Containing, restraining, restricting Horsetail Equisetum Hyemale
I am interested in planting horsetail next to a swimming pool but I am concerned about its invasiveness. I have several large boulders at the deep end of the pool and I would like to contain the equisetum hyemale to an area just behind the boulders. What I need to know is how deep the root barrier must be to prevent spreading. A woman at a local nursery (which did not have any to sell) told me 4" is all that is required but I am skeptical of this advice. Does anyone have any direct experience? I know it is not a grass but some of the ornamental grass books mention it.