Stake them and pinch off most of the buds from the lowest section of stem. The blooms at the stem tips will be stronger and larger, and will make lovely long-stemmed cut flowers. As long as chrysanthemums have good drainage, they are not picky about soil. Don't be surprised if mums that were compact when you bought them in pots a year ago grow much larger when set free to grow in your garden. They're just showing their appreciation for a long growing season on unrestricted roots.
Climate plays a role in plant height, too.
High temperatures slow stem growth in midsummer, but cool temperatures and heavy rain can make stems lanky. In mild, moist climates, many mums won't stay upright without support. To keep them from flopping, support them with thin bamboo or metal stakes before the flowers open. Alternatively, pinch growing tips more often to stimulate development of stockier, bushier plants. By the time garden mums have finished blooming, you may want to cut them back.