Ferns

Of the many plants in the redwood forest, ferns are one of the most common types. Their green fronds can be observed year-round in the damp, cool shade of the towering trees. Muir Woods is host to thirteen species of ferns from six different fern families, ranging from delicate, tiny plants that grow on trunks of trees to hardy larger specimens scattered across the forest floor.

What Are Ferns?

Ferns have been around for more than 300 million years, and have a worldwide distribution on all continents except Antarctica and most islands. Three primary characteristics define ferns:

Ferns are vascular plants, meaning they have specialized structures that move nutrients and water (contrast to mosses, which do not).
Ferns reproduce using spores. They do not produce flowers or seeds and come from an ancient lineage that predates flowering plants.
New growth unfurls from fiddleheads. Note that some non-fern plants also do this, so it is not unique to ferns.

Other seedless vascular plants include horsetails and club mosses - not technically ferns, but "fern allies" or close relatives that meet similar criteria. Ferns have megaphylls (large leaves with several veins). Fern allies have microphylls (small leaves with a singular vein). Another difference is that fern allies have a specific branch used for reproduction called sporangia which looks like a pod. Ferns use the spores found on the back of their frond (leaf) for reproduction.

Fern Reproduction

Ferns evolved millions of years before plants developed flowers or seeds to reproduce, instead using spores. The spores emerge from a structure called sori, typically found on the underside of the fern plant's fronds. These blow away in wind. With enough moisture, the spores will grow into a prothallus, short-lived miniature heart-shaped structures with reproductive sex organs.
Also called a gametophyte, some prothallus have sperm, others have eggs, and some have both. The sperm are flagellated, meaning they have tails that wiggle for forward motion, and require water to move. For up to an hour, these sperm can swim in search of eggs contained in other prothallus.
In this way, ferns exhibit an alternation of generations between the prothallus or gametophyte phase and the fern plant, also known as the sporophyte - or spore producing - phase. However, ferns can also clone themselves through vegetative reproduction like their neighbors, the redwoods.
While redwoods come from specialized burls at the bottom of their trunk, ferns use underground horizontal stems called rhizomes to reproduce. The rhizome will break off and grow roots to create a clone. When succession (ecosystem change) happens, ferns can help the environment by taking over an area and provide habitat for many species.

Ferns at Muir Woods

While the delicate fronds of ferns may look the same at first, this plant family offer a unique yet inviting challenge for visitors to distinguish the species from each other. By learning the names and descriptions of plants around them, people can connect deeper with the natural world. This fosters a stronger relationship with the natural world, increasing responsible stewardship and motivating conservation efforts. As you visit the park, notice the different types of ferns scattered through the forest. Below is a guide to help you in doing so.

Some of most common at the park include sword fern, chain fern, five-fingered ferns, and gold back ferns. Horsetail are also often observed at the park, though they are a fern ally and not a true fern. To best distinguish ferns, some bigger vocabulary words are used. For a breakdown of fern vocabulary, check out the bottom of this page.

Coast redwood (Sequoia sempervirens) trunks and sword ferns. — Western sword ferns are evergreen, or green all year. They can develop dense clusters underneath the towering trees.
NPS

Western Sword Fern
(Polystichum munitum)

Shade-loving western sword ferns are one of the most common in the park and surrounding Mount Tamalpais area. A lobe at the base of each pinna resembles a sword hilt, giving the plants its name “sword fern.” They are evergreen, meaning they have green leaves all year, with fronds that arch downward. During droughts, they will have shorter fronds to conserve water.

a backlit image of a fern frond with spores visible — This backlit fern frond illustrates the plant's namesake: two lines of sori organized like a chain on each pinna.
John. W. Wall

Giant Chain Fern
(Woodwardia fimbriata)

This large fern always grows near water. Its fronds are two times pinnate, and each frond is more separated than the fronds of the lady or bracken fern. Notice that the lobes are pointed.
On the back of each lobe are large, oblong sori which are more rectangular in shape than those of the sword fern. These sori, also known as fruitdots, are organized in two parallel lines a way that resembles a chain, hence the plant's name.

lady fern grows near a stream of water — Lady ferns will almost always be found near a stream or water source.
California Academy of Sciences/ Gerald and Buff Corsi

Lady Fern
(Athyrium filix-femina)

This fern always grows near the creek or by a source of water. Lady ferns die back every year. The fern's delicate, lacy looking fronds are three or more times pinnate, which means the number of times the fern leaf branches off.
Lady Ferns are up to 3 feet tall with fronds up to a foot wide. Ferns have fiddleheads (tightly curled part) which will unfurl creating a new frond.

common bracken fern in dirt — The common bracken fern's triangle shape is a key distinguishing characteristic that differentiates it from the lady fern.
Barry Breckling

Common Bracken Fern
(Pteridium aquilinum)

The bracken fern looks at a glance very similar to the lady fern with delicate fronds that die back every winter. However, it is distinguished by the triangle-shaped fronds that branch several times. It is able to survive in heat better than many other ferns - though it still needs a moist environment. It is also known to be poisionous, causing internal bleeding for grazing cattle.

goldback fern green front and gold powered back — The goldback fern's namesake powder appears on the underside of mature fronds. It can rub off onto skin, paper, or other surfaces to make an imprint.
California Academy of Sciences/ Charles Webber

Goldback Fern
(Pentagramma triangularis)

This fern is only seen during spring and early summer in drier areas, like the Canopy View Trail, before it dies back for until the next winter. Its name references the gold-colored powder dusting its back or underside.
The fronds of the small fern grow on a black, wiry stem from 4 inches to one foot tall in a cluster from a small root crown. Though fronds may appear triangular, they are actually five-sided, like a pentagram, hence this fern's latin name. It is one to three times pinnate, depending on the size it grows.

a five finger fern under a frond of western sword fern — Here, a five-finger fern with a black stem and dainty leaves, can easily be distinguished from the waxy sword fern growing above it.
NPS/ Jace Ritchey

Five Finger Fern
(Adiantum aleuticum)

This fern is a type of Maidenhair fern native to the west. The sori are on the top margin on the back of the pinnule. It has very delicate-looking palm-shaped fronds, which hang down from the stem. The step is dark and shiny, giving the fern an appearance of a lack of connection.
This fern grows in very moist areas, like the banks of streams. It was formerly used as an ointment for inflammations of the skin and the stems are used in traditional basketry.

polypody ferns growing on rock — This image of California Polypody illustrates how the fern will grow on rocks or other areas with limited soil. The plant uses aerial roots to collect moisture.
California Academy of Science/ Gerald and Buff Corsi

Polypodys
(Polypodium spp.)

Muir Woods has several polypods, including the California Polypody (Polypodium californicum), Licorice Fern (Polypodium glycyrrhia), and Leather polypody (Polypodium scouleri). Their name is from the Greek Polys (many) and pody (foot) refering to the numerous knoblike branches of the underground stem. You may find individuals as well as hybrid examples of these two ferns at Muir Woods.
These ferns are epiphytic, meaning that they grow on other plants, such as big leaf maple trees. However, other plants are only used for mechanical support - the ferns are not taking energy or resources from the trees. Instead, they have aerial roots which absorb water from the rain and fog.

horsetail stems emerge from the ground — Northern great horsetail can be seen in Muir Woods for much of the year. Visitors can understandably mistake this unique plant for tree saplings, but this plant only reaches about three feet tall.
NPS/ Jace Ritchey

Northern Great Horsetail
(Equisetum braunii)

Horsetail is a fern ally closely related to true ferns. They grow vigorously in damp or marshy areas and are common in the redwood forest, especially in the spring and fall. Their "bristles" are made of stem tissue but are green to perform photosynthesis. Their actual leaves are tiny and typically don't produce energy for the plant.
Much of their stalk is rigid and hollow. Some are sterile while those with a cone shaped growth are the fertile spore-producing structures.
Great horsetail belong to an ancient lineage of plants, the equisetum family. According to research on fossils, this family started growing over 300 million years ago. During the Jurassic period, horsetail grew almost 100 feet tall!
This plant has also been called Equisetum telmateia, with a vast range and our local northern great horsetail being identified as a subspecies. However, botanists in 2024 determined it is a unique species. Equisetum braunii has a range from California to Alaska and is distinguished from Equisetum telmateia by its photosynthetic green stems, which are reportedly only a feature of the northern species.

a fiddlehead unfurls from a fern — Like many ferns, the western sword fern fronds develop from small, tight fiddleheads.
NPS/ Jace Ritchey

Fern Vocabulary

Frond describes the large leaf of a fern which typically has many divisions.
Pinna are the primary leaflet of a frond.
Pinnate refers to the number of times the frond is divided.
Sporangia are structures that produces spores, typically on the underside of fronds.
Sori contains the sporangia. Sori come in different shapes and can be a diagnostic tool when identifying a species.
Indusium is a covering over the sori that protects developing spores. Indusia can dry out, shrivel, or tear when spores are ready, facilitating release. A false indusium describes when a fern relies on curled or rolled over edges of leaves to protect the spores.
Rachis describes the main axis of the frond (also means the central shaft of a feather).
Pinnule are the secondary leaflet (levels of division can help with identification).

Ferns

What Are Ferns?

Fern Reproduction