Killing Weeds with Vinegar
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Question. I don't use chemicals on my lawn, but just learned that my wonderful daughter and son-in-law have hired a chemical lawn care company to get rid of their weeds. I've heard you speak about using a vinegar with 20% acidity to achieve the same end. I went to a large garden center, but they didn't have it. Please tell me where I can get 20% vinegar. And please provide any 'man-to-man' information I can share with my son-in-law; my daughter wants my grandchildren to be able to walk barefoot on their lawn, so we really need your sensible advice.
- ---Barbara in Cherry Hill, NJ
Will they definitely be harmed? What's the absolute risk? To me these are meaningless questions; chemical herbicides are 100% unnecessary, making any risk a foolish one.
That's right—unnecessary. The Dirty Little Secret lawn care companies don't want you to know is that the most important factors for keeping a lawn as free of weeds as possible have nothing to do with chemicals or organics; it's all in the cutting, watering, seeding and feeding.
If your son-in-law has a lawn composed of cool-season grasses—the norm in New Jersey and similar climes—and cuts it or allows it to be cut below three inches, weeds are inevitable. If he feeds it or allows others to feed it any time other than Spring and Fall, weeds are inevitable. And if he waters it for short periods of time, weeds are inevitable.
To achieve the least amount of weed in a lawn in the North, the grass must be three inches high after cutting. Never cut the lawn during a dry heat wave or feed it in the summer. If Nature has not provided an inch of water that week, water deeply overnight, ending at 7 or 8 am. Do those things—all free of cost and risk—and you will have prevented a good ¾ of potential weed problems; maybe closer to 90%.
Now to directly answer your question. An Internet search of the phrase 'horticultural vinegar' brought up a dozen organic products that are readily available ranging in strength from 8% acidity to 20%. Because they're liquid, they can be expensive to ship; so if vinegar you want, try and get a local garden center that offers organic options to stock a few brands.
Just be aware that while residual vinegar won't harm people or water supplies, vinegar in the eyes will. If you choose this weed control option—which I sometimes do for weeds in my gravel driveway—be sure and wear eye protection (which you should do when spraying ANYTHING other than pure water).
But vinegar is just one chemical free weed control option—and probably a poor choice for lawn weeds, as it is a 'non-selective' herbicide that will kill any grass it soaks as well as weeds. An organic herbicidal soap might be a better choice; and the newest kids on the block, herbicides made from iron, are designed to be very kind to adjacent grasses.
But again, care for the grass correctly and weeds should become a serious non-issue.
Question. Does horticultural vinegar change the pH of the soil, thus killing the targeted weed? If that weed is close to a plant I wish to keep alive, will the desired plant suffer the same consequences as the weed, i.e. death? Or is it simply the spraying of the foliage that causes the death of the weed? Thanks
---Nick in Reinholds, PA Answer. Super-excellent questions, Nick! And the answer to them all is 'yes'.
As with any contact herbicide, chemical or organic, you'll get the best results when you coat the surface of the undesired plant with a high-strength vinegar at the hottest height of a hot and sunny day when it hasn't rained for awhile and is not predicted to rain within the next few days. And, as with any contact herbicide, care must be taken to avoid or protect adjacent plants.
As you seem to have figured out, high-strength vinegars also have a quasi-systemic action. They aren't absorbed into the cellular structure of a plant like some chemical herbicides, but large amounts can lower the pH of the soil the plant is growing in below its survival range. In my opinion, this is the best use for the horticultural vinegars; finishing off tough weeds like bamboo and thistle after you've stressed their above ground growth by repeatedly cutting it back.
To achieve this noble end, cut the monster weed back several times beginning in Spring, and then drench dry soil around their root systems with vinegar on the hottest, driest day of the year. Stress them enough, time it right, use a good amount of the highest acidity vinegar you can find, and you'll achieve an impressive knock down.
Yes, you'll knock down some soil life as well, but that life will return as rain and time ease the soil pH back up. After a month or so if you like, you can dust some wood ash on the area to get the acidity levels closer to normal a little faster.