I put in 2 plants last spring...to use for herbal remedies and such...and the chickens ended up getting most if it...they're crazy for the stuff! This year we have bunnies too, and I'm starting new compost bins so that comfrey's gonna be working overtime
I dunno about seeds either...I ordered plants. I rather doubt it'll spread much here either...I think it's pretty cold tolerant but it does like moisture and that we just don't have here in the high desert. Having to water seems a fair trade off for not having it take over though 
maybe headed towards the blackberries. I gave some to the rabbits, but they did not like it that much and then read not to give them very much, so I quit giving it to them. Before I "inherited" the rabbits, the owner would throw rhubarb to them, leaves and all. He had some chickens too and the rabbits and the chickens ate all the leaves. I thought they would be dead, but they lived on. I would never give them rhubarb leaves and decided aganist the comfrey, so now it just attracks bumble bees and I put it in the compost pile. It gets tall. I think they were almost 6 ft tall when a storm came with wind and blew them over. I cut them off at the ground and they grew right back up again before end of summer.