How Come Herbicides Don't Kill the Grass?
Or, The Strange Magic of Broad-Leaf Herbicides and Other Chemicals
Self-heal (Prunella vulgaris), a common edible & medicinal lawn “weed”
Working for three organic or naturally-grown veggie farms taught me plenty about quality compost, foliar feeding with seaweed emulsion, mulching and intercropping to suppress weeds, and encouraging beneficial insects for pest management. However, it did not teach me a lick about conventional herbicides.
To gain a better grasp of safety when harvesting in the vicinity of treated lawns--whether physically or time-wise, months or years post-treatment-- I'll offer a series of articles delving into lawn chemicals. Here, we'll look at the categories of herbicides that are commonly used today and how they work.
If you're as jazzed about chemical enlightenment as I am, you won't find this article dry in the least. If you feel it's lacking some juice, take comfort in knowing that defining these terms now will make reading future articles easy-breezy. You don't need to remember all this info! My intention is to orient you to the herbicide world as I understand it.
Pre-emergent vs. Post-emergent Herbicides
Pre-emergent herbicides kill weed seeds as they sprout. To do this, a chemical barrier is created on the surface of the soil. To be effective, pre-emergent products are applied evenly across an entire lawn or property. The barrier can be compromised by digging into the soil after application, such as when new plants are planted (University of Illinois). The barrier is usually ½ to 1-inch thick, depending on the product, the soil, and whether the product is simply top-dressed (laid on top of the soil) or incorporated. Since most weed seeds germinate in the top 1/2 inch of soil, pre-emergent herbicides kill weeds as they germinate within the barrier (Oregon State).
According to Green Care Turf Management, pre-emergent herbicides control annual weeds "by inhibiting cell division in the young root [or shoot] system. The failure of the root system to develop results in the death of the young seedling weed shortly after germination" (2019). Pre-emergent herbicides won't kill existing weeds whose roots are established. This is also why pre-emergent chemicals don't usually harm established garden plants and grasses unless they're already stressed by drought or disease (University of Illinois Extension).
When doing a google search for pre-emergent herbicides, the product at the top of the list was Nitro-Phos Barricade. It offers "pre-emergent control of grassy and broadleaf weeds in established turfgrass, gardens, flowerbeds and many other non-crop growing areas. " Its ingredients are listed as "Prodiamine, .58%. Other ingredients, 99.42%." More on this ingredient information (or lack thereof) in a coming article.
Post-emergent herbicides kill existing weeds, but won't stop new weed seeds from germinating (University of Illinois Extension). True Green explains that some of these chemicals "work by traveling down the plant stalk and into the root system to kill the weed. So to achieve control with post-emergent herbicides, weeds must be actively growing" (2019). More on these migrating chemicals below.
Cell membrane disruptors are one type of post-emergent herbicide used to control both broadleaf weeds and grasses. They cause the plant to quickly dry out by destroying cell membranes (Baumann et al).
According to a publication by Texas A&M University, "Some herbicides are applied to the soil and are taken up by seedling plant roots or shoots. They are said to have soil activity. Herbicides that are applied to plant foliage have foliar activity. Some herbicides have both. Herbicides with a high degree of soil activity usually are applied preemergence" (Baumann et al).
Mullein (Verbascum thapsus), a medicinal broadleaf “weed.”
Selective vs. Non-Selective Herbicides
Herbicides that indiscriminately damage or kill plants are called non-selective. They may be used for killing weeds growing in sidewalk cracks or in pathways, for example. Where weeds and cultivated plants are growing close together, these chemicals may be difficult to use and more precision is required (University of Illinois Extension).
Examples include:
Aromatic amino acid synthesis inhibitors are "non-selective and control a broad range of annual and perennial grasses, broadleaves and sedges. Roundup Ultra® [glyphosate], one of the most commonly used herbicides on the farm and around the home, is in this category....These herbicides bind tightly to soil clay and organic matter and have no soil activity. For that reason they may be less effective when plants are dusty or when application water is dirty" (Bauman et al).
Amino Acid Synthesis Inhibitors: "This new category of herbicides can be used at extremely low rates, controls both grasses and broadleaf plants, has soil and foliar activity, and is essentially non-toxic to mammals and most nonvegetative life forms. [I'll investigate this claim in a coming article!] Amino acid synthesis inhibitors bind to a specific enzyme and prevent the development of amino acids essential to plant life" (Baumann et al). Examples include bensulfuron, imazapyr, and diclosulam.
Selective herbicides kill some plants, but not others. For example, some selective herbicides target broadleaf weeds, leaving grasses and grass crops (like wheat and corn) relatively unharmed if used as directed (Baumann et al).
According to a University of California article, "many postemergence broadleaf herbicides are prone to drift in the air and may contact desirable plants, especially when applications are made in windy conditions" (2019).
Some examples:
Growth Regulator Herbicides are chemicals that disrupt hormonal activity regulating respiration, cell division, protein synthesis and more (Baumann et al). 2, 4-D is one example. In an interview with agronomy professor Alex Martin, he explains that 2, 4-D affects broadleaf plants because they use a different style of transport than grasses (Ganzel).
Photosynthesis Inhibitors do just that. Many chemicals in this category are more effective on broadleaf plants than grasses (Baumann et al). In Living Downstream, Sandra Steingraber notes that "triazine herbicides, of which atrazine is one, poison a chain reaction that takes place within the chloroplasts of leaves. (The chloroplasts of grass species, such as corn, rely on a different chain reaction and so are far less susceptible to the poisoning powers of atrazine.)" (2010). Photosynthesis inhibitors may be used for pre-emergence, such as atrazine, or post-emergence, such as pyridate (Baumann et al).
