Perennial penstemons are easy to grow and maintain, even for beginners, and perform particularly well in borders and mass plantings. Nearly carefree, they thrive in full or dappled sun and love evenly moist, well-draining soil. Penstemons really don't require pruning for the sake of their health. However, they may grow unattractively leggy and become woody if you don't prune them annually.
Feed your penstemons a good, all-purpose fertilizer in early spring. This will prepare them for their performance during the growing season. Apply a 2- to 3-inch layer of composted mulch.
Use clean, sharp shears to cut the plants back by half in late April.
Begin watching penstemons for the production of new shoots from their bases. When the new shoots soon appear, cut off any remaining old bloom stalks at ground level. Don't remove any foliage. New flower spikes suitable for cutting should soon emerge.
Shorten stems of plants that are not producing new basal growth. Locate the lowest sets of healthy leaves, and make your cuts just below. Leave the foliage intact.
Deadhead your penstemons as soon as the blooms fade, unless you want them to stop blooming and produce seeds. Clip the flower stalk back to the first set of healthy leaves below it. This will encourage your plants to bloom again later in the same season and will be all the pruning they'll need to remain attractive throughout the season. Furthermore, removing spent blooms will direct their energies and resources toward re-blooming, rather than producing seeds.
Allow some of your penstemons to retain their spent blooms at the end of the season so that they can produce seeds, if you like. You can just let nature take its course, and the plants will re-seed themselves for a repeat performance next year.
Clip unwanted spent flower stalks with seed pods attached after they've browned and matured in the fall, and start a small wildlife pile with them. Add weeds, leaves, twigs and other garden and yard refuse as you acquire it. Wild birds and other creatures will appreciate the readily available seeds and other food and winter nesting supplies in the pile.
Cut your penstemon plants back by about a third in the fall, after they're through blooming for the year.