Is it true I can use vinegar as a weed killer?
Answer: Vinegar can kill weeds—but it's not the same vinegar that you find in your kitchen. Kitchen vinegar is 5% acetic acid. To be effective against weeds, vinegar must be distilled to 10 to 20 % acetic acid. Such concentrated vinegar exists as commercial food-prep product—such as that used by pickle manufacturers. However it isn't labeled/bottled for home cooks and it isn't labeled as an herbicide.
There are a few gardening products using "horticultural vinegar" that are labeled for home use as an herbicide, but they aren't available everywhere (they must be registered state by state). If you look for a vinegar-based herbicide at the garden center, make sure it is registered with the EPA, and follow the instructions carefully. Concentrated acetic acid can burn the skin and damage the eyes. Keep the area closed off until the spray has dried.
Finally, you may have heard that homemade concoctions using kitchen vinegar (5% acetic acid) do kill weeds. It's true to a short extent. 5% acetic acid can kill certain types of weeds when they are young. However it can also damage nearby plants, and it doesn't kill the roots, only the top growth. So perennial weeds will return.
As hard as it can be to accept, the most effective (and safest) weed control is hand pulling.
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From Horticulture Magazine