From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156723-521759
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!