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Using vinegar for weed control?
July 19, 2019

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It seems like now a days you can find several homemade remedies for any problem on the internet, including weed control solutions for your backyard. After getting several calls on the topic I want to share an interesting article written by Richard Zollinger, NDSU Extension Weed Specialist, Andrew Kniss, University of Wyoming Weed Scientist and Kirk Howatt, NDSU Weed Scientist. These three weed scientists have put a common home remedy for weed control to the test a vinegar + salt + dish soap mixture. Below is the article they wrote with their findings:

From web sources there appears to be a recipe for making your herbicide from common household products. Sources call it a "magical, natural, weed killing potion" with high "safety, effectiveness, and naturalness" and recommend as "an alternative to chemical weed killers." The recipe is a derivative of: gallon of vinegar + cup of salt + 2 tablespoons of dish soap.

For those avoiding use of chemicals to kill weeds - vinegar and salt are chemicals! Vinegar contains acetic acid, a chemical that has been investigated by USDA and academia with herbicidal properties. It has been used as an organic herbicide. Salt (sodium chloride) is a chemical that predates 2,4-D for use as a herbicide, and soap (detergent) is another name for "surfactants" that is applied with herbicides to stick spray droplets (retention) on leaf surfaces, reduce droplet surface tension (spread the droplet), and aid forming a herbicide deposit (close interface of the chemical active ingredients with the leaf surface). Most commercial herbicides formulated as a liquid also contain detergents/soaps (emulsifiers) for this purpose.

As this mixture of acetic acid, salt, and soap can kill many SMALL annual weeds, this presents the questions, "how does it compare to commercial herbicides namely Roundup/glyphosate?" This question is especially relevant since several websites tout the mixture as a safe and an inexpensive alternative to glyphosate. Effectiveness is relative to the situation the homemade herbicide can simulate glyphosate activity if spraying SMALL, annual weeds. The vinegar + salt solution will simulate a contact herbicide and may burn/desiccate weeds faster than glyphosate with full sunlight and hot temperatures. Weed researchers at NDSU conducted trials with acetic acid in the early 2000s but were not able to duplicate favorable results shown from other sources. We found several important factors that affect performance:

1.Table vinegar contains only 4-5 percent acetic acid while industrial vinegar contains ~20 percent acetic acid. Table vinegar was not effective in any treatment while industrial vinegar gave greater results but still considered unacceptable weed control.

2.Grasses were much more difficult to control than broadleaf weeds.

3.Sunlight and temperature was a very important factor penalizing northern regions where temperature is lower than in the mid-west and southern regions.

4.Complete coverage and spray volume were the most discriminatory factors. Spray volume of at least 60 gpa was required to get a significant weed response while 80 to 100 gpa would be considered minimum. Incomplete coverage of the plant leaves with the vinegar + salt solution would allow plants to re-grow from the living tissues. The burning action of vinegar + salt solution is not effective on perennial weeds. It will burn off the top growth of perennials (which may be desirable), but it will not provide long-term control.

Glyphosate is systemic, that is, it will travel throughout the plant down to roots and up to forming buds and seeds (known as "sinks) to effectively kill all plant parts. This difference between systemic and contact herbicides is very important in how to best use each product. Because glyphosate travels through the plant, it can control perennial weeds, such as Canada thistle and quackgrass. Coverage with glyphosate is less crucial, since the herbicide molecule will travel to parts of the plant that were not sprayed. Glyphosate will be more effective on large weeds, perennial weeds, and when applied under cool, cloudy conditions.

The contact nature of the vinegar + salt mixture can be a benefit, though. If you need to kill weeds in close proximity to a desirable plant (say, killing chickweed in a flower bed), then glyphosate can be problematic. Only a few stray drops from the glyphosate spray bottle onto a flower might be enough to kill the entire plant. A few stray drops of the vinegar + salt solution may only produce a few localized spots/speckling but won't kill desirable plants. The exception would be if you continually spray salt in the same area, you can end up with too much salt in the soil and will damage all plants. Acetic acid will break down quickly in the soil and won't cause long-term soil problems.

