Illustris is so hardy it will survive unless you have a hard freeze which is very deep in the ground. I am in zone 7 and it has been very good about coming back. Last winter it was left in an iron pot above ground and it came back. It is sometimes late but the ground must be very warm. Illustris doesn't form large bulbs, more like small bulblets. You should try alocasia macrorrhiza. It will survive outside in zone 7 with protection. I have one planted under the drip of the greenhouse roof. We get about 36 inches rain from Nov-March and we get hard freezes in the teens and 20's. My alocasia keeps coming back. It is planted in a 4 inch raised bed for drainage. I cut the leaves off after the first light frost. I fill a large garbage can with chopped leaves and turn it upside down over the stumps for mulching and insulation.

Thanks for the advice tsmith! I haven't had much luck keeping any of my alocasia or colocasia bulbs over the winter.

As for overwintering outdoors. The water table in my area is high. During the winter if I take a walk in my backyard it is like walking on a wet sponge. Puddles form wherever I have stepped. I do not think the bulbs would make it in these soggy conditions.

Most of the bulbs I try to store, rot over the winter. I've tried several different locations and temps. Many of the bulbs that do make it thru to the spring begin to rot before they start growing. It may be a losing battle.

I will try again this winter and if my results aren't any better I may give up on these plants. I've recently moved one of my Illustrus indoors and already I'm noticing some leaf droop. I do however have a few small Alocasia Macrohizza plants that made it last winter and I am also trying to keep them indoors. They seem to be doing fine so far.

I've read some of the expert growers say they keep them overwinter on heat mats. I think the cold soil is the problem. I'm not experienced with keeping these 'cause most the ones I grow are winter-hardy here, but I think it's partly because they store up so much heat in the summer, and then because it never gets more than chilly in the winter. Usually light freezes once or twice--last year we had a light freeze after things had started to grow in the spring that did more damage than we would have had with more cold earlier.