(Editor's Note: This article was originally published on April 9, 2010. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your quesitons.)
Common comfrey (Symphytum officinale) is a perennial herb native to Europe and Asia but now found in the wild worldwide. Because of its traditional uses in herbal medicine, comfrey has also been cultivated in thousands of farmyards and kitchen gardens. It's an easy plant to cultivate and propagate; perhaps too easy. Comfrey survives rough treatment and especially thrives in moist areas. In spring, the cold hardy rootstock pushes out large oval, hairy, textured leaves of deep green. By summer, flowering stems rise to three feet or more, topped with clusters of dainty downfacing buds. This perennial is better described as bold than beautiful. The coarse texture, floppy summer habit and relatively small blooms combine to make a plant better suited for a casual, distant backdrop than a closely viewed or formal garden. Gardeners who want to show off their comfrey could find it worthwhile to seek out a commercially bred hybrid or another species of comfrey. (You might begin your search here in PlantFiles or PlantScout for more refined comfrey cultivars.)
Comfrey's reputation for medicinal qualities stretches way back to ancient Greek and Roman times. Throughout the history of herbal treatment, and like many herbs, it seemed comfrey was good for lots of things that "ailed you." One nickname for comfrey is knitbone, alluding to an ability for comfrey leaves to heal broken bones. It's written that chunks of meat will stick together if boiled with comfrey roots. Comfrey leaves contain cell-stimulating allantoin. That chemical has healing attributes supported by modern science. External comfrey leaf applications are said to be useful, and widely accepted in some places, for treating bruises, sores, and a variety of skin ailments. Comfrey may even have antibacterial action.
Comfrey rode the wave of herb popularity of the 1970's. Many herb newcomers began eating, brewing or otherwise experimenting with all kinds of plant parts. And medical researchers examined comfrey more closely. What they found (certain alkaloids suspected of liver damage) has tempered the enthusiasm for comfrey consumption. Comfrey plant parts are no longer officially considered safe as a tea or green edible. As with many herbal products, definite proof of specific benefits and risks is hard to come by. One source says that the toxic alkaloids in comfrey leaves are very low while the plant is in bloom, possibly making its use as a tea or vegetable during that time a less dicey proposition. (It amazes me that herbalists like Aunt Bett apparently knew that timing was critical in collecting comfrey leaves.) And surprisingly, comfrey (usually Symphytum x uplandicum, Russian comfrey ) is sometimes grown as animal browse. Those alkaloids don't seem to cause illness to livestock.
Comfrey is very useful in a very different way for farmers and gardeners. Growers can use comfrey leaves to make a fantastic, potassium rich, compost additive or liquid "tea" fertilizer. Tough far-ranging roots make Symphytum hardy, and hard to remove. But those same roots extract vital potassium (the K in fertilizer's N-P-K) from deep soil layers and it is distributed throughout the comfrey plant. Plants need potassium for many metabolic functions; keeping crops supplied with potassium is essential for their overall health, and flower and fruit production and quality. Comfrey roots also hold extra potassium.
I've looked through several pages of "hits," trying to find a university-based page about using comfrey plant parts in the garden. So far I haven't found one. I have found numerous links to discussions, anecdotes, and even videos on making and using comfrey leaf tea or juice. The general idea is this: harvest a lot of comfrey leaves, fill a bucket with them and let them rot. Then drain off the resulting dark liquor. Dilute it and use it as fertilizer, foliar feed or compost boosting liquid. The-Organic-Gardener.com says that raw comfrey leaf tea has an N-P-K ratio of about 8:3:20. That site recommends diluting the tea with fifteen to twenty parts of water before pouring or spraying it on garden beds or crops. Another writer dilutes with only ten parts of water. Without more specific advice, you'll have to use your judgement. Understand that the nutrients in diluted comfrey tea are soluble and quickly absorbed by plants. Frequent applications of a weak tea are better for the plant's health than one concentrated treatment. One other caution: hold off on comfrey tea treatment until plants are near maturity (ready to flower or set fruit.) Younger plants are more sensitive to excess nutrients that alter their growth.
If comfrey tea isn't quite your cup of tea, reap the gardener's benefits of comfrey by using freshly cut leaves. The leaves are unusually high in potassium, and contain nitrogen (N) and phosphorous (P) as well. Established comfrey plants can be cut close to the ground a few times each summer and will rebound without harm. Use the fresh leaves as a mulch, or chop them and mix them into the soil near growing plants. Use juicy green comfrey leaves to balance the dry "browns" of your compost. Comfrey roots hold potassium too, but might resprout. Roots can be used if completely dried. If you plan to harvest comfrey in quantity, keep your plants thriving by supplying them with plenty of water, sun, and rich soil.
Like me, you may have been only vaguely aware of comfrey in the past. I hope this introductory article has somewhat satisfied your curiosity. I recevied my first comfrey plant last year from another Dave's Garden-er, and found comfrey hardy and interesting to grow. I've just confirmed that it survived the winter, and am excited about my first experiments this year with some of its uses!
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Thanks to fellow writer Bev Walker (Sundownr) for generously sharing comfrey pictures from her garden!
For further reading (in addition to links above in text)
Kowalchik, Claire and William H. Hylton, eds. Rodale's Illustrated Encyclopedia of Herbs. Emmaus, Rodale Press, 1998, ISBN 0-87596-964-X
Brown, Deni. New Encyclopedia of Herbs and Their Uses. New York, DK Publishing Inc., 2001. ISBN 0- 7894-8031-X