By Dan Stein and Kay Rumsey
Controlling weeds is never easy. Getting the upper hand on weeds usually takes a lot of elbow grease and a good plan. Serious organic growers and people who simply don't want to use herbicides around their families and homes need to be especially resourceful as they come up with strategies to tackle weeds. Until recently, there were no "natural" or organic herbicides at their disposal. That's starting to change.
In the last few years, some new herbicides have reached the market that contain vinegar, lemon juice and other plant-based ingredients. Some of these products contain both natural and synthetic components, while others contain all natural ingredients. These products can be used along fencelines, on driveways, in flowerbeds, and elsewhere, but only some may be used around fruit and vegetable plantings. If you are interested in using certified organic products — with naturally derived ingredients that meet national standards for organically grown food — look for product labels that display "OMRI Listed," "USDA Organic," or other certification icon.
Pesticides, including herbicides, must be registered by the U.S. Environmental Protection Agency (EPA). However, special rules allow for minimum risk pesticides to be developed and sold without EPA approval.1 Some of the new herbicides are EPA-registered while others meet the criteria for minimum risk status. In the case of vinegar-containing products, this distinction makes a big difference.
In minimum risk herbicides, the vinegar component is pretty much the same as the vinegar found in your pickles and salad dressing. Household vinegar contains about 5 percent acetic acid2 while minimum risk products are allowed to have vinegar with up to 8 percent acetic acid used as an "inert ingredient."3 This strength of vinegar can irritate your skin and eyes just like vinegar in your kitchen, but is considered safe.1
By contrast, acetic acid concentrations greater than 8 percent must be EPA-registered as an "active ingredient" in a pesticide product. (EPA uses the word "vinegar" only for the lower concentrations). That's because the corrosive effects of acetic acid can be serious. Concentrations of 11 percent or greater can burn skin and cause permanent eye damage - even blindness.4 Goggles or face shields as well as waterproof gloves, long sleeves and long pants are required when using the stronger vinegar herbicides that contain 20 to 25 percent acetic acid. Acetic acid can be harmful if swallowed or absorbed through skin.5 A parks employee reported that a vinegar herbicide corroded the bottom seal on his backpack sprayer, spilling the contents onto his pants.6
High concentrations of acetic acid can harm aquatic organisms by affecting pH levels.7 Applying 20 percent acetic acid as a soil drench (pouring it directly onto soil) can lower soil pH (make it more acidic) for at least a month.8 This may not be the sort of stuff you want to have around the house, especially if there are children.
Do vinegar herbicides work?
The answer to this seems to depend largely upon the concentration of acetic acid in the product used, how much of the product is used, and the type and age of the weed being treated.
One important characteristic of vinegar herbicides, and in fact, all of the so-called natural herbicides, is that they are contact herbicides. When vinegar is applied to the leaves of a plant, the acetic acid destroys the cell membranes. But only the parts of the plant it contacts are injured so the roots are not affected.4 Results are quick. Within 24 hours the plants look dead.9 However, the effects are not always lasting.
Some perennial weeds have the ability to regenerate from reserves in their root systems.10 Grass and grassy weeds will show initial signs of damage from vinegar but recover quickly. Broadleaf weeds tend to be more susceptible to vinegar than grasses.2,4,11,12 However, using vinegar on dandelions and Canada thistle, which have heavy duty roots, may only result in top kill. Unless the weed is very young, it's likely to grow back.
Studies show that vinegar works best on very young weeds that have only one or two leaves.4,11,13 Weeds with more than three or four leaves are likely to survive treatment,11,12,13,14 but using more of the product can improve weed control.2,4,11,14 Multiple applications can also improve control.9,13
Kitchen-strength vinegar may work on a few types of young weeds. Research on varying concentrations of acetic acid show that stronger is almost always better when it comes to weed control, so 20 percent acetic acid is more effective than a 5 percent or 10 percent concentration.2,9,14,15
Studies on the effectiveness of vinegar have turned up promising results for some weeds. Vinegar can work well at controlling broadleaf plantain,9 carpetweed,4 common chickweed,13 cutleaf evening primrose,2 ground ivy,9 ladysthumb,13 oriental mustard,11 pale smartweed,13 tumble pigweed,4 spiny amaranth,4 and even crabgrass!2,4,9
Some Tips for Use
If you decide to use a vinegar herbicide, read the label carefully and follow these guidelines:
- Target young weeds. Don't let the weeds get away from you and expect to have good results.
- Spray the weeds thoroughly. It is not good enough to get a little on each weed.
