Penstemon plants (Penstemon) are herbaceous perennials belonging to the snapdragon family (Scrophulariaceae). The plant's name stems from two Greek words meaning "five" and "thread," referring to the fact that penstemon plants have five stamens. Penstemon typically thrive in dry, well-drained soils in fully sunny locations. According to Southwest Colorado Wildflowers, more than 250 Penstemon species grow in North America.
Penstemon barbatus, also called southwestern penstemon, works well in rock gardens and borders. Native to New Mexico, Arizona and Utah, this penstemon variety is winter hardy in U.S. Department of Agriculture zones 4 to 8. Penstemon barbatus typically grows between 2 and 3 feet in height with a 1- to 1 1/2-foot spread. Tubular, red flowers bloom in May and June. The narrow, pointed leaves reach up to 6 inches long and look similar to willow tree leaves. This penstemon plant occasionally suffers from leaf spot disease.
The penstemon caespitosus plant, also called the beard tongue, is native to Colorado, Wyoming and Utah. This mat-forming penstemon species only reaches about 3 inches in height and 9 inches in width. Delicate purple flowers bloom abundantly from May through July. Hardy in USDA zones 4 to 7, penstemon caespitosus is often used in native plant gardens and rock gardens.
Penstemon canescens, often called gray beard tongue, is native to the Appalachian Mountain regions of the eastern United States. This clump-forming penstemon variety reaches between 1 and 3 feet in height with a slightly smaller spread. Violet flowers bloom in May and June. This penstemon plant features stiff, gray stems and hairy, green leaves. Leaf spot and root rot are sometimes problems. Winter hardy in USDA zones 5 to 8, penstemon canescens plants are commonly used in native plant gardens and borders.
The penstemon cobaea plant variety, also called the dew flower, is a clump-forming plant that reaches up to 2 feet in height and 1 1/2 feet in width. Winter hardy in USDA zones 5 to 8, this penstemon species naturally thrives in North American prairies and rocky bluffs. The pink, purple or white flowers bloom in May and reach up to 2 inches long. This penstemon plant features stiff, downy stems and pointed, downy leaves. Penstemon cobaea works well in wild gardens, native plant gardens and rock gardens.
Penstemon digitalis, more commonly called beard tongue, is indigenous to the fields, woods and prairies of the southeastern and eastern United States. These plants display tubular, white flowers from April through June. Reaching between 3 and 5 feet in height and 2 feet in width, this penstemon variety is hardy in USDA zones 3 to 8. The penstemon digitalis plant grows well when mass planted in borders, native plant gardens and naturalized areas.
Penstemon heterophyllus, also known as foothill penstemon, naturally grows in California's grasslands and forests. This penstemon plant thrives in USDA hardiness zones 6 to 10. Blue flowers bloom from May through July on stems reaching up to 1 1/2 feet in height. The penstemon heterphyllus works well when planted in dry soils on hillsides or in rock gardens.