It might be too late to delay their flowering (summer is their normal bloom time if not pinched every year). Usually, mums are pinched (lightly trimmed on the stem tips) from late spring or so up until the Fourth of July (perhaps mid-July at the very latest). This forces the plant to regrow, staying more compact in the process and developing more buds than they would have produced without pinching. If you want to experiment and pinch them now, you might get close-to-normal flowering this autumn, but we can't be certain. Otherwise, enjoy the flowers now and make a note to start pinching them next spring once they reach just under a foot high (refer to the linked University of Illinois Extension page).

Miri