There are a lot of herbicides on the market today. Many are the same chemical with a different brand name, or a combination of familiar ones, and many target the same plants. How does one decide which herbicide to use?
Various methods are used to classify herbicides: sometimes by their mode of action or their selectivity, other times they are lumped into broad chemical categories.
When making herbicide decisions talk with your distributor but do your own research, too. What plant are you targeting? Do you want to use this herbicide for more than one type of plant? Is the target plant herbaceous or woody? Do you want a foliar application, stump application or basal application? Will you be doing the work in winter or in the growing season?
Aside from these basic questions, I research other key aspects, such as soil residual, toxicity and the chemical’s mode of action. I read the label and the Safety Data Sheet (SDS) and I request a copy of the Technical Bulletin from the company representative. I have a “checklist” of important considerations I want answered before I buy. Beware of unsubstantiated claims. The chemical company must follow particular testing guidelines; until there is an independent herbicide testing company, their data is the most accurate.
My list and their explanations follow:
Soil Residual
Soil residual is also known as the “half life” of the chemical in the soil. This tells how long it takes the original chemical amount to be degraded by half in the soil. Generally, the manufacturer of the herbicide will provide an average time range because their testing is done in a laboratory and herbicide response is highly dependent upon environmental conditions. Some soil details help us to sort out this variability:
- The half-life is longer in dry soil.
- Leaching (movement into groundwater) is more likely in wet soils. Water competes with herbicide binding sites leaving more herbicide free to be leached from the soil. The herbicide’s water solubility is measured as a Koc A Koc value lower than 1900mL/g is more likely to leach into groundwater.
- What is the soil pH? A pH below 7 is more acidic and provides fewer sites for binding and fewer microbial bacteria which are responsible for herbicide degradation.
- What is the texture of the soil? Finely textured soils and soil high in organic matter leads to longer persistence than coarsely textured soil or soil low in humus.
Toxicity
The EPA tests for lethal doses to non-target biota such as mammals, birds, fish and invertebrates. You can search by chemical or brand name at the Pesticide Action Network (PAN).
Toxicity considerations include lethality by dose (LD50) and by concentration (LC50). LD50 measures what amount is required to kill 50% of a test population. It’s measured in mg of chemical to kg of body weight. For example, Milestone’s LD50 in rats is 5000 mg/kg. The popular mosquito repellent Deet has a LD50 of 1800 mg/kg in rats. Which one would you rather have on your skin? LC50 measures the amount required per volume of air or water to kill 50% of a test population; this is expressed as mg per liter (mg/L).The values of LC50 range from 10 being extremely toxic to 100,000 being relatively harmless if breathed in or ingested; these values are not correlated to humans but are used to determine toxicity in species such as fish or honeybees.
For a thorough explanation of how toxicity is described and defined on labels, the National Pesticide Information Center has a fact sheet on Signal Words.
Herbicides Mode of Action
How does the chemical effect the plant? Most herbicides I use in restoration work are systemic, the plant’s transportation system moves them to the target kill spot. Contrast this to contact herbicides, such as vinegar, which simply top-kills without translocation to the roots
Mode of action is one way to classify herbicide, although I prefer to know both the mode of action and the selectivity. Selectivity is based on what the herbicide targets (i.e. broadleaf, grass, nonselective) or can be based on postemergence or preemergence. I don’t use preemergence herbicides because they affect seed germination and often aren’t selective.
Mode of Action is classified in the following ways:
Growth Regulators are the most common. They mimic a plant growth hormone known as auxin, which is responsible for cell elongation. This herbicide upsets the natural hormone balance, causing a cascading, disruptive effect. Soil pH has little influence on this category of herbicides. Examples include triclopyr and aminopyralid.
Amino Acid Synthesis Inhibitors are the second most common. They inhibit the production of the ALS enzyme, an essential amino acid for producing new cells. If the pH in the soil is low, there can be a long residual effect. An example is metsulfuron.
Lipid Synthesis Inhibitors, the third most common, are grass-specific herbicides, which target production of lipids, resulting in new cell production. Examples are fluazifop, clethodim, sethoxydim.
