Many hollow or pithy plant stems provide excellent homes for cavity nesting insects like the small carpenter bee in the photo above. Simply cutting back dead flower stalks in varying heights of 8-12 inches will provide valuable habitat to these tiny but important pollinators.
Check out this Xerces brochure, How to Create Habitat for Stem-nesting Bees, for advice on when and how to cut your plant stems for maximum benefit to cavity-nesting bees.
Photo of small carpenter bee by Heather Holm.