Tussiemussie's Garden

About this plant

Genus: Symphytum.

Species: Symphytum officinale.

http://www.allotment.org.uk/vegetables_and_herbs/assets/comfrey.PDF
I make my own comfrey liquid feed for tomatoes, also a good feed for Runner beans and potatoes, see above link. Soak a tub full of comfrey leaves till it's liquid, strain and use diluted as a liqud feed. Trials on potatoes showed this was more effective than manure or normal fertilisers.
Apparently these can be invasive, not in my experience if they are cropped regularly before they seed to make fertiliser. They are very deep rooted,, so choose the site with care as they are not easy to dig up and move.