Marlene:
I prune my penstemon's after bloom time (they have similar growth characteristics).Cut the plant back to one node above the foliage line of your bloom spike.This will help
to encourage new growth from the base(Crown)of your plant.
Should also help with rebloom (weather permiting).
Art

The advice sounds correct, but Marcus is a form of Salvia nemerosa, yes?

Marlene & Rich:
Yes ,S.nemerosa according to the information I have seen.
Art

Thanks, Rich and Art.
Just to clarify: would I cut most of the spike and stop where the first node is? I left the the spent blooms and spikes a long while after they first bloomed this spring. As a result many spikes were just dried stalks when I pruned this week. Just wanted to know how best to prune Marcus for future trims.

Don't know what your temperatures are running and
how much sun you have them in(should have asked that first).Yes,most of the spike will be gone.You will cut most of the spike so sunlight can go to the crown. If some have dried already you should see growth regenerating already.

Art