As a permaculture designer in the Bay Area, Comfrey (Symphytum officinale) is one of my favorite and most used plants in designing a food forest. It's deep taproot draws nutrients from sub-soil levels that other plants cannot reach thus making large amounts of potassium available in its leaves. The carbon and nitrogen levels are well balanced in it's plentiful leaves which generate four to five pounds of leaf matter per harvest. When designing a food forest we will typically place comfrey plants around drip line of fruit trees. A couple times a season the leaves can be "chopped and dropped" allowing the bountiful nutrients stored inside the leaves to decompose and be absorbed by the orchard. Comfrey, originally native to Europe, is widely known and used for it's medicinal qualities. Leafs can be directly applied topically to encourage skin regeneration and decrease inflammation. Allatonins in the plants leaves stimulate cell growth. When ground into a poultice the herb is helpful in treating bruises, swellings and sores. A tea can also be made from comfrey, but the USDA suggests not to ingest the comfrey for longer than 10 days in a six week period due to the levels of hepatotoxic pyrrolizidine alkaloids.
Some Additional videos about Comfrey:
Liquid Comfrey Fertilizer: