If you can get away with leaving your regular dahlia tubers in-ground, let it stay outside, highly protected from the elements; but do not let it freeze.

This advice is coming from someone who let their beautiful D.I. freeze two years running, so proceed with caution. I didn't really have a choice year 2, hit by a sudden, very prolonged cold spell. Nothing wrong with a nice winter sleep for the tubers, cut off and stored in a box of either shavings or sawdust. I believe they say to leave 12" stalk on the D. Imperialis, so a taller container may be in order.

I don't know that I would even bother to divide it next year. They are an awesome plant to watch grow by leaps and bounds, and the more stalks the merrier. I hope whatever you do, it survives.

Thanks Poochella, It's a double white so I sure don't want to lose it. Most of my dahlias have to be dug except the ones in raised or bermed beds because I've got lousy drainage. It's very bushy but only about 8 inches tall and in a pot slightly larger than a 4" so I'm afraid if I leave it outside it will freeze since there's little soil around the roots/tuber. I think I better go ahead and let it die back and store it until spring.