Lipid synthesis inhibitors are unusual because they target grasses, rather than broadleaf plants. Most are used post-emergence. These chemicals disrupt the formation of lipids in grasses. All plants require these fatty acids for proper function. Inside the cell wall, plants contain cell membranes composed of lipids, and lipid synthesis inhibitors prevent these membranes from forming. The chemicals are quickly absorbed, and "within an hour they can not be removed by rain" (Baumann et al). The enzyme inhibited in grasses is "insensitive" to these chemicals in broadleaf plants (Louisiana State).
According to Texas A&M University, selectivity has as much to do with the timing and "placement" of the chemical application. "Most herbicides can be harmful, even to normally tolerant plants, if the dose is too high" (2019).
Contact vs. Systemic Herbicides
Contact herbicides only damage the part of the plant they touch. For example, if the above-ground parts of dandelions are sprayed with a contact herbicide, the plant may regrow from the untouched roots (University of Illinois Extension). The chemical doesn't migrate within the plant once uptaken.
Once a systemic herbicide is applied, the plant absorbs the chemical and it spreads to untouched parts of the plant, ensuring that the whole plant dies (University of Illinois Extension).
Pigment Inhibitors are one category of systemic herbicides. These chemicals are absorbed through the roots and migrate to new shoots. Here they inhibit the formation of carotenoids, which protect chlorophyll molecules required for photosynthesis. Examples include norflurazon and clomazone (Baumann et al).
Some products may be used as contact or systemic herbicides, depending on how they're applied (Baumann et al).
Burdock (Arctium sp.) is a wild edible and medicinal that may be targeted by broadleaf herbicides.
Reviewing What We've Learned: Two Popular Herbicides
Each herbicide product possesses its own combination of the traits above. Note that some products on the market combine several active ingredients, broadening the range of plants they will kill.
Here are two examples:
Roundup ProMax is a popular systemic, post-emergent herbicide. Yellowing and death of weeds usually happens within a week of application, though cloudy or colder weather may slow this process (Monsanto). Glyphosate, the active ingredient, inhibits an enzyme "found only in plants and microorganisms that is essential to the formation of specific amino acids" (2). The Roundup label states that the product results in "no soil residual activity" (2). This means that weeds must already have emerged when the product is applied; it won't kill weeds that germinate after application (2).
Monsanto states that Roundup is intended for "non-crop, industrial, turf and ornamental weed control." I tried to get in touch with the company to find out why they don't recommend using it where crops grow, and I didn’t hear back.
Ortho Weed B Gon is a "combination pre- and post-emergent herbicide." It is a contact herbicide, and is selective for broad-leafed plants (The Scotts Company).
Wrapping Up
If you want to go deeper and read more about herbicides, there are lots of resources on the web. Companies post their herbicide product labels online, and there are a few examples in the sources list. This site from Penn State offers a chart that's helpful for visualizing different categories of herbicides. This Texas A&M article was one of my favorite herbicide websites. It offers more examples of commonly used chemicals, and describes signs of herbicide damage on plants.
It's not guaranteed that an herbicide is the culprit when you see plant damage. Nutrient deficiencies and disease can result in similar symptoms. However, when foraging in a place we're less familiar with, it doesn't hurt to avoid plants with yellowed, bleached, or curling leaves, or any other unhealthy-looking characteristics. Just because a plant looks good doesn't mean it's chemical-free, so inquire into the chemical history of the area before noshing.
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Sources:
Baumann, Paul A.. et al. Texas A&M University. "Herbicides: How they Work and the Symptoms They Cause." http://cotton.tamu.edu/Weeds/B-6081%20Herbicides%20How%20They%20Work%20and%20Symptoms.pdf
Ganzel, Bill. "The Herbicide 2, 4-D." Wessel's Living History Farm. https://livinghistoryfarm.org/farminginthe70s/pests_04.html
Green Care Turf Management. "Weed Control 101: Pre-Emergent vs. Post-Emergent Pesticides." https://www.greencareturf.com/weed-control-101-pre-emergent-vs-post-emergent-herbicides/ Updated 2019.
Monsanto Company. "Roundup ProMax Herbicide: Complete Directions for Use." https://www.solutionsstores.com/amfilerating/file/download/file_id/2419/
Oregon State University. "Top Reasons Your Herbicide Failed." Viewed March 5, 2020. https://oregonstate.edu/dept/nursery-weeds/feature_articles/top_10/Top_10_reasons.html
Pesticide Action Network (PAN). Pesticide Database. “Product Name on Label: Nitro-phos preemergence weed control with 0.58% barricade herbicide." Viewed March 5, 2020. http://pesticideinfo.org/Detail_Product.jsp?REG_NR=03280200067&DIST_NR=054343
The Scotts Company LLC. "Ortho Weed B Gon label." 2020. https://www.ortho.com/sites/g/files/oydgjc116/files/asset_images/products/CFLS/OR-WBG-WeedKillerforLawns-Concentrate2-2015-CFL.pdf
Ross, Merrill A., Daniel J. Childs et al. "Herbicide Mode of Action Summary." Cooperative Extension Service, Purdue University. https://www.extension.purdue.edu/extmedia/WS/WS-23-W.html
Steingraber, Sandra. Living Downstream. Da Capo Press, 2010.
University of California Agriculture and Natural Resources. "Weed Management in Lawns." 2019. http://ipm.ucanr.edu/PMG/PESTNOTES/pn74113.html#BEFORE
University of Illinois Extension. "Understanding Herbicides." Viewed March 5, 2020. https://web.extension.illinois.edu/hortihints/0406d.html
Louisiana State University Ag Center. "Lecture: Inhibition of Lipid Synthesis." pg. 2. viewed March 30, 2020. https://www2.lsuagcenter.com/weedscience/pdf/AGRO4070/Handout18.pdf
