I would stop the miracle grow until they have sprouted. My hops are gang busters and I don't do any winter prep. However mine are in the ground. How did you store your potted hops over the winter? They can tolerate the winters, but they can get damaged from extreme cold with out protection.
If you plan on the potted method in the future put the plant next to the house. Keep them covered with a pile of mulch, leaves or other material that can offer some protection. A 15 inch pot is going to limit the size of the rhizome too. Get a bigger pot or plant them in the ground if you can. They are most likely root bound too.