Taken from an article in the Los Angeles Times

Why vinegar might be your secret weapon to fighting weeds

Boiling water or full-strength white vinegar can kill the leaves and stems of most tender green weeds, such as spotted spurge, crab grass, sow thistle and prickly lettuce — especially when they’re young. The vinegar is easier to use, and some people dilute it 50-50 with water. It can be used at full strength to drench the plants “because it’s so cheap.” White vinegar is particularly useful at getting weeds in gravel paths, rock walls and gaps in the concrete, which are otherwise nearly impossible to pull.
This was suggested by a Douglas Kent. Who is an ecological land manager, adjunct professor at Cal Poly Pomona and author of several books about California-friendly gardening and fire-resistant landscaping.