If it's not dormant, it's too early. I'd suggest you put a little dirt in the bottom of the container, dump the root mass in there and put some more dirt on top. Then fill it with water and leave it sit till it goes dormant. Leave it where it will get colder faster . . . be that out of or in the pond. Then as soon as that sucker looses all it's leaves, yank it up, and divide it. Plant your tuber properly for next year and store the rest in a pot of wet dirt in a cold place till trading time in the fall.