Rocky Mountain Penstemon (Penstemon Strictus) Zellajake Farm and garden offers Rocky Mountain Penstemon. Its spikes of bright blue flowers and evergreen foliage is one of the easiest-to-grow Penstemon. Long lived this beauty thrives in most any well drained soil with full sun exposure. Easy to grow attracts hummingbirds bees and butterflies. This wildflower is also rabbit resistant. Days to Maturity: 85-95 in full sun.