The many species of penstemons show a great difference in the size of the flowers. Penstemon cobaea probably has the largest blossoms. A bed of them in full bloom is a spectacular sight and one not soon to be forgotten. The blooming period lasts from two to three weeks.
This herbaceous species is of medium height. The large soft leaves are thick and a glossy green. The variation in the markings and coloring of blossoms is both interesting and tantalizing. They vary from almost pure white with lavender to purple veinings to a good lavender. On some, the lower petal lobes may be lighter in color than the rest of the blossom or the amount of color in the throat will be deeper.
Seeds of Penstemon cobaea start well if planted in late Fall in a seed frame. They will germinate in late March or early April. As soon as the little plants are large enough to handle they can be reset in the garden.
Penstemons have a delightful blooming arrangement. Instead of a sequence of the bloom with single buds from the base of the spike to the tip, there are clusters of buds in the axils of the bracts, and one flower and then another opens in each cluster at one time. Thus one has a full spike of flowers in bloom instead of just one part.
Penstemons have a wealth of blossoms and are usually heavy seeders. This is thought to be the reason why some species appear to be almost biennial instead of perennial. It is a good idea to allow only a stem or two to form seeds and to remove the rest of the withering flower stems promptly just below the chin of the lowest blossom. Thus the strength can be utilized in species like cobaea to make new rosettes after blooming which will usually live over another year. It is also often helpful to apply a top dressing on such kinds that have woody stems. This top dressing could well be of sifted compost and sand. Apply this top dressing after the plants have finished blooming to encourage them to make new growth.
If one has a large planting of Penstemon cobaea with a number of colors, one can arrange a lovely bouquet by using a very large, heavy, dark vase. Commence with the darkest lavender purple for the base and continue with lighter shades, using the whitest one for the outer part of the bouquet. Stems should be charred over a flame or placed in a couple of inches of hot water for a few moments, then plunged into cold water. If the stems show signs of wilting, repeat the hot water treatment.
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