A friend told me about this, says it works great.

Mix 1 gallon of white vinegar, 1 cup table salt and 1 Tablespoon
dishwashing liquid together and spray on weeds. To do so, remove
approx 2 cups vinegar from the jug, pour in the salt and dishwashing
liquid, then return the 2 cups of vinegar to the jug. Close the lid
and shake to mix.

Transfer to a spray bottle (after shaking to mix the ingredients as
needed). It works as well, if not better, than chemicals but is much
cheaper. BUT be careful, it will kill whatever you spray it on!