When my PD Penstemon stops blooming how much do i trim it?

Answer 1

MASTER GARDENER TO THE RESCUE

PRARIA DUSK Penstemon

i have been a Master Gardener for the univ of Calif for over 15 years & i enjoy growing penstemon in my garden.
there are some 250 species of penstemon. only a few are widely available. P. ambiguus (prarie) is a native to southwestern u.s. if given too-rich soil and too much water it may die quickly. if cared for correctly you should have blooms from early summer to early fall.
yes, remove the dead clusters if you want the blooms to continue. don't over water! it is a short lived plant, 2 or 3 years max. hummingbirds love it.

Answer 2

If your talking about it being finished for the season, cut it back to the basal leaves. If you look close to the ground you will see a cluster of leaves grouped in something similar to a rosette. These are the basal leaves. Trim the entire stalk back to just above that group of leaves.
If your are talking about during the season and you are doing whats called deadheading, then you will cut back to the 1st set of leaves.