I notice that Christmas ferns in the wild are generously spaced. I have a shaded slope where I would like to increase them to use as a ground cover, so I've been dividing and interplanting, replacing invasive vines as I go. Is this likely to work, or is there something about these ferns that needs the wider spacing? If so, what should I do in the "empty" space? It's not an area I would like to mulch...