Why not ask this question in the pesticide/herbicide forum? Perhaps you should consider getting certified so you can leagally apply pesticides?
And, BTW, there are some things at Lowes and Home Depot that work pretty well, but unless you can ID the target, know what weed it is you want to kill, know when and how to apply, taking shots in the dark will likely mean more missed targets.
This is exactly why some training that leads to getting certified for pesticide applications is the law. The time is coming when people who are not certified won't even be able to buy pesticide concentrates at retail outlets.
There is a pinned topic in the pesticide forum that will provide links to what one has to do to get legal in every state. Each state is different. Might want to look into that. In the meantime, you might lurk in that forum. There is a lot of great info there, but if you aren't legal, I would avoid posting anything.