It's best to prune 1/3 off the top of the plants in spring to keep them in control. If you cut into the woody growth, you risk losing the plants. You can divide them in the fall.

Newt

* * * *
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.