You want to get roots. Do not get the kind that can be propagated by seed. Seeds end up where you don't want them and comfrey has a very deep root and is hard to eradicate.
Get roots, plant it where you want it. Want more? Wait a year and divide your plants. Just a small root will grow.
This stuff is tough, I ordered mine last year from
Coe's Comfrey and my 10 roots arrived in March. We were in the process of buying a house and due to close about the time the roots arrived. The deal fell through and we did not get our house till the middle of June.
I took the roots with me the day after closing and was going to plant them after I did some work inside the house. I fell and broke my ankle badly and the roots sat in the refrigerator for another six weeks until I could coerce my son into putting them in some dirt.
He took a big round tub and filled it half full of dirt scraped up out of the chicken yard, watered it thoroughly and left. It was not until the end of August when I was finally able to get around on my own outside that I realized he had not put a drainage hole in the tub and the water was up to the top, no growth. I took my drill and made a hole and four of the ten roots grew at that point. The chickens loved it.
It died down over winter but is sprouting already. I will be planting it around the chicken yard. It would probably grow great under rabbit cages as straight urine is not too strong for it. Chicken poop can go on straight as well.
It really is great stuff, but be mindful where you plant it.