Florida is home to many native fern species, including the Boston fern (Nephrolepis exalta) and giant sword fern (Nephrolepis biserrata), which can be difficult to distinguish from non-native ferns that grow in the same environments, such as Tuberous sword fern (Nephrolepis cordifolia) and the Asian sword fern (Nephrolepis multiflora). Each of these are often still sold in the nursery and landscape trade, and often confused or misidentified as the native species of fern.
The native Boston fern (Figure 1) has erect fronds that can reach up to 3 feet long and 6 inches wide. The round sori (clusters of spore-bearing organs) are in two rows near the underside of the pinnae (leaflet). It is commonly found in humid forests and swamps of Florida, although is native to other regions such as South and Central America. It is grown outdoors as well as indoors for ornamental value; their high humidity tolerance makes them a good candidate for both indoor and outdoor use.
Figure 1. Native Boston Fern
The native giant sword fern (Figure 2) has fronds that extend several feet and can be found in moist to wet soil. The species name comes from tiny teeth that alternate with larger teeth along the edge of each lance-like pinna. Underneath each pinna, round sori occur evenly around the entire edge. The petioles (stalk) are sparse to moderate with reddish to light brown hair-like scales. Tubers are never present in this species.
Figure 2. Native Giant Sword Fern
Since the non-native ferns can be invasive and disruptive to native plant communities, it is very beneficial to be able to recognize the differences between them. The Asian sword fern and Tuberous sword fern are sold under various names, often ones of native origin.
Tuberous sword fern (Figure 3) sometimes produces tubers, and it is the only one of the four ferns mentioned that is capable of doing so. The presence of these tubers alone is a distinct way to identify the species. The presence of scales on the upper side of the rachis (stem) that is distinctively darker at the point of attachment is another way to distinguish the tuberous sword fern from the other three species. Native sword fern has scales on the upper side and are homogenously colored.
Figure 3. Non-Native Tuberous Sword Fern
Tuberous sword fern can be distinguished from Asian sword fern (Figure 4) by its glabrous central vein of the pinnae contrasted by the presence of short stiff hairs that occur on the central vein of the pinnae of Asian and giant sword fern. The most distinguishing characteristic for Asian sword fern is a dense covering of dark brown, pressed scales with pale margins on mature petioles. Petiole scales of tuberous sword fern are dense, spreading, and pale brown, while those of native sword fern are sparse to moderate, reddish-brown, of a single color or slightly darkened at the point of attachment and have expanded bases with small hairs.