Comfrey (Symphytum officinale) is an herb with myriad beneficial properties.
It has a very long tap root and therefore mines deep into the soil for minerals that lots of plants cannot access. For this reason, it is known as a dynamic accumulator.
It's been called "knitbone" for centuries because of it's amazing ability to heal wounds and mend broken bones.
It's easy to grow and propagates well from root cuttings. Comfrey is very mucilage, or gummy when you break open stem. It is demulcent, expectorant and astringent.
The leaves are very large, like football size. However, they dry out quickly once harvested, nearly overnight. You can put the leaves in water for about a week and they breakdown to make a very pungent liquid fertilizer. The stems are stringy and rubbery, they will stretch quite a bit before breaking.
Much has been made of the link to liver damage from pyrrolizidine alkaloids, which comfrey contains in small quantities.
The oft cited study from 1968, force fed rats highly concentrated doses of these alkaloids for months on end. When the rats wouldn't eat the powderized food any more they injected them with it. The plant used in the study was not comfrey, it was Senecio. There's some fascinating language in the document for those that care to read it.
Based on these studies the FDA in 2001 issued a strongly worded statement advising that the internal use of comfrey as a medicinal could lead to liver disease. Foods that can contain pyrrolizidine alkaloids like milk, eggs, honey and grains are not regulated. Herbs bad, food good.
It's doubtful that pyrrolizidine alkaloids consumption will ever be a contender for the number one spot for causing liver disease, that title remains safely with the reigning champion, alcohol.