From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1904-342386
Question: It is now the beginning of July and our chrysanthemums are
already flowering. I recently read that I was supposed to cut them
back to 6 inches in July. Now that they are flowering is it too late
to cut them back? I would like to have flowers this fall.
Answer: Many gardeners pinch back or shear off their mums every few
weeks starting in early spring and continuing up until about the
fourth of July. This forces the plants to become thick and bushy and
also delays flowering until the fall. Since yours are already
flowering, I would enjoy the blooms.
You may be able to get a second flowering from your mums if you pick
the flowers for bouquets and then cut the plants back by about a
third, but do it right away so they have the maximum amount of time to
regrow and set buds. Then, make sure they receive adequate moisture
and nutrients for the rest of the summer and do not pinch them. With
luck they may bloom again but very late this fall.
Good luck with your mums!