4 cups of white vinegar - 1/4 white table salt - 1 to 2 tsps of liguid dish detergent. Mix together and put in a spray bottle. Use fine spray and spray weeds between patio cracks and whereever weeds grow. Do not spray near grass or flower beds. Avoid walking over where you sprayed and keep children away, as the spray can be tracked on your shoes. It is cheap, safe for the environment and works. Rain will wash it away, so use on a sunny day. I've used this for years, and it works great.
