Comfrey is a hardy perennial herb in the borage family, long prized for its traditional use in healing bones and soft tissue injuries. It grows between 50 and 150 centimeters tall (roughly 2 to 5 feet), produces clusters of bell-shaped flowers, and has deep taproots that make it nearly impossible to remove once established. The plant has a rich history in folk medicine stretching back centuries, though modern science has revealed both genuine healing properties and serious safety concerns worth understanding.
How to Identify Comfrey
Comfrey grows from a thick, branched taproot that can extend several feet into the soil. The entire plant is covered in spreading, downward-curved bristly hairs, giving the leaves and stems a rough, sandpapery texture that’s unmistakable once you’ve touched it. The large, lance-shaped leaves can reach over a foot long, and they get progressively smaller as they climb the stem.
The flowers hang in curling clusters and come in a surprising range of colors: deep purple, pale purple, blue, pink, or cream. Common comfrey (Symphytum officinale) is the species most often referenced in herbal medicine, but Russian comfrey (a hybrid of common comfrey and prickly comfrey) is the variety gardeners typically grow. The two look similar, though Russian comfrey tends to be larger and more vigorous.
Why It Was Called “Knitbone”
Comfrey’s long list of folk names tells its story. “Knitbone,” “boneset,” and “bruisewort” all point to the same traditional belief: that this plant could help broken bones heal. The name “comfrey” itself likely derives from a Latin root meaning “to grow together,” and the old German name, Beinwell, translates literally to “bone well” or “bone healer.”
The 17th-century herbalist Nicholas Culpeper wrote that comfrey was “so powerful to consolidate and knit together” that boiling it with separated pieces of flesh would supposedly rejoin them. He recommended decoctions of the root for broken bones, wounds, and internal injuries. By the 20th century, external comfrey preparations were formally indicated in European herbal medicine for bone fractures, promoting callus formation (the new tissue that bridges a break), strains, contusions, and pulled muscles and tendons. Germany’s Commission E, a respected authority on herbal medicines, gave comfrey root a positive rating for these external uses.
What Makes Comfrey Work Topically
The compound most credited with comfrey’s healing reputation is allantoin, which promotes cell growth and tissue repair. Comfrey roots contain between 7 and 25.5 milligrams of allantoin per gram, a notably high concentration compared to most plants. Allantoin works alongside rosmarinic acid (which reduces inflammation) and mucilages (gel-like plant sugars that soothe irritated tissue) to produce the effects people have observed for centuries.
Modern comfrey creams, ointments, and salves are widely available and used for joint pain, sprains, bruises, and muscle soreness. A typical application is 2 to 4 grams of cream rubbed into the skin three times a day. The key safety guideline: don’t use comfrey cream for more than 10 consecutive days, and only apply it to intact, unbroken skin.
The Liver Toxicity Problem
Comfrey contains a group of natural chemicals called pyrrolizidine alkaloids, or PAs, that can cause serious liver damage. These compounds are present in both the roots and leaves, with roots generally containing higher concentrations. When PAs are processed by the liver, they form reactive molecules that damage liver cells and can cause a condition called hepatic veno-occlusive disease, where small blood vessels in the liver become blocked.
Researchers have identified more than a dozen specific PAs in comfrey, with lycopsamine and intermedine among the most toxic to human liver cells. Lab studies show these compounds can inhibit liver cell viability even at relatively low concentrations, and combinations of PAs are more damaging than individual ones. At 75 micrograms per milliliter, a mixture of intermedine and lycopsamine reduced liver cell survival to just 33%, compared to roughly 48% for either compound alone.
This is why every major regulatory body restricts internal use. The U.S. FDA issued a safety communication in 2001 flagging comfrey as a source of pyrrolizidine alkaloids, and comfrey-containing dietary supplements intended for ingestion were pulled from the market. The European Medicines Agency permits only external use of comfrey root on intact skin in short-term treatments of up to 10 days, with strict limits on PA exposure set at 0.35 micrograms per day. Drinking comfrey tea or taking comfrey capsules is not considered safe.
Common Comfrey vs. Bocking 14
If you’re considering growing comfrey, the variety you choose matters. Common comfrey (S. officinale) has a clumping and spreading habit. It self-seeds freely and can become invasive, popping up in places you never planted it. Its deep taproot means that even small root fragments left in the soil will regrow into new plants.
The cultivar most recommended for gardens is Bocking 14, a selection of Russian comfrey developed at a trial ground in Bocking, England. It’s sterile, meaning it produces no viable seeds, and it’s comparatively well-behaved, reaching about 1 meter in height and spread without aggressively colonizing your garden. Because it can only be propagated from root cuttings, it stays where you put it. For anyone who wants comfrey’s benefits without the commitment of a plant that never leaves, Bocking 14 is the standard choice.
Comfrey as a Garden Resource
Beyond its medicinal history, comfrey is one of the most useful plants in organic gardening and permaculture. Its deep taproot pulls minerals from subsoil layers that shallow-rooted plants can’t reach, concentrating them in its fast-growing leaves. Russian comfrey foliage contains exceptionally high levels of potassium (nearly 53,000 parts per million in one Cornell study) and silicon, both well above the thresholds that qualify a plant as a “dynamic accumulator” of soil nutrients.
Gardeners use comfrey leaves in several practical ways. Chopped leaves can be laid directly around plants as a nutrient-rich mulch that breaks down quickly. Leaves steeped in water for a few weeks produce a dark, potent liquid fertilizer especially high in potassium, the nutrient most important for flowering and fruiting. That same Cornell study measured potassium in the resulting liquid fertilizer at 889 ppm. Comfrey leaves also make an excellent addition to compost bins, where they act as an activator that speeds decomposition.
A single comfrey plant can be cut back several times per growing season, producing large volumes of leafy biomass each time. This combination of rapid regrowth, deep nutrient mining, and high potassium content is why permaculture practitioners often plant comfrey around fruit trees or along the edges of garden beds as a permanent, self-renewing fertilizer source.