June is a great time to pinch back many of your perennials to keep them more compact. Since shorter stalks are more resilient to the wind, this is a great way to prevent your gorgeous blossoms from breaking off. Upright Garden Phlox can be cut in half to produce shorter, more compact plants. Joe Pye Weed (Eupatorium) can be pinched to produce fuller plants, sturdier stalks and more flowers. Other perennials that benefit from early summer pruning include Boltonia, Chrysanthemums, Hibiscus, Obedient Plant (Physotegia), Russian Sage, Veronica, and Yarrow (Achillea).
For detailed information on how and what to pinch, check out the University of Illinois Extension for suggestions.