There are a couple of ways to make more of the ferns in your garden. You can wait for them to grow (some spread faster than others) and divide them. Or collect and sprout their spores.
Spores are like little seeds, though they're much smaller and slower to germinate and grow. They're found on the fern fronds instead of a seed pod, capsule, or fruit. Spores appear as little bumps, often black or brown, lining the underside of some fronds.
Step 1: Gather the Spores To collect spores, place a mature fern frond on a piece of smooth white paper. The ripened spores will fall from the frond and onto the paper after several days.
Step 2: Plant the Spores Carefully fold the paper so the fern spores fall into the crease. Then sparsely sprinkle the spores over moist seed-starting mix. Mist the seed-starting mix with water after planting.
Step 3: Be Patient Cover the container with clear plastic after planting to keep the spores humid. Make sure the spores stay moist, but not saturated or soggy. Be patient. Spores can take several months to sprout.
Step 4: Plant the New Ferns Once the spores have sprouted, keep them humid until they're large enough to transplant. For most ferns, this is after they have at least three fronds.