Mine are blooming too, next year cut them back in late spring.

This year, you could probably go ahead and trim them down and they will bloom this fall....

Not sure if you should wait until after they bloom this first flush or not.

here in kansas my mums will try to bud also and bloom...but i dont want that of course.....so when plants are up around 8 inches tall.....i give them a good trim all over a couple inches.....and round them as i shear them.then every two weeks until mid july ...i shear them again....lightly...then let go.....and they should be rounded and not so tall as to fall over for fall bloom.