My response to that familiar “organic weed killer” meme became a popular post on Facebook, so I’ve added a bit of information, links, and posted it here as well.

Please, stop encouraging people to use the vinegar and soap mix because
1. It won’t kill most weeds, and,
2. It does kill beneficial insects (including bees).

I will once again point out that:

• Kitchen vinegar will provide top kill, barely, on some plants. The plants will usually survive though, because you’re only killing the foliage, not the root.
• Horticultural vinegar will burn plants and you very badly, including your lungs. Stay away from it.
• Epsom salt is basically magnesium. Magnesium is added to deficient soils to help certain plants grow better. It can’t help plants grow better and kill them at the same time.
• Dish soap is actually “detergent,” and will remove the waxy cuticle of leaves AND of insects, killing the insects but usually only top-killing the plant. The detergent breaks through the cell membranes, causing the cells to leak out. They’re also toxic to certain fungi, including beneficial fungi.
• “Dish soap” is technically a “detergent,” not a soap. Think dish soap is non-toxic and safe? Try spraying it on a bee and see how it responds. (Better yet, don’t, because if you spray one, you’ll slowly kill the bee and other soft-bodied pollinators.) (Insecticidal soap is potassium salts of fatty acids – an actual soap which is safe to use on plants.)
• This mixture is non-selective, meaning it will damage whatever it comes into contact with.
• All of the above are “chemicals” that can harm you. Try spraying some kitchen vinegar on your kid’s arm and let me know how that goes.

If you’re only spraying on a patio, it is hard to argue that you’re going to do any harm by spraying kitchen vinegar, but it might be just as effective to use a weed whacker/string trimmer, because in many circumstances, the weed is going to come back anyway. Why?

Research has found that 5-10% acetic acid herbicide products can give viable control of very small, young weeds that have only 1-2 leaves (or within 2 weeks of germination).

https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/ipmnet/Vinegar-AnAlternativeToGlyphosate-UMD-Smith-Fiola-and-Gill.pdf

Your weed has more than two leaves? Vinegar probably ain’t gonna do the job.

When people shout “save the bees!” yet promote home-made herbicides like these that are capable of killing a wide variety of pollinators, it isn’t just frustrating and ironic – it is putting those same pollinators, as well as other beneficial insects, at risk.

Even if you spray glyphosate on a bee, it won’t die, but that bee will die if you spray dish soap on it, according to Xerces.

The soap disrupts cell membrane permeability, causing cell contents to leak, leading to death. Mortality may occur if directly applied to foraging bees, however no residual toxicity exists. Apply only to non-blooming crops, or apply at night, or when bees are not present. Where managed pollinators are maintained, hive entrances should be closed.

Xerces is actually referring to the effects of using “insecticidal soap,” which is SOFTER and LESS HARMFUL than typical dish soap. Remember, “insecticidal soap” is meant to be used on plants to kill the insects but not the plant.

The same effect occurs on all soft body insects. If you’ve spent any time in the garden, you know there are always insects present – you just need to look closely. Not every beneficial insect is a thumb-sized bumble bee.

Pulling weeds is more effective, and even simply cutting it at the soil surface is arguably more effective than this mix, both with the added benefit of not harming any surrounding plants or organisms. (Nor will it damage metal furniture on your patio.)

Please, stop sharing this recipe. Just like using anything, if you don’t know what you’re doing or why you’re using it, there’s a good chance you may be causing harm. I don’t believe for a minute that people who use or share this recipe believe they may be harming beneficial insects.

More regarding the efficacy of vinegar as a weed killer: http://cru.cahe.wsu.edu/CEPublications/FS161E/FS161E.pdf