Pioneer Valley Fern Society

Winter Ferns 2024-2025

Rock Polypody vs Christmas Fern

Rock Polypody (Polypodium virginianum) and Christmas Fern (Polystichum acrostichoides) are two common native evergreen ferns we have in our area. From a distance they may appear similar, but up close they are easy to tell apart. They both can be found growing on rock, but not usually right next to each other. In this photo, the Rock (or common) Polypody is on the left, and the Christmas fern is on the right.

The easiest way to tell them apart is by the pinnae or leaflets. The Rock Polypody leaflets are cut almost to the rachis or stem, but are not separate from each other. They are still slightly attached along the rachis all the way up and down the blade, The technical term for this is "pinnatifid". The Christmas Fern blade has many individual pinnae or leaflets, each attached separately to the rachis. This is called "pinnate". Christmas Fern pinnae have very distinct "auricles" or ears near the rachis. Depending on which way you look at the pinnae, they can look like Santa's sleigh or a Christmas stocking.

Both ferns have fairly leathery pinnae that remain green through the winter. Other distinctive features include: the Rock Polypody have beautiful golden colored round sori on the back of the pinnae on the upper half of the blade, while the Christmas Ferns have a modified top of the blade with much smaller pinnae with small brownish sori that cover the back of the leaflet. Also, the Rock Polypody stems are smooth, and the Christmas Ferns are scaly. Lastly, I noticed that the Rock Polypody pinnae curl inward (showing the sori on their backs) when they are cold, while the Christmas Ferns curl backwards, hiding the back of their pinnae. That is why the photo shows the back of the Rock Polypody and the front of the Christmas Fern, since both were curling from the cold weather. In order to do a closeup of the pinnae, I was not able to include the fertile pinnae, which were on the top of both of these ferns.

Hope you are having a good winter!

Posted: to PV Fern News on Tue, Jan 14, 2025
Updated: Tue, Jan 14, 2025