Here's a cheap solution for unwanted plants and weeds, especially those in walkways, driveways and stone patios. If you spray it on ant's nests it gets rid of them too! Just don't spray it on your lawn or any plants you want to keep. Although it's made from fairly benign ingredients keep it away from kids too.
Here's the recipe:
1 gallon Pickling vinegar-at least 7 or 10% acetic acid( Available in your grocery store)
1 cup salt
1 tablespoon dish soap
Mix it together and pour into a spray bottle, preferably one with an adjustable nozzle.
Squirt it full strength onto those pesky plants and they'll wilt in a day or two.