I planted three starts in a small group way back on the end of my lot. I mow over it every week. The spot is now about six feet by six feet. It will not die. I will before it does! I would not give one to anyone. LOL
To anyone considering planting Comfrey - it is VERY difficult to get rid of and spreads. My father planted some about 30 years ago and it has spread along the back length of my lot. It has tap roots that get enormous and also self-seeds. And it has no problem growing in the half clay / half rock that is my yard!
Thanks for raising the red flag! I had read some call it invasive, (and thought I had said more about that in the article than "perhaps too easy [ to grow]). But weekly mowing and it still spreads--that is extreme!
Some commercial ones, and the Russian species, are supposed to be less- or non- self sowing. But, famous last words like that have been said about a lot of plants.