Hundreds of penstemon cultivars are available to gardeners. Some are only a few inches high and regrow from their crown every spring. Others can reach 2 to 3 feet high and produce new leaves on old growth. Needless to say, you must know how your penstemon grows in order to prune it properly. However, if you accidentally prune a penstemon that grows on old growth to the ground, it should regrow (but it will do so as a stubbier version of itself).
Prune herbaceous or perennial penstemons in mid-spring after all danger of frost has passed.
Cut penstemons that regrow from the crown back to the ground (just above the crown).
Shorten the stems of penstemons that leaf out on existing stems making cuts just above a healthy bud or leaf. Ideally reduce the height by one-quarter to one-third, for sturdier stems, and to keep the plant from flopping over during the summer.
Deadhead penstemons (cut back faded flower stalks before seeds form) in late summer or early fall when the flowers have started to fade and the flower stalks have begun to die. Remove flower stalks as close to the crown as you can.
Remove dead leaves and broken or dead stems when you deadhead flowers. You might be tempted to cut back your penstemon now--but don't do it. Penstemons need their stems left intact to protect the crown from winter damage. The stems also provide visual interest and height to winter-dormant gardens.