So there are certainly some scenarios where the homemade herbicide mixture might be preferable to glyphosate for practical reasons. Comparing effectiveness between the two herbicides is difficult; they both have a potential fit depending on the situation. But what about the "inexpensive" and "safe" claims? A quick trip to Wal-Mart reveals pricing of all products.

Walmart stores selling a half-gallon of glyphosate based Eliminator Weed & Grass Killer Concentrate for $27.97 is more expensive than a gallon of the homemade mixture; however, to mix up 1 gallon of spray solution, you only need to add 1.5 fluid ounces of the concentrated product. At that rate, the cost of the glyphosate solution is only $0.66/gallon. The label states that for "Tough Weed Control" you can mix up to 2.5 fluid ounces per gallon, raising the cost to $1.09/gallon. Even then, glyphosate is actually less expensive than the homemade mixture on a per-gallon, ready-to-spray basis.

Toxicity

In toxicity measures, acetic acid is more toxic than glyphosate. Salt is more toxic to rats compared to glyphosate when exposed orally. The dermal toxicity numbers are a little more difficult to interpret, since for both glyphosate and salt, the values are listed as greater than a value. This typically means that the experimenters did not kill enough of the test rabbits at the highest doses used in the studies; so we know that glyphosate is safe at least up to 2,000 mg/kg and salt is safe at least up to 10,000 mg/kg. But we can determine from this data that acetic acid is more toxic than either glyphosate or salt. Pound per pound, glyphosate actually appears to be less acutely toxic to the mammalian test organisms compared to acetic acid or salt.

But this is only half the story with respect to toxicity. To estimate the actual risk of these products, we need to know not only the toxicity, but also the use rate; the dose makes the poison. Even highly toxic substances can be used safely if the dose is sufficiently low, and seemingly safe chemicals can be problematic if the dose is too high.

To figure out the actual risk, we need to calculate the amount of the toxic substances being applied. Most distilled white vinegar is 5% acetic acid (50 grain). At this concentration, one gallon of the homemade mixture would contain 6.4 fluid ounces of acetic acid (the active ingredient). One gallon of acetic acid weighs 8.74 lbs; so 6.4 fluid ounces would weigh 0.437 lbs; so there is 0.44 lbs of acetic acid per gallon of homemade mixture. To convert this to similar units as the LD50 values, 0.44 lbs equals 198,220 mg.

Eliminate Grass & Weed Killer contains 3.7 lbs of glyphosate acid per gallon; or 0.0289 lbs glyphosate acid per fluid ounce. At the higher labeled rate of 2.5 fluid ounces of product per gallon, there would be 0.07 lbs of glyphosate acid per gallon of mixed product. Similarly converting this to the same units as the LD50 values, 0.07 lbs equals 31,751.5 mg. So it appears that glyphosate, the less toxic chemical, is being applied at a rate 6-times lower compared to acetic acid.

Let's do one more calculation to put these toxicity numbers into perspective. Male rats can weigh up to 500 g, or 0.5 kg. One gallon of the homemade mixture contains 198,200 mg of acetic acid, or approximately enough to kill 59 rats, if administered orally. One gallon of mixed glyphosate solution contains 31,752 mg glyphosate, or enough to kill 6 rats. The acetic acid in the homemade mixture is nearly 10 times more lethal than the glyphosate in the Eliminate mixture.

And this doesn't include the salt. The internet has ample documented and suggested information on benefits and liabilities of glyphosate but a quick google on sodium chloride, acetic acid, and salt can also produce less than positive information regarding effect on mammalian life.

If you are not worried about the safety aspect but simply don't want to purchase a Monsanto product - don't forget that vinegar is often made from corn, and most corn in the US has the Roundup Ready trait developed by Monsanto. So the vinegar you are using to spray your weeds is probably made from corn that was sprayed with glyphosate: the very herbicide you were trying to avoid.

Conclusion

Both the homemade vinegar + salt mixture and Roundup are safe when used properly, they're both relatively inexpensive, and both can provide effective weed control in the appropriate situation.

For more information, contact the Pierce County Extension office at 701-776-6234, email yolanda.schmidt@ndsu.edu or "Like" us on Facebook at www.facebook.com/ndsuextensionpiercecounty. NDSU is an equal opportunity institution.

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