I have a Dryopteris intermedia (intermediate wood fern) that has been planted for a few years, and it is growing soooooo slowly. It's growing under some pawpaw trees in dappled sun, clay soil, lots of organic matter put around it every fall, well drained. Other plants in that area grow well (hosta, pawpaws, aconite, daffodils, sweet rocket). I keep the area around the fern clear of other plants, though the pawpaw roots are throughout the area. Are these ferns just slow growing, or is there something I can adjust?

I'm in zone 6, Bethlehem Pennsylvania. I started the fern from spores over 5 years ago.