Comfrey is a 3- to 5-foot-tall perennial herb native to Europe and Asia. Its botanical name is Symphytum and it has been cultivated since 400 B.C. Comfrey has a long history as a medicinal plant used for healing wounds and broken bones. It was known as "knitbone" in earlier times and its name is derived from the Latin word for "grow together." Comfrey contains allantoin, a hormone-like substance that stimulates cell division and supports growth of new tissue. There are three types of comfrey: common comfrey, prickly comfrey and blue comfrey. Comfrey is commonly used as a green fertilizer crop and as animal food.
Comfrey is grown as a biennial and a perennial herb plant. It has a black turnip-like root, broad hairy leaves and small white or purple flowers. It grows 3 to 5 feet in height and prefers 3 foot spacing between plants. Comfrey grows wild on damp riverbanks and ditches and spreads very easily.
Optimal conditions to grow comfrey are full sun and compost-rich soil. Choose a garden bed that gives access on all sides. Comfrey spreads very easily and needs to be dug up when it has grown into other growing spaces. When dividing comfrey take care not to dispose of root fragments in the compost pile because they will re-root quickly. Weed the bed well and add one shovelful of compost per square foot.
Comfrey is propagated by root cuttings, crown divisions and transplants. Place cuttings 2 inches deep and 2 to 3 feet apart. Water thoroughly and keep moist until plant shoots appear. Plants will grow quickly and produce large quantities of leaves. Do not harvest comfrey until the second year of growth. At that time it can be harvested four to five times per year.
Care and Harvest
Water comfrey plants regularly. Comfrey has no common disease or pest problems. When plants are 2 feet tall they are ready for harvest. Cut the plants off to 2 inches above the ground. Use a sickle or scythe to cut the plants. Gardeners familiar with comfrey recommend wearing heavy garden gloves because the leaves have hairs that irritate the skin. Comfrey leaves are now ready for use as cattle and rabbit food. When grown as a green manure it is tilled into the ground to increase soil fertility.
Comfrey should not be used internally. The alkaloids in it are reported to cause liver damage if used over a long period of time. Excessive ingestion of comfrey is toxic and has been shown to cause pre-cancerous conditions in rats. Comfrey can be used externally as a salve or poultice. The blue cultivar has the highest allantoin content for use as an external medicine. It is sterile and will not reproduce by seed.