Cell Membrane Disrupters cause cell membranes to rupture. I don’t use these because they are very persistent in the soil and can cause respiratory problems to humans. Examples of this are paraquat or lactofen.
Nitrogen Metabolism Inhibitors inhibits an enzyme that allows the plant to convert microbial-created ammonia formulations into nitrogen. The only example I found is glufosinate.
Pigment Inhibitors inhibit chlorophyll production. Another herbicide I seldom use because it’s restricted to coarse textured soils. Examples are clomazone or isoxaflutole.
Photosynthetic inhibitors have three types, which disrupt photosynthesis in different ways. These are applied to the soil, have long persistence, and can leach into the groundwater. One example is atrazine.
Cost
Calculating costs is not only about the actual financial outlay; it’s about effectiveness, too. If the financial cost is lower, but I have to apply it two or three times for effectiveness, this changes both cash and environmental costs. Costs also include negative environmental impacts or known human health issues – I avoid these herbicides. All herbicides should be treated with respect, and most concerns are mitigated through proper use and handling yet with some herbicides, there are health or environmental issues regardless of proper handling (e.g. 2,4-D, atrazine).
Storage and Cleaning of Herbicides
Always keep your herbicides in their original containers with the labels attached. Storing mixed herbicide longer than the growing season reduces its potency and effectiveness. Don’t allow liquid herbicide to freeze because it can change the chemical structure and reduce effectiveness. If you don’t have access to a storage area that doesn’t freeze, consider an herbicide in granular form.
Clean all spray equipment when finished with it or when changing herbicides. Cleaning is best done with an herbicide neutralizer and cleaner, then rinsing containers three times (i.e. Fimco Neutralizer and Cleaner). I apply the diluted rinse water to the edges of our gravel driveway. If this application isn’t an option, consider keeping the rinse water in containers and using it as your dilution for the next mixing.
Surfactant or adjuvant
Some other aspects to consider are whether to use a surfactant or adjuvant. These increase the effectiveness of the herbicide to disperse more thoroughly across the leaf when foliar spraying rather than bead up and roll off; some allow for easier penetration into the leaf. Knowing the physiology of the target plants helps with understanding this. If the leaf is waxy or pubescent (hairy), using the herbicide without a surfactant could diminish its ability to penetrate and reach the target location to kill it. A link to a great article by David Cordray on this subject is here.
A few other considerations
All herbicides are photosensitive to some degree. Application is best on cloudy days but not always practical. For very photosensitive herbicides, don’t store them mixed; mix them immediately before application and put them in opaque applicators. The concentrated herbicide should be kept out of the sunlight as well.
Keep an eye on the weather. You don’t want it to rain immediately following herbicide application because it reduces effectiveness and could increase leaching into the groundwater. Check the weather, and check the rainfast chart. North Carolina State Cooperative Extension provides a rainfast chart on its website.
The volatility, or vapor pressure, of an herbicide’s active ingredient(s) is required by EPA. For granular herbicides, this doesn’t apply until they are mixed. Information on the volatility is found in Technical Bulletins. These are not the same as the label or the SDS. Technical bulletins are written by the companies to provide application use and background information to the end user. You’ll need to ask your company representative for these.
No herbicide piece would be complete without discussing safety. Personal protection equipment (PPE) goes beyond chainsaws or brush cutters! Your skin is the main contact for herbicide. Be sure to wear gloves when mixing and cleaning; the nitrile type are inexpensive and allow for dexterity. I wear two types of glasses – my prescription and either sunglasses or safety glasses. Wearing a face mask when applying herbicide should be a consideration, too. Finally, unless I’m in a protected area, I don’t spray on windy days. This causes collateral damage to other biota, including me!
For more info:
The National Pesticide Information Center has a wealth of information.
Weed Science Society gives a tutorial on How to Read a Label
Techline News provides a free newsletter packed with info.
Pesticide Environmental Stewardship shows how to calculate correct application amounts
Wisconsin Dept of Ag, Trade and Consumer Protection maintains a database of pesticides. You can search in a variety of ways, such as by trade name, by active ingredient, by target, etc. It’s an easy way to pull up labels.
Dow has labels and SDS information on their website.
