Salt. I learned from Pinterest about using the following as a weed killer. Into a spray bottle put 1cup of salt, add white vinegar almost to the top, shake until salt dissolves, add a squirt of blue Dawn dish detergent, shake again.
I sprayed same onto a dandelion who stood proudly in the crack of my walkway. It was not a particularly hot day. When I checked it three hours later - no so proud!!! Could hardly believe it worked so quickly. And now we are using a second bottle. Yes, we do have weeds. Are we the only ones?