I'm in Melbourne Australia, and have comfrey throughout my front and backyard. We are in heavy clay and it grows no problem.
I'd just caution that if you ever dig it up - you'll always leave at least a bit of root behind, which will sprout again. you really don't need much to propagate and spread it. So, don't plant it where you don't want it!
I'm aware that most of you are above the equator, but down here, my species of comfrey grow to about 70cm (i don't know what that is in feet!), and then die down. I can cut it down 3-4 times in Summer alone, no problem, and it will shoot up again. It does die down in Winter, and the spikey leaves can be a little irritating to the skin if you're taking the leaves for compost / moving it for where ever you need. I tend to just chop and drop.
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