"Drought tolerant" is a vague term and means different things to different people. Penstemons are not as "drought tolerant" as a cactus, but some do grow and bloom in the same deserts as many cacti. Most penstemons have some degree of drought tolerance; even eastern and northern species require less water than do roses or annuals. Plants have many mechanisms to protect them from drying out such as woody stems, small leaves, thick leaves, and hair and other coatings on the leaves. They also have long roots that grow deeply into the soil if the soil is penetrable. These need to be watered deeply and infrequently. Generally, plants with large, thin, bright green leaves require more water than those with small or leathery or hairy leaves. The amount of water you need to apply will depend on the water-holding capacity of your soil, depth of the soil, natural humidity and precipitation and the origin of the varieties you have chosen. If you are growing local species and have improved your soil to some degree so that water sinks through it rapidly, you will rarely have to give them supplementary water after they have become well established. Creating wells around the plants is not necessary in soils that drain well. They can use extra water in the months before blooming and during the bloom period if precipitation is low. Drip irrigation, soaker hoses or hand watering are preferred watering methods. After blooming, watering can be reduced. Many species are surprisingly low in water usage in the west when given suitable light and soil conditions. Most hybrids bred for garden conditions are not as drought tolerant as species and perform better with regular watering. The large-flowered hybrid species should never be allowed to dry completely.

Selecting, Growing and Caring for Penstemons
From Chapter 41

Lindgren, Dale and Wilde, Ellen. 2003. Growing Penstemons: Species, Cultivars and Hybrids: American Penstemon Society. 519 West Lancaster Avenue, Haverford, PA 19041-1413 : pp. 105-110