July 21, 2015 - 11:33am -- smith.263@osu.edu

“Rain makes grain” is an ages old saying you might hear any time two or more farmers are gathered and a mid-summer shower pops up. While that adage may be true, it’s also common knowledge that rain makes weeds!

This year’s record setting rainfall has created lots of very healthy, rapidly growing weeds in the landscape. Along with those weeds come calls from homeowners asking for ‘safe’ ways to control them. One product that is frequently asked about for control of landscape weeds is vinegar. A simple question regarding vinegar commonly evolves into a conversation about toxins, pesticides, the legality of its use, and exactly what ‘safer’ means.

Let’s begin by saying that vinegar does have some weed control properties, and presently there are three vinegar products labeled in Ohio. “Labeled” means they are legal for use to control pests, but only one of the three in Ohio is labeled as an herbicide. For some it may be hard to imagine, but common household vinegar is not “labeled” or legal for use as an herbicide in Ohio.

Regardless, when we take a look at what happens when vinegar is applied to a weed, we realize the acetic acid in the vinegar ‘burns’ through the wax coating of the weed’s leaf surface and destroys those leaves. If the weeds are annuals – like foxtail, crabgrass or ragweed – and are small at the time of application, perhaps one treatment with the 20% acetic acid vinegar that’s labeled will kill the weed (note that household vinegar is only 5% acetic acid). If the annuals become larger before treatment, it could take more than one application. It should be noted that when sprayed on perennial weeds such as ground ivy, vinegar will burn the leaves and then the plant will likely grow new leaf . . . vinegar may ‘control’ but seldom kills a perennial.

To this point we’ve talked about acetic ‘acid’ in vinegar and the plants that it ‘kills.’ It’s important to point out here that if a product – in this case, what some would call a ‘natural’ herbicide such as vinegar - kills a plant, it obviously has some toxic properties! So, is it safe . . . or, can it be ‘safer’ than a synthetically manufactured commercial herbicide? I’ll let you decide as we continue.

As we consider that question, we need to understand toxicity. The EPA conducts studies that determine the toxicity, or Lethal Dosage (LD50) Values of all pesticides and many other products we commonly use, including what many like to call ‘natural’ products like vinegar. An LD50 is the standard measurement of acute toxicity used for comparison of all the products tested. The LD50 is stated in milligrams (mg) of pesticide per kilogram (kg) of body weight and represents the individual dose required to kill 50 percent of a population of test animals (e.g., rats, fish, mice, cockroaches).

Most callers who ask about the use of vinegar as an herbicide are wanting a comparison to glyphosate – commonly sold by the trade name Roundup. Like all pesticides, EPA has tested glyphosate and given it an LD50 value. Likewise, acetic acid that’s exactly the same as that in vinegar has been tested by EPA and given an LD50. When rats were used in the test procedure, the LD50 value for glyphosate was 5600, and the LD50 value for acetic acid was 3310.

If we keep in mind that an LD50 value represents the amount of individual dose required to kill 50 percent of a population in the test, we realize the lower the number, the more toxic the material. When equal amounts were given orally and compared, it took less acetic acid to kill rats in the laboratory test that it did glyphosate. The acetic acid in even household vinegar was MORE toxic than Roundup!

Going one step further, in this case a comparison of rate of application is a moot point. A 1% solution of glyphosate will kill most any annual weed listed on the label, and also the majority of perennial weeds. It may take more than one application of a 20% acetic acid product to kill, at best, only a portion of the annual weeds we see in the landscape.

This discussion isn’t meant to suggest vinegar is not an acceptable herbicide. The intent is to create awareness that regardless the origination of a material – be it considered ‘natural’ or a synthetically manufactured product – if it has the ability to kill plants or insects, it is a toxin. All toxins should be handled with care, in accordance with the label and their intended purpose, and at the rate of application that has been determined to be acceptable. When used properly, both natural and synthetic herbicides can be safe and effective.

- Stan Smith, OSU Extension PA, Fairfield County