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.
Stem pith from bee tunneling, Penstemon ‘Husker red
Plugged bamboo nests, Bee Research Lab
Delphinium stems left for stem nesting bees. New foliage and flowers grow up to conceal old stems.
Flower for Pollinators 
One thing to note is that the bee life cycle takes a full year. Year one: stems grow, year two, bees lay eggs, year three eggs hatch. The stems need to be kept in tact thru egg hatch the 3rd season.
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