I'd be interested to see the answers to this as well. I experience the same thing when I try to grow them in pots past a certain length of time. Even repotting and fertilizing doesn't seem to improve them. Once I got them planted into the garden soil and I finally learned to start judiciously pruning them throughout the season to make them fuller they seem to do be happy, but I'm terrible with potted plants.

Danielle J.

Mums are short term bloomers, especially in containers. Careful watering can help, I think. Avoid soaking the flower heads every time you water to prevent fungal problems from starting and spreading. Keep up with the deadheading from the very beginning.

Once planted in a permanent location in the garden and cut back pretty heavily at the right times of years can result in a twice a year (spring and fall) display. Full sun with maybe some protection from the hottest afternoon rays would be good in your location.