Are you near a nuclear power plant, by any chance? :-)

Do you have a lot of comfrey, or only the one plant? I planted mine 15 or more years ago, and sometimes I marvel that I actually paid money to buy that first plant. It would be everywhere if I let it. I dig them up every spring and literally toss them across the roadÂwhere they take root immediately. I like comfrey, and leave some on this side of the road, too. But man, does it spread! Ranks right up there with lemon balm, which I paid money for, too . . .

I keep reading about common comfrey's spreading powers, and I believe you all, but the one plant that I planted in my home garden in 1988 is still just 1 plant in the same place! Guess it's the horrible damp, clay soil in that spot that I've never improved, because then it would travel all over! I added a lot of gypsum, compost and other organic matter to that clay soil about 20'from that comfrey
where a small apple mint lived, which became a monster and attacked my garden big time before I finally got it under control!! I had the same problem with lime balm (just like a lemon balm, but more limey than lemony smell). I just got on Fri. a lovely blue English/Quaker Comfrey from Barb Steele (Alloway Gardens, Littlestown, PA), which I have not planted yet. I have been looking for this plant for 20 years to replace the one that my neighbor mowed down. If I remember correctly, it does not have the bad manners of the purple one. The lovely blue flowers reminded me of Virginia Bluebells. I do want to place it carefully, in case it decides to take off on me! By the way, I highly recommend Alloway Gardens for perennials as well as herbs.
Their display gardens alone are worth a trip and her plants are reasonable as well as correctly named. I've been a customer for over 25 years...