Here is something that I picked up a while back.
You'll need a dowel, lid off of a can of spray paint, cheap kitchen sponge and a screw about 2" long.
Cut the sponge to fit inside of the lid. Drill a hole through the center of the lid to fit the body of the screw, you may want to drill a starter hole in the end of the dowel as well to keep it from splitting. Put the screw through the center of the sponge, then through the hole in the lid and into the dowel.
Wet the sponge with Round-Up and you have a handy little dabber that will put the Round-Up just where you want it. There is no overspray, so it is good for windy days and you can move it through your plantings and hit just what you want dead.