Answer: It is too late to do anything this year except to stake them gently. After frost has killed off the tops you can trim them back to about four inches. Next spring, trim them back again before you divide them and remember that the outer sections of each crown will make the most vigorous plants. With regard to height, they can be pinched at regular intervals up until early July, then allowed to grow on and develop their flower buds. This will result in the bushiest possible plants, but the ultimate height is basically determined by the variety of mum you have -- they are available in a wide range of heights. Sometimes mums flop open regardless of what you do -- if this is the case, either be prepared to stake them each year, or try a different variety.
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