It has been my pleasure to grow penstemons experimentally for the past few years. I am glad to be able to report favorably on those adapted to the midwest and the Great Plains which I have grown from seed. There are some important facts I have learned which should be of interest to beginners. Because I so strongly believe penstemons should be more generally grown in our gardens, I will be more than glad to pass on such knowledge as I have learned from actual experience. For the life of me I cannot understand why people would have synthetic grass when they can grow live plants.
Choose seed which other gardeners use successfully in your own locality to begin on. Later on you can experiment a little. It is not enough to be able to germinate the seed – to be successful, the plant should survive the winter and bloom. Belonging to a Round Robin or the Penstemon Society is a wonderful help in solving problems. In my case, I have found the Glaber group and the Grandiflorus seem to be the hardiest here. They are good to begin on. There are three distinct colors of Grandiflorus, a clear pink, a pure white and the more common though beautiful lavender. Glaber has many members in its large group in shades of blue and rose. If you have a rather meager, well-drained soil in a sunny place you are well fixed to grow these penstemons.
I like to sow seed very late in fall or early in spring, as I like early spring germination best. Then I can do the necessary planting in late June after a good rain. The Glaber I like to set out in small clumbs, but the Grandiflorus being more sturdy may be planted out individually. However, I like to plant each kind together in separate small beds, as they make a better showing that way.
Because penstemons have very long roots, the seed box or flat is better without a bottom; that is it should rest on the very ground itself. I also like very deep, large, leaky old dishpans to plant seeds in; they can be moved around at will. Somehow or other, I have all kinds of trouble with coffee cans, they seem to get spilled or dry out or do not germinate well. Others report perfect results with them – I suspect it is just me who is to blame. I just use the garden soil for the seed bed and give the flat a little protection by throwing over it some loose branchy material. This is to keep dogs from running over it; it also serves to prevent the wind from blowing off the top soil.
So far I have not found penstemons to be true perennials. Usually after the third year they begin to dwindle off. For this reason it is best and safest to treat them as biennials, planting new seed each year.