I just dug out of my garden five ostrich ferns and put them in a 24 inch pot. They will grow on my porch all summer. Come winter they will die out. Next year when I go to replant new ferns in the pot I will find the soil in last years pot one big dense, massive root ball. I will need to water them regularly/ daily so they get all the water they need. I am not sure how long you plan to keep this potted fern or what you want it to look like. My ferns will not multipy in the pot this summer. A few years ago I planted an old cooler with ferns. We had a mild winter and they all returned the next summer. This year when I went to replace the dirt I could not believe the hard mass of roots in a cube forum that came out. I dumped it in the woods and I noticed yesterday on of the crowns is growing out of that block! I do not think the crowns have spread and multiplied, in a pot for me ever.
Does this help at all?