Here is a popular remedy for weed control -- using vinegar as an herbicide. Does it really work? Well, yes and no.
First, your kitchen variety of vinegar is only concentrated to about 5% acetic acid (the active ingredient that kills the weeds). Studies by the USDA's Agricultural Research Service back in 2002 (Spray Weeds With Vinegar?) showed that major weeds such as common lamb’s-quarters, giant foxtail, velvetleaf, smooth pigweed and Canada thistle could be killed with common household vinegar, if the weeds were less than two weeks old.
Further research showed that weeds older than 2 weeks old required vinegar in greater concentrations, i.e. commercial vinegar at 10% to 20% acetic acid concentration. The proplem with vinegar containing such a high concentration of acetic acid is it can cause burns upon skin contact. Eye contact can result in severe burns and permanent corneal injury. 25% acetic acid concentrations registered through EPA and states for commercial use all have restricted entry intervals of 48 hours and list personal protection equipment to be used by the applicator. In other words, vinegar at the higher concentrates needed to kill weeds can be harmful to humans and animals coming in contact with the treated area up to 2 days after application.
If you want to try using vinegar to kill weeds look for a vinegar-based herbicide at the garden center, but 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. And be careful about how you apply the product -- it can also damage nearby desirable plants.