Many species and hybrids of penstemons can be divided from established plants. It is usually a much surer and quicker way to have more of those very desirable plants.
Gently loosen the soil around the base of the plant to determine if all the roots are coming off a single stem or if the crown divides into several rosettes, each with some roots. For example, Penstemon digitalis is easy to divide whereas Penstemon grandiflorus can be more challenging to divide. You may also want to divide plants that become less floriferous after several years when the clumps have become quite woody.
To begin the division process, water the plant well and dig it up the next day. Take it to a cool, shady location and break it apart into sections so that each section has some roots and a rosette of leaves. In temperate climates, it may be replanted in the ground immediately. In harsher climates, place each of the sections into a pot of moist soil, water well and place in a shaded location where it can recover. Gradually bring it into bright sun and wind. When wilting no longer occurs, it can be replanted in the ground. Water transplants regularly until well established. The time to divide will vary with the plant and the climatic conditions. One technique that some use to divide penstemon is to mound some soil in the center of the crown of the plant in the spring. This encourages rooting of the crown. The plant is then divided in the fall.
In some locations, dividing plants in early spring is best because the progeny have a chance to recuperate before hot weather. In other situations, right after blooming is a good time to divide or it may be desirable to wait until the worst of the summer heat is over before dividing. This will depend on your climate and the plant you want to divide.