The easiest way to organically propagate the crown-forming ferns, such as shield fern (Dryopteris) and ostrich fern (Matteuccia struthiopteris), is by dividing the crowns in mid spring.
To begin, carefully dig up a clump of ferns with a fork and cut off the fronds.
With a small clump it may be possible to pull the crowns apart with the hands. If the clump is large, use two garden forks. Push them back to back into the center of the clump. Taking care not to damage the actual crowns, push the forks into the clumps between the crowns, not through them. Gently push the handles together; then pull them apart until the clump is broken into two parts. It may sometimes be best to make the final division with a sharp knife to minimize possible damage.
Subdivide the two new clumps by the same method to produce several new plants, each with its own root system.
Rhizomatous ferns can also be propagated by division. In mid spring dig up a clump of ferns and cut off the fronds. With a sharp knife cut the rhizome into sections, each with at least one growing point (the point from which new shoots are emerging). Plant these individual sections and add organic material to help growth and drainage. Each of these will form a new fern.