Christmas Fern is one of the most common ferns found in the forests of New Jersey. If you’ve come across a dark glossy green fern as you wander the woods chances are you are looking at Christmas Fern.
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Key indicators for identification.
Each pinna or leaflet in Christmas fern has a projection (auricle) at the proximal end, pointing towards the apex of the frond. It reminds some people of a Christmas stocking, which is a helpful mnemonic.
Within its core eastern North American range (I think it gets as far as Mexico, surprisingly!), Christmas fern is pretty distinctive. Small ones can be confused with ebony spleenwort, but are distinguished by the spiny teeth along the margins and the green scaly, rather than lustrous dark-brown, rachis.
Polystichums in general have that upward-pointing auricle. Most are more lacy than Christmas fern, but a few are also once-pinnate. I believe there is some range overlap between Christmas fern and P. lonchitis, northern holly fern, which typically has shorter pinnae with more prominent, spreading, spines at the margin. (They’re fairly subtle on Christmas fern unless you’re looking for them). P. munitum, western sword fern, and to a lesser extent, P. imbricans, are western North American species that can be confused with Christmas fern by a careless computer vision user. Other than range, P. munitum pinnae tend to be a bit longer and are somewhat falcate near the tip, while P. imbricans pinnae are often rotated out of the plane of the blade and end in a distinct bristle.
Thank you choess. I certainly learned much more after reading your post!