I quickly found after a couple of years that they spread quite quickly. What started out as a couple of clumps became a dominant row. It was interesting to see how they started to blend their colors. However, they also prevent bulbs from coming up and are not easy to cut back in the late fall or early spring. Sometimes when I cut out the dead stalks, it leaves an empty hole. Perhaps I am not always doing that part right.
I do keep my mums in my garden. They were in place before I moved in and I have a lot of other plants around them. I am trying to get more to grow up in between them and am trying to contain them. I just don't feel like adding anymore.