I find it best to either cut them down by half around July 4th or to continually pinch them until then.* You can start pinching off an inch or so when they're about 4" tall.* Whichever is easier for you.* They'll flower a little later, but will be more compact and usually have smaller but more flowers.*
If you wait to cut them by half in July, you can then take the cut ones and make new plants.* Just let the cut ends calous over in the shade for a day.* Then insert into moist potting soil in pots.* By the time winter arrives you can either sink the pots into the soil for the winter, or plant in the ground if they have developed a good root system.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.