Greetings
I would add that I. japonica can have a " stem " ( arial rhizome ) that can be up to 6 inches long or more in old neglected plants or when growing in compitition with larger plants. Although the plant is hardy, many times the flowering stem can be killed long before it has emerged from the fan in winter from cold, even in zone 8 ( where I live ). It often has a reputation here of flowering infrequently, tho not in our recent non-winters.
I've read that the I. wattii usually grown is a direct descendent of one collected by Major Lawrence Johnson ( the creator of the incomparable Hidcote Manor gardens in the English Cotswolds ) is a hybrid and not the pure species.
Jerry
Vashon Island near Seattle, WA,USA zone 8.