Rhizome depth question

zpenguin

I am looking for the rhizome depths of several herbaceous forest planties. I've looked all over the place without much luck and am hoping some of you have suggestions on where to find this info. The species include: Epigaea repens, Goodyera, Hepatica americanum, Osmorhizza claytoniana, Oxalis Montana, Panax trifolia, and Streptopus roseus. Any suggestions would be helpful. Many thanks!