Have a look at Anchorman's blog of November 2010 where he lists some long flowering plants.
Strange that your penstemons aren't doing well - how long have you had them?

I was wondering the same as I think they lose some flowering impetus when the get woody so I take cuttings every year and have plants of a variety of ages.

That's useful info Barbara. How long do yours take to get woody? Mine aren't showing any signs of it yet. How hard back do you cut them?

Hi Steragram, re penstemons. It seems to depend on the variety. I have mostly big flowered ones like czar and a red one with white throat that I have lost the label. However it's taken about 4 years to get woody. I cut them down in spring to new shoots at the base and try to then cut out the woody stem. Mine are still flowering well up here including one in a container . I have never mulched them as the RHS book suggests.

I have a czar about three years old, and have noticed its getting a bit woody at the base. will see how it goes next year.