Ask Mr. Smarty Plants

The Maidenhair ferns that cling onto the limestone almost always are embedded in a small (or very small) pocket in the stone.  I have had good luck carefully prising these out using a small- tipped trowel or a screwdriver.  There is generally a little bit of soil around the root, and this will protect the root until the fern is placed in moist soil.  I am much more successful when selecting the smallest fern plant that I can find, perhaps ones with a single frond only a few inches long.  

You didn't say if the grotto is on your own property (mine is!).  Be careful not to create a hole in the beautiful green face of the grotto wall when you remove a plant.  I spend much time looking for just the right little plant to remove without spoiling the beauty of the cascading ferns.

The transplanted ferns should be kept in a nearly saturated soil out of direct sunlight until they begin to grow.  But the soil should not be waterlogged.  The ideal site is one on a porous stone or slope where water trickles down over the ferns and keeps the roots well oxygenated.

Maidenhair fern can also be propagated from spores, which form in summer on little dots regularly spaced on the underside of the leaves.  When these dots look black, remove the leaves and dry them in a paper bag.  The almost microscopic spores will be released into the bag.  If these are sprinkled onto moist peat-enriched soil and kept moist in a transparent plastic-covered container the spores will germinate into tiny plantlets.

Once the Maidenhair fern is established it becomes fairly resistent to drying.  The tender fronds will turn brown, but the roots will remain viable and send out new fronds when moisture returns.  Consult Internet sites such as this for more information on these beautiful plants.