- Make applications on warm, dry days when rain is not forecast.
- Be very careful to not harm valuable plants with drift. Vinegar herbicides don't discriminate between good and bad plants.
- Read label carefully for safety requirements for goggles, gloves, etc.
Many DIY recipes for weed killers can be found online, using a mixture of vinegar and soap. The general ratio is as follows:
- 1 Gallon 5% (or stronger) vinegar
- 1 cup Castile Soap (like Dr. Bronners)
- 1 Garden Sprayer
A Magic Bullet?
While organic farmers will appreciate having a new tool to use against weeds, herbicides alone, whether "natural" or not, are not magic bullets. Weed control calls for using a "big picture" approach. This includes the use of mulches, timely cultivation, flaming, soil solarization, cover crops in the off-season to smother weeds, weed barriers, etc.16 When weeds pop up, catching them when they are young is easier and produces better results - whether you're reaching for the hoe, the hand weeder, or the natural herbicide.
U.S. Environmental Protection Agency. Undated. Minimum risk pesticide: Definition and product confirmation. https://www.epa.gov/minimum-risk-pesticides/minimum-risk-pesticide-definition-and-product-confirmation
Webber III, C.L. et al. 2005. Vinegar as an organic burn-down herbicide. Proceedings of the 24th Annual Horticulture Industries Show. pp. 168-172. http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=176571
U.S. Environmental Protection Agency. Undated. Inert ingredients overview and guidance. http://www.epa.gov/opprd001/inerts/section25b_inerts.pdf
Webber III, C.L. and J.W. Shrefler. 2006. Vinegar as a burn-down herbicide: Acetic acid concentrations, application volumes, and adjuvants. 2005 Vegetable Weed Control Studies, Oklahoma State University, Division of Agricultural Sciences and Natural Resources, Department of Horticulture and Landscape Architecture. http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=195808
Pharm Solutions, Inc. Undated. WeedPharm Common Use Label. https://pharmsolutions.com/index_htm_files/Weed-Pharm-Food-Use-Label-Comm-1gal.pdf
Chirillo, S. 2008. Non-herbicidal Weed Control Strategies Implemented by City Parks Staff in the Northwest: Maintaining Shrub Beds and Landscaped Areas. Northwest Coalition for Alternatives to Pesticides. https://d3n8a8pro7vhmx.cloudfront.net/ncap/pages/48/attachments/original/1437762972/Shrub-Beds.pdf?1437762972
National Center for Biotechnology Information, PubChem Database. Undated. Compound Summary: Acetic Acid. https://pubchem.ncbi.nlm.nih.gov/compound/176
Radhakrishnan, J. et al. 2003. Agricultural applications of vinegar. http://www.ars.usda.gov/research/publications/publications.htm?seq_no_115=142484
Chinery, D. 2002. Using acetic acid (vinegar) as a broad-spectrum herbicide. Cornell Cooperative Extension of Rensselaer County. http://ccerensselaer.org/resources/using-acetic-acid-vinegar-as-a-broad-spectrum-herbicide
Mugaas, B. 2006 Salad dressing or weed control? An organic non-selective weed control product evaluation. Yard & Garden Line News (University of Minnesota Extension) March 1, 2006. v.8n3 http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Mar0106.html
Johnson, E. 2005. Efficacy of vinegar (acetic acid) as an organic herbicide: Final report. Saskatchewan Agriculture, Food and Rural Revitalization, Agriculture Development Fund.
Masiunas, J. and A. Bicksler. 2006. Evaluating organic herbicides, summer annual cover crops and mowing for Canada thistle control. New Agriculture Network 3(10), 1-5. http://www.new-ag.msu.edu/issues06/8-23.htm (link no longer active)
Miller, T. and C. Libbey. 2004. Effect of postemergence organic products. https://www.pharmsolutionsinc.com/researchAssets/Acetic_Acid_Weed_Trials_WASU.doc (link no longer active)
Evans, G.J. and R.R. Bellinder. 2005. Evaluating the herbicidal properties of acetic acid: Preliminary results. Cornell University. https://pharmsolutions.com/index_htm_files/cornellreport.pdf
Chase, C.A. et al. 2004. Preliminary evaluation of nonsynthetic herbicides for weed management in organic orange production. Proceedings of the Florida State Horticultural Society 117, 135-138.
Curran, W. 2004. Agronomy Facts 64: Weed management in organic cropping systems. Penn State Extension. http://pubs.cas.psu.edu/freepubs/pdfs/uc187.pdf