If you want a chemical-free landscape and also want to defeat lawn weeds, the best defense is prevention. But weed prevention can take some time and, on occasion, you may need herbicides.
Organic herbicides are a viable option for weed removal today, with more than 30 on the U.S. market. But as with all tools for the lawn and garden, we need to know the benefits and limitations. Visit the Organic Materials Review Institute to get names of herbicides and other products that meet organic food production standards.
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. (See more about this below.)
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. Here are some points to consider:
How Organic Weed Killers Work
- 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.
- 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.
- Conversely, some say that iron-based broadleaf herbicides can discolor grass when used at higher temperatures.
- Some products, such as Finalsan, work across a broad temperature range.
What Organic Herbicides Do to Weeds
- Organic herbicides reliably kill the top growth of both annual and perennial weeds.
- Very few organic herbicides kill roots directly. Perennial weeds with deep roots, therefore, are likely to reemerge. In my research, I found only one organic product that claims to reach plant roots: A.D.I.O.S. by Herbanatur. (See Note 1 below.)
- Several products also list algae, moss, and lichen among their targets. Examples are AXXE, Monterey Herbicidal Soap, Bayer Natria Weed and Grass Killer, and Finalsan.
- 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.
- 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. (See EPA’s complete list of low-toxicity herbicide ingredients.)
- Some organics are required to have EPA registration, the same as synthetic chemicals. Always look for the "warning," "caution," or "danger" messages.
It may be surprising, but an organic weed killer 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 veggie garden. Also see the EPA’s list of food-safe herbicide ingredients. Then check the ingredients on 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.
Note 1: 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 downward into the roots.