Fern spores are notoriously tiny, which is how these plants easily spread worldwide. Some fern species are now circumpolar in their distribution as ferns continue their conquest to colonize the entire world. In our garden, we occasionally find new ferns growing thanks to a stray spore. Most of these, we remove, but occasionally one lands in just the perfect spot. One of our favorite popups is Cyrtomium falcatum (holly fern) that occasionally lands in our dry stacked rock wall. This is not where I would have ever thought to plant one, but they look absolutely fabulous there. Here is one that spored into the wall over a decade earlier and looks especially great during the winter months.