Many of those fall mums are no longer hardy. They bred the hardiness out of them because they want you to buy them every year.
Buy some small plants in the SPRING/EARLY summer from a source like Bluestone Perennials or Mums of Minnesota that specialize in truly hardy mums. By fall they will not be small!!
Greene is right, mums do need good drainage. But sorry Green, DO NOT CUT THEM BACK UNTIL the next SPRING! Yes they look ugly over the winter, but they will reward you the following year. I have grown mums for many many years (I have over 100 varieties!) and last year was the only year I ever cut them back in the fall. I just couldn't stand the messiness. Thank heavens I did not cut them all back. Because most of the ones I cut back did not survive.
As Greene pointed out they do need to be pinched several times. The general rule of thumb is three times before the Fourth of July. You can even save the parts you pinch off and root them into new plants. They are as easy as coleus when it comes to rooting a cutting.
