I have 2 patches of comfrey. One planted approx 30 years ago, one approx 20
years ago. The first one spread only when I tried to dig it up (made it
mad, I guess.) The second, planted from roots of the first, spread only
when woodchucks decided to nest under it. They are both now in part shade,
increasing year to year. The are having to compete with brush and nettles,
in increasing tree shade. Deer mow it close in the cool fall weather. I
have good aquaintance with several invasive exotic species; this comfrey is
NOT one of them !