Comfrey is one of the "permaculture all-star" plants because it has so many uses! One must take care to plant it in the right spot, because once established it is very difficult to discourage. Its roots go so deep that they are almost impossible to dig up... but usually that's a good thing, because it makes comfrey one of the hardiest plants in the garden. It tolerates full sun, full shade, drought, flooding, excessive nitrogen, salt, and mowing. As a nitrogen scavenger, it is a good plant for spots where dogs have killed off the grass.
The nutritious (to plants) leaves can be used as mulch, though they quickly turn black and unattractive. They can be pressed into a "comfrey tea" which is as good a fertilizer as raw sewage and smells the same. I find the best use for comfrey leaves is as a compost additive, either in sheet mulch or a traditional compost pile. The only plant I've found that breaks down faster in a compost is ragweed.
Russian comfrey is sterile and spreads only by division.
