I live in Indiana and have grown A. konjac for nearly eight years.
They grow all summer and get dug up for the winter, since they tend to
freeze if left in the ground. The tubers (bulbs, corms?) multiply nicely
by way of rhizome-like growths, but never really get any bigger than
onions. However, the two small tubers I gave a friend are now enormous,
having increased in size about five times within a couple years. I
believe the secret to growing Amorphophallus is sunlight. Mine grow in
heavy shade, and my friend's plants seemed to get full sun, at least half
the day. Apart from this, they got the same treatment.
Roger L. Sieloff