Last year I mentioned that I was leaving a one hundred fifty gallon
stock tank with lotuses out for the winter in zone 5/Chicago, and that
I would report if it froze. I surrounded it with fiberlglass
insulation on the sides and white foam board on top, sank a one
hundred watt aquarium heater into it, and covered the whole thing with
a tarp. when I opened it the Spring, it was perfect-- covered with
duckweed and with the cattails and lotus pushing against the cover.
So, it didn't freeze. In fact, I think the lotus went back into
hibernation because the water cooled after I opened it in the end of
March. Based on my experience, you don't have to dig or bring in for
the winter.