Little pruning is needed on penstemons but removing stems after flowering is completed helps to encourage the formation of new basal shoots and prolong the life of plants. Some species and many hybrids will have a second flush of bloom if cut back, particularly in long-summer areas. Many large-flowered hybrids and bedding types need only dead-heading rather than cutting back to bloom continuously. It is a good idea to leave a few of the lower seed capsules on a couple of stems to dry if you would like more plants of the same appearance. You can allow seeds to fall in place or capture them before the capsules open for growing more plants or sharing with other gardeners. Often birds will find fallen seed and plant it for you in surprising locations! Large species, such as P. palmeri and P. clutei form thick, woody stems in their second and third years from which new shoots sprout at many closely spaced nodes. These should be thinned out to maintain the plant for four or more years. Bushy plants such as P. ambiguus, P. triphyllus and P. diphyllus should be cut back to a few inches in fall or early spring and they will grow new herbaceous blooming stems. The northwestern Dasanthera species, cultivars and hybrids frequently have leaves turn brown or lose most of their leaves when exposed to winter sun and dry conditions. They need to be cut back, but not until late spring to be sure you are not removing live wood. Sometimes old plants do not produce new growth in the center. This can be remedied by piling a loose, very gritty soil mix on top of the decumbent stems in the open area to encourage new growth. Continue this top-dressing throughout the year to mimic the scree conditions in which they are at home.

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