Hi twithcher, I was hoping someone with direct experience with Elepaio will chime in but I'll take a shot. First of all I think your mounding of soil or compost is a good idea. The root zone should probably be a minimum of 4" or so deep. I've noticed that some C. esculenta might take two years to form a sizable tuber here in z6a. I've also read that the runners may take even longer if ever. Is Elepaio a runner? Brian, of Brian's Botanicals, replying to an email noted that many of his less cold tolerant, potted Colocasia are overwintered dry in a greenhouse.
One of the pieces of advice I've read and follow is to modify the type of fertilizer applied to 14-26-6 in the fall when the tuber is in active growth. See "Breakfast, Lunch and Dinner - Soil fertility by the seasons" by LariAnn over on DG.
