Answer: Ferns grow from spores which are released from the fronds when they are mature. That's how ferns naturalize an area. As long as there is little competition, little fern plants will pop up all around the parent plant. So, to encourage their growth, keep other plants cut back so the ferns won't be overtaken.
You can collect fern spores and germinate them yourself, but it takes lots of time (30-180 days) and special treatment of the dust-like spores. Fern spores need a fine film of moisture over which to swim in order to complete the process of reproduction. It takes up to 5 months for a jelly-like algae to form on organic matter onto which the spores have been broadcast before they can begin the germination process.
I'd encourage the existing ferns to naturalize on their own by keeping competing plants cut down and making sure there's a nice layer of organic matter under each fern so the spores produced will have a place to develop and grow.
Q&A Library Searching Tips