If you want a chemical-free landscape and also want to defeat lawn weeds, the best defense is weed prevention. But weed prevention can take some time and, on occasion, you may need herbicides.
About Organic Weed Killers
There are more than 30 organic herbicides and other products on the U.S. market that meet organic food production standards. That's good news, but organic herbicides do have their pros and cons.
One benefit of organic herbicides is that, after they do their work, they dissipate in a matter of hours―not weeks or months. They are not persistent in soil. In general, they do not change soil pH or pollute water through runoff. Many of these products are safer around people, pets, and pollinators than synthetic alternatives.
One downside of organic herbicides is that they often require multiple applications. (That's sometimes true for conventional weed killers as well.) Multiple applications mean more labor and greater expense.
Impact of Organic Weed Killers on Lawn Grass
Most organic herbicides are non-selective, killing any weed or grass they touch. They work best when leaves are drenched. Multiple sprays are often required, depending on the variety and age of the target plants. For lawns, these non-selective herbicides should be used only where complete removal of all vegetation is the goal.
A few organic herbicides target only broadleaf weeds, leaving grass safe. Selective weed killers include Fiesta, Iron-X, and Bayer Natria Lawn Weed Killer, all based on iron HEDTA. (Some say that iron-based products can harm bentgrass, however.) A.D.I.O.S., a product based on sodium chloride, works on broadleaf weeds but not grass.
Effectiveness of Organic Herbicides
Very few organic herbicides kill roots directly. Perennial weeds with deep roots, therefore, are likely to reemerge. There is one organic product that claims to reach plant roots: A.D.I.O.S. by Herbanatur.
Only specific herbicides, such as AXXE, Monterey Herbicidal Soap, Bayer Natria Weed and Grass Killer, and Finalsan will work on algae, moss, and lichen. In addition, only one product claims to eradicate poison ivy, poison oak, and poison sumac. That is C-Cide, a citric acid product.
Legumes such as crown vetch, black medic, or clover can be difficult to eradicate. Even with synthetic chemicals, they often require a combination of repeat-mowing and multiple herbicide applications. Among organic herbicides, the iron-based products may be the most effective against legumes.
How to Use Organic Herbicides
If you decide to use organic herbicides to manage weeds, it's important to use them correctly. These tips will help you decide when and how to use your herbicide of choice.
- Organic herbicides reliably kill the top growth of both annual and perennial weeds, so most of these products work best on short, newly emerged plants.
- Organic herbicides are helpful as a second "punch" at weeds that have been “scalped” very low to the soil by a mower or weed whacker. They also work well as a follow-up after "smothering" weeds under paper or cardboard for a few weeks.
- Weather conditions make a difference. Since these products work by contact with leaves, use them on dry days. Those based on acetic acid (vinegar) and fatty acid (soap) work better in warmer weather (over 65 degrees). Examples in this category include Weed Pharm, AllDown, Elimaweed, Green Match, Green Match EX, Scythe, AXXE, Safer Fast Acting Weed and Grass Killer, and Monterey Herbicidal Soap. Some manufacturers suggest that direct sunlight also helps some products do their work.
- It may be easier to kill plant roots during the warmest summer weeks and early fall than in the spring. Plants draw food downward at the end of the growing season, creating greater potential for weed killers to be drawn down into the roots.
Some organic herbicides are exempt from EPA registration. These are also called “25b” products in EPA registration lingo. Examples include Ecosmart Weed & Grass Killer, Burnout II, Weed Zap, C-Cide, Matratec, Matran EC, and Phydura. EPA 25b products can be used without special permission on school grounds and playgrounds, according to a Cornell University report. Other organics, however, are required to have EPA registration, the same as synthetic chemicals. Always look for the "warning," "caution," or "danger" messages.
To be safe, bear these tips in mind:
- See the EPA’s list of food-safe herbicide ingredients. Then check the ingredients on the products you are considering.
- Any herbicide should be used when bees and pollinators are least active. Some organic herbicides can be harmful to pollinators such as bees. Read the label.
Organic weed killers may be considered safe for lawn weeds and not safe near food crops. Read the label to learn if a product is safe around the vegetable garden.
Organic Herbicides. SDSU Extension.
Allowable Herbicides for Schools & Day Care Centers. Cornell University