When deadheading flowers, leave stems about 18″ long; in other words, don’t cut them down you the ground. Many bees can use them for as nesting sites. They will lay an egg in the stem and then pack it with food, plugging it with grass or mud. Bees will also tunnel out stems like elderberry or, as in this case, Penstemon. Now, I know you are thinking, “But I don’t want ugly dead sticks in my garden”. Plants, like this Delphinium, will grow up around last year’s stems, concealing them with foliage and beautiful blooms.