Duane, I agree with you that I.japonica is not really suited for Zone
3-4 but it does survive and might flower. I am finding quite a few
plants grown and flower when they are not designated for Zone 4B. They
just did not read the fine print in the books!
I have I. lacustris but at the moment it is two (or may be three) very
small clumps which is just getting established. It did flower but the
flower fell off before opening. May be in it second year it will do
better. I. cristata and tectorum do well here.
I. delevayi is beautiful, very tall and stately with lovely purple
flowers which are about a foot above the leaves. I have two new, small
clumps and do not think that it can be divided unless it grows very well
this summer. It is certainly well suited to your climate as its native
home is the mountain regions of China. It grows very well from seed.
Ian, in Ottawa