San Francisco Ferns

Ferns make up one of the most popular groups of plants, exhibiting a tremendous variety of forms, shapes and unique life cycles. Approximately 11,000 species of true ferns have been classified (in comparison with the more than 325,000 species of seed-bearing plants) ranging from tiny aquatic forms less than one half inch in diameter to giant tree ferns growing over 80 feet high. The stems of most species found in the temperate zone, however, are reduced to underground creeping rhizomes. It is estimated that the ferns first appeared on Earth around 320 million years ago.

The true ferns belong to the division Pterophyta. California’s true ferns belong to the class Filicinae, which consists of four orders, nine families, 24 genera and 74 native species. While ferns are typically associated with moist areas such as forests and streams, many of California’s species occur in dry situations.

As you might expect, there isn’t a lot of good fern habitat left in San Francisco. However, 12 native species in eight families can still be found within San Francisco County and another two are recorded historically but have not been seen recently. Although none of these taxa is listed as rare by CNPS as rare within San Francisco, the habitat for most is severely restricted and the distribution of naturally occurring populations is highly threatened.

Just because winter is just around the corner, doesn’t mean that we have to shut ourselves indoors waiting for spring’s bounty to appear. The California polypody and goldback fern can be seen only in the rainy season and quickly go dormant when the rains cease. Both the polypody and goldback fern can be seen only in the rainy season and quