Tromping through our woods in December in search of a Christmas tree, I often notice an evergreen fern, one of the few green plants on the forest floor this time of year, other than young conifers. An easy fern to identify, it grows in fountain-like clumps and has glossy, dark green leaflets or pinnae. This is a Christmas fern (Polystichum acrostichoides), named for its evergreen habit and the shape of its leaflets. Each leathery, lance-shaped leaflet has a lobe at its base, creating a leaf shape my college botany professor described as looking like Santa’s boot. Others have noted the leaflets’ resemblance to Christmas stockings.
Christmas fern grows throughout eastern North America. It is related to the uncommon Braun’s holly fern, which grows in cool, moist forests in much of New England. Occasionally, the two species hybridize, producing the rare Potter’s fern, named after Henry Potter, a Vermont farmer and botanist.
Christmas fern grows in a variety of woodlands and on shady, rocky slopes. It tolerates a range of soil acidity and moisture levels. This fern is most abundant in northern hardwood forests, especially those with rich, calcareous (limey) soils. I once visited a red oak-northern hardwood forest with rich soils and the tallest and healthiest-looking Christmas ferns I’ve ever seen, some with twisted leaves.
In winter, the fronds of these ferns are often flattened against the ground. Studies have shown that the first hard frosts stimulate the development of a hinge zone at the base of the stem, which causes the fronds to fall over. The prostrate fronds benefit from the warmth and protection of leaf litter, which helps to prevent the leaves from freezing. Humidity is trapped beneath the ferns’ leaves, reducing desiccation. The leaves continue to photosynthesize during winter at a reduced rate. When fiddleheads emerge in spring, covered with silvery scales, the old fronds die.
Spring Christmas ferns include fertile fronds in the center of the clump, which grow taller than the sterile fronds. The uppermost leaflets of the fertile fronds are smaller than the others and bear spores in tiny cases called sporangia. If you check the undersides of fertile leaflets in summer, you can see brown masses of these spore cases. When the sporangia are ripe and dry in late summer or early fall, they split open, and the wind disperses the spores.
Each fern produces millions of spores, but only a few will land in suitable habitat. In contrast to flowering plants, ferns and their relatives, such as club mosses, undergo two very different developmental stages during their lives (sometimes called alternating generations). There is a gametophyte, or sexual phase, and a sporophyte, the familiar, spore-producing phase. According to the “Peterson Field Guide to Ferns” by Cobb, Farnsworth, and Lowe, if a spore lands in a moist, shady spot, it develops into a gametophyte: a flat, green, heart-shaped body with sex organs that grows underground. When stimulated by water, sperm from one gametophyte swim to the female organ on another gametophyte and fertilize an egg. The egg divides and grows into a tiny fern, a new sporophyte anchored to the gametophyte. Ferns also reproduce vegetatively; horizontal stems, or rhizomes, grow outwards and develop new fronds and roots. When the connecting rhizomes die, the ferns become separate plants.
Christmas fern is easy to grow and makes a good ornamental for shade gardens. It’s best to purchase nursery-grown plants so as not to deplete wild populations. People have used this fern for a variety of medicinal purposes, including to treat rheumatism, stomachache, pneumonia, and toothache.
Wildlife also eat Christmas fern. Ruffed grouse feed on leaves in fall and winter. Deer may browse the fronds during harsh winters when food is scarce. Wild turkeys eat the young spring leaves. Ground-nesting songbirds such as ovenbirds and veery sometimes nest in clumps of Christmas fern.
When you walk in the winter woods, look for the splashes of green made by the hardy Christmas fern and the stocking shape of its leaflets.
Susan Shea is a naturalist, writer, and conservationist based in Vermont. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: nhcf.org.
