Symphytum officinale (common names: Borraja)

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.

Was everywhere in the garden - only a few clumps left. Regrows from tiny pieces of root, and seeds itself, too.

Planted Jan 2008 in herb garden

Another one of my 'it was here when i moved in and that's my excuse' plants. It's a tall pink-flowered one, coarse because of its rough leaves yet stately because of its height which hits 4ft, maybe more. A sort of poor man's foxglove. I let it stay behind my greenhouse for two reasons...

1. Bees love it when it's flowering, and frankly they need all the help they can get.

2. Good as a fast breaking down plant for the compost heap or for making vile smelling comfrey 'tea' as a fertiliser. The leaves are supposed to be good as a mulch for currant bushes and bad for pieris - although, not remembering where I read this, it could be some old wives' tale. I think it may have something to do with their potash content in both cases.

I don't let it self-seed though. I don't believe it is the sterile variety that some organic gardeners use, as I've had some stray seedlings on occasion. Now I dead-head it after flowering.