I’ve heard that I can prune my chrysanthemums in the summer so that they are fuller. When is it appropriate to do this?

Yes, for a fuller plant with more blossoms you can prune your chrysanthemums two or three times before mid July. You should pinch the tips out of the tops of the stems so that about four inches of growth remains. This will encourage side branching and more blooms. If you are interested in reading more about mums, go to http://ohioline.osu.edu/hyg-fact/1000/1219.html