I grow a number of Penstemons in the garden, they are evergreen shrubby perennials, they flower from early summer and continue into autumn until the first frosts.
Each Spring, I cut them back, usually mid April or end of the month. Cutting back last year’s growth allows the new shoots from the base to develop and prevents the plant getting too leggy and improves the display of flowers.
See my video below to see how I do it.
Ciaran The Gardener 