Penstemons need to have their crowns dry so a mulch that absorbs and retains water can be very damaging. If you want to mulch a bed that contains penstemons, choose a mulch that protects the soil from the sun but does not hold water. Hard shells, such as pecan, are attractive and are not generally water-holding unless applied to great depth. Shredded bark or ground tree trimmings, pine needles, pine and cedar duff can also be used. They should not be pulled right up to the crown. There are many types of gravel and grit available in most areas and they may be the best choice in arid climates. Look for regional products that do not retain water.

Selecting, Growing and Caring for Penstemons
From Chapter 41

Lindgren, Dale and Wilde, Ellen. 2003. Growing Penstemons: Species, Cultivars and Hybrids: American Penstemon Society. 519 West Lancaster Avenue, Haverford, PA 19041-1413 : pp. 105-110