This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Perennial scabrid herbs with thick fleshy roots. Radical leaves stalked, cauline sessile or decurrent. Flowers white, red, purple, blue or yellow, in terminal bracteate cymes. Calyx 5-lobed or -toothed. Corolla tubular, inflated, shortly 5-toothed, the throat closed by ciliated scales. Stamens 5, included. Nuts 4, smooth. There are about fifteen species in Europe and West Asia. The name is said to be an altered form of a Greek word signifying to cement, in allusion to the healing properties of some species. S. officinale, Comfrey, a British species, is a tall herb with ample foliage and yellow or purplish flowers. This species was formerly employed in domestic medicine.
1. S. Bohemicum. - This is scarcely distinguishable from the common Comfrey, except in its bright reddish purple flowers, which appear in Summer. .
2. S. asperrimam. - A tall-growing species, remarkable for the prickly bristles with which it is closely beset. The flowers are red in bud and eventually blue. A Caucasian plant, bloom-. ing in Summer.
S. Caucasicum, from the same region, is a dwarf-growing species with bright blue flowers.