I've had I. fulva here in Zone 5 for two years, and the person I received it
from had grown it in Zone 4 for 10 years. It grows well for me, and I'm not
mulching it, just to see how tough it is.
For best results, plant in spring so roots are well established by winter, or
grow in a pot, brought inside an unheated garage or cellar first winter, then
plant out the following spring.
Use a winter mulch of 6-12 inches of white pine needles, or straw.
It is a lovely thing-good luck!
Kathryn Mohr, near Albany NY, Zone 5