gardeners increasingly seek out native plant species for their gardens and landscapes. Many believe that native plants are better choices because they are adapted to local conditions. The U.S. Environmental Protection Agency (EPA), among other federal, state, and local agencies, perpetuates this belief by making many unscientific statements about native plant benefits. Among these claims are that native plants are superior to introduced species in their ability to withstand local climate conditions, to resist pests and disease, and to require less water, fertilizer, and other forms of maintenance (EPA 2017). None of these claims have been supported in published research relevant to home gardens and landscapes.” https://pubs.extension.wsu.edu/are-native-trees-and-shrubs-better-choices-for-wildlife-in-home-landscapes?fbclid=IwAR1AcM_FpFx5zVipLg_oRkl2FxdHBbgzmA_Pg_iSGJRZA0XdoD8eksQ9FSI
Protecting native wildlife and flora seems like a worthy goal to me. Native species are not being pushed out by climate change as much as human development and invasive species takeover in nearby areas is a factor. The native habitat has been cut up like the land has been. From what I understand reading Doug Tallamy’s work and all sorts of other studies and scientific reports, we ARE losing species due to the habitat loss caused by our development and the invasive species that come with us that spread from our developments.
Webmaster: In fact, there are NO plant extinctions in the continental US that have been attributed to introduced plants. (Marcel Rejmanek, “Vascular plant extinctions in California: A critical assessment,” Diversity and Distributions, Journal of Conservation Biogeography, 2017)
I have read most of Doug Tallamy’s publications. He consistently makes two fundamental errors: (1) He exaggerates the degree of specialization among insects, and (2) He underestimates the speed of adaptation and evolution. In fact, insects are capable of using non-native plants that are chemically similar to the native plants they have used in the past and they are capable of making rapid transitions to non-native plants when they arrive.
Using native plant species in our landscapes we can create more habitat and sequester more carbon and save water and reduce the need for pesticides, fertilizers and other inputs.
Webmaster: Native plants do not sequester more carbon, save water or reduce the need for pesticides. Carbon sequestration is unrelated to the regional origin of a plant. It is largely proportional to above-ground biomass, which is unrelated to the plant’s regional origins. Water consumption is also largely a function of the size of a plant, regardless of its regional origin. It is a contradiction to claim that native plants are more useful to insects than non-native plants and also make the claim that they need less pesticide. Think about it. These are physical properties that have nothing to do with the nativity of a plant.
We can change policies to restrict invasive species to prevent introduction at the least and encourage gardening and landscaping with native plants. I do not believe it is the lost cause you seem to think it is, though you are certainly not helping haha. I will look further into your skepticism as I am a skeptic and think we should weigh the costs and benefits of all our actions. I’ve been focused less on killing invasives these days and more on creating native habitat for the wildlife that have not (yet.. perhaps) adapted to live in the invasive, novel ecosystems. So in a sense I agree we should be less focused on killing all the invasives which is not happening anytime soon. But I wonder if that’s just because we just don’t care enough as people. I think controlling invasive species introduction and spread from our developed areas is a worthy goal; but I am skeptical of the amount of resources put into that and how sensible it is at times given a whole system type cost benefit analysis.