For more than 2,000 years, comfrey, or knitbone, had a reputation as a healer of wounds, and broken bones (because its parts were so mucilaginous and sticky).

More recently, we’ve learned that comfrey contains allantoin, a substance that causes cells to multiply — which should be good in healing wounds and knitting bones. But it also contains pyrrolizidine alkaloids, or PAs, and beginning in the late 1970s, researchers began to connect PA with a disease that causes major blood vessels from the liver to clot.

Ever since, most herbalists advise never to use comfrey internally. Anyone with liver problems and pregnant and nursing women shouldn’t use it externally, either. Others can use it as a compress or poultice for about any insult to the body’s outer covering, from bruises to cuts and scrapes.

Erect and brawny, comfrey rises to at least 4 feet (1.2 m) tall and wide, is floppy in wind and rain, and bears 10-inch (25-cm) lower leaves covered in itch-producing hairs. The stems have pronounced “wings.” Still, its forked tassels of bell-shaped flowers in blue-violet, pink, or pale yellow are hard to resist.

It blooms from late spring to early summer, occasionally much longer. Large and made quite permanent by a taproot that can burrow as deep 6 feet (2 m), comfrey needs loose, deep soil, rich with organic matter. Comfrey doesn’t produce many seeds, so start it from divisions or root cuttings.

Hardy through Zone 3, comfrey can grow in partial shade, but full sun makes the stem stronger and less apt to fall over on blustery days.

Click here to find out what Zone you are in.