It's very possible to get as plants ... but it is only just starting to grow. (It dies back in the winter, and starts regrowing about now. I got mine from eBay) There are loads of posts on comfrey..have a go with the search button for more info.
Most people recommend the sterile variety Bocking 14. The "seed" stuff is a weed... you'll be digging it out forever. The Bocking 14 stays where you plant it.

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