I grew comfrey for years. At first, because I thought it was really quite good looking. After that, I grew it because I couldn't kill it. If it isn't were you planted it - it is a weed. kill it, quick.
Comfrey is indeed invasive. It self-seeds too, so keep your patch away from the main garden.
On the other hand, it is a generous, wonderful plant that deserves respect. Chickens will eat it
IF it is vertical, either as a live plant, or hanging from the rafters.
A friend of mine makes an incredible liniment based on comfrey, poplar buds and other herbs.
I used it when I was recovering from a broken ankle and had to get back to work on my feet. I'd
limp into the shower, in pain, apply the salve, and be painfree for the day. I gave a jar to my
sister-in-law whose broken ankle was a long time healing. It did its magic there too.
I have NO financial interest in this one. http://flyinghandsfarm.addr.com