The Many Faces of Winter Ferns

Photo: Kevin McKeon

By Kevin McKeon, Maine Master Naturalist

During our winter walks, we can see that some ferns keep their greenery to remind us that there’s much life still happening in the woods during the winter. In New England, there are four common evergreen ferns: polypody, Christmas, marginal wood, and intermediate wood ferns.

The first on our list is the polypody, or rock cap fern. This small fern grows in mat-like clusters on the tops of exposed boulders and cliffs. Its leaflets (pinnae, or “once divided”) are elongated, with smooth edges, flared at the base where they meet the stem. In late spring and summer, the fern produces a leaf (frond) with reproductive spores called a fertile frond. Flipping over that frond and looking at its upper part, or blade, you will find some spore cases (sori) that turn from green to brown as the spores within mature. However, polypody’s primary reproductive mode seems to be by sprouting shoots from buds along its shallow rhizome, the roots of which anchor the plant to its rocky substrate. Polypody’s botanical name refers to this trait: Poly, or many, and pods, or feet. This pretty little fern serves as winter food for animals such as deer, wild turkey, and ruffed grouse.

Sometimes these ferns, along with many other evergreen plants, will droop and/or curl their leaves in reaction to winter’s freezing temperatures. This phenomenon, called thermonasty, deserves a little column space of its own.

Originally, the drooping and curling thermonastic reactions were thought to reduce the plant’s exposure to the dry cold — slowing water evaporation from the leaves and helping to conserve water and mitigate desiccation. This is probably one part of plant thermonasty, but recent studies suggest that protection from sunlight may be the primary reason. Most thermonastic evergreen plants grow in partial or full shade, on the forest floor under its leafy canopy, so protection from direct sunlight during winter’s leaf-drop is gaining prominence as the primary theory for this survival adaptation for the plant’s leaves.

Christmas ferns, or Polystichum acrostichoides, have a different tactic for winter survival. They lie down and let the snow’s insulation help protect them. But since they grow up to 3 feet tall, their fertile fronds — adapted to grow higher to capture spore-dispersing winds — can often be seen poking through the snowpack. This is one of the easier-to-identify ferns by its slight upward bump at the base of each leaflet near the stalk, which makes it look like a miniature Christmas stocking to some folks, or like Santa’s sleigh to others. Those attributes, along with being green at Christmas time and being used by colonists as yule decorations, gave this beauty its name.

Christmas ferns grow as slightly raised clumps from a central point, with new yearly growth originating from rhizoid budding, forming fiddleheads. As the new growth matures, the previous year’s greenery dies back into the forest floor duff and humus layers, adding soil stability, slowing surface water flows, and adding organic matter to feed micro-invertebrates — so Christmas fern is useful for erosion control on slopes. This large fern provides habitat for small mammals and amphibians and nest sites for birds, and it’s a great food source for grouse.

More winter greenery is added by the marginal wood fern, Dryopteris marginalis, called “marginal” due to its sori growing at the margins of the fern’s pinnules, or spikes. Growth is similar to the Christmas fern, but at about 2 feet tall, it’s smaller and a bit less erect. Like the

Christmas fern, it has pinnules, but these pinnules’ margins are deeply lobed, giving it a more lacy appearance. Also distinguishing is the color difference between the top and bottom of the frond; the bottom is a noticeably duller, light blue-green than the top. And of course, there’s no “Santa’s sleigh” on this fern!

Our final evergreen fern to look at is the intermediate wood fern, Dryopteris intermedia, commonly called the evergreen wood fern. This beauty is even lacier than the marginal fern because its fronds are “thrice divided.” Its leaves have leaflets that have their own leaflets! Also, the intermediate fern’s pinnules have sharper, more serrated margins, making it look “toothier” than the more rounded margins of the marginal fern. This lacy-looking fern is often used in floral arrangements due to its ability to maintain freshness after being cut. It can also be a sizable 3 feet tall.

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net