Naturalization and Spread of Four of the Most Marketed Ornamental Ferns: Cyrtomium falcatum, Dryopteris cycadina, Dryopteris erythrosora and Polystichum polyblepharum

Little research has been done to determine market influences on the naturalization and invasion of ferns. A 2021 paper led by McCulloch-Jones identified four species of ornamental ferns of particular concern due to their potential to naturalize and invade, based primarily on their current high-level presence in the horticultural trade and their recent naturalization in a few countries. A 2021 survey of ferns in nature preserves and uncultivated situations in parks in the Atlanta Georgia region, USA, found all four species: Dryopteris erythrosora, Dryopteris cycadina, Polystichum polyblepharum, and Cyrtomium falcatum. Dryopteris erythrosora (autumn fern), recently documented to be naturalized in the southeastern USA, was found at 56/58 of the surveyed sites. At half of the sites (28/56) where the fern occurred, it was abundant and often the most common fern present. The long naturalized Cyrtomium falcatum (Japanese holly fern) was found at 15 surveyed sites. Polystichum polyblepharum (Japanese or Korean tassel fern), previously collected in the USA at single sites in Alabama and Georgia, was found at 9 sites. Dryopteris cycadina, previously collected in the USA at a single site in Georgia, was found at 6 sites. Attempts to learn when these ferns began to be marketed in North America were not successful due to their absence in historical nursery catalogues. All four were considered commonly available by 1994. The number of nurseries selling these ferns in North America in 2004 and internationally in 2021 were 31 and 55 for D. erythrosora, 27 and 35 for P. polyblepharum, 5 and 28 for C. falcatum, and 13 and 13 for D. cycadina. The degree of naturalization of these ferns detected on the surveys appears to be influenced by historical and contemporary levels of market intensity. In addition, all four ferns are polyploids and apomictic, biological characteristics that aid their colonization and spread after they have been introduced. The discovery of these ferns at so many sites suggests that they have probably naturalized elsewhere in North America.