Should I just chop off the just ex-flowers or should I cut the entire shoot back to the ground?
I must admit I never dead-head my penstemons Aqui (right or wrong - I don't know), I wait until the spring to cut them right back for new growth in the summer. Maybe someone more knowledgeable on these things will let me know if I am right :)
I am not an expert but I prune the Penstemons back in spring.
They are not 100% winterhard so I take cuttings in July (better and stronger dan those from Sept) . From the Barbatus I take seed, sow in March indoors, outside in early May in the sun.
If you remove faded flower stems just below the last ex flower (just above a leaf node), you can encourage side shoots to develop and a second flush of flowers. If nothing else it will keep the plant tidy if you dont want seeds.
Of course, prune as normal in the spring