About Penstemon xm. ‘Red Rocks’: The Penstemon ‘Red Rock’, Penstemon x mexicale, has constant small, bright, rich, magenta flowers that bloom all summer long with narrow, lance-shaped evergreen foliage. ‘Red Rock’ has a plant height of 15″ and a spread of 12″. It is best to plant Penstemon in a full sun location, having a moist, well-drained area. Being drought tolerant, ‘Red Rock’ looks great in a perennial border, in a naturalized setting, in rock gardens or for cuttings. Hummingbirds and butterflies love this perennial, not to mention being deer resistant. Zones 5 to 8.