Mums are phototropic. This causes flowering to be triggered by long, uninterrupted nights. If you have any night (or traffic) lights that will interfere with total darkness at night, you must avoid exposure starting August 1 through Sept 3. Either cover the plant with a box or move it into a dark garage, etc. Be sure to return it to the sunny location every morning.
Late August or early September you might notice a single flower bud or a cluster of flower buds forming on the tip of each stem. If more than one bud, when they are the size of a pea, gently remove by rolling out all but the central bud or if that is damaged, the next best bud.
Thus, you will end up with only 1 bud/stem (1-3 blooms/plant). Stop feeding when the buds show color and enjoy watching your beautiful disbudded blooms develop. As the blooms enlarge, protect from wind and rain.