spraying a plant
PHOTOGRAPH BY VALLEFRIAS/GETTY

Is Vinegar A Safe Herbicide?

It may sound like a good alternative to chemicals, but in concentrations strong enough to be effective, it's hazardous and can cause environmental damage.

May 26, 2015

The idea sounds well-intentioned: Stop using chemical weed killer, and spray vinegar instead. But as with many home-brewed garden remedies and even packaged organic ones, we need to ask whether science supports the theory—and if it’s safer. Hint: There’s no scent of salad dressing in my garden (and no chemicals, either).

Related: Organic Weed Control Methods

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Vinegar’s active ingredient, acetic acid, can be produced naturally through bacterial fermentation, as in apple-cider vinegar, or industrially, via chemical reactions from such source materials as methanol. In concentrations strong enough to be effective against anything but the youngest, most tender weeds, vinegar is no longer a food product but an herbicide called horticultural vinegar, and not so friendly.

Moreover, vinegar is a contact or “burndown” herbicide, killing what it touches within hours or days. It looks like it’s working, but then weeds resprout from the roots, particularly perennial species.

That partial success worries Jeff Gillman, a contributor to the Garden Professors blog at Extension.org and author of The Truth about Organic Gardening. The gardener—seeing results but not entirely satisfied—trades up to higher concentrations, replacing household vinegar (5 percent acetic acid) with a horticultural product (typically 20 percent).

Related: What Makes A Foreign Species Invasive? 

Despite the signal word danger on most such labels, gardeners may instead just see vinegar and be careless. Sobering details: In concentrations over 11 percent, acetic acid can burn skin and cause eye damage; 20 percent is corrosive to tin, aluminum, iron, and concrete and can even cause blindness. Such herbicides are meant to be applied while wearing goggles and protective clothing.

And then, Gillman says, there is potential environmental damage—such as to the toad or salamander shading itself beneath those weeds.

“If you’re talking about just-sprouted seedlings, and you go after them right away with household vinegar, fine,” says Gillman. Otherwise, it’s better to reach for a hand cultivator than a spray bottle.

Do This Instead
Wary of horticultural vinegar? Preventive mulches, well-timed cultivation, and Jeff Gillman’s and my favorite remedy—hand weeding—are indisputably effective, and 100 percent environmentally safe.

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Longtime organic gardener Margaret Roach, whose Hudson Valley garden has been open for 20-plus years for Garden Conservancy Open Days, creates the popular AWayToGarden.com website and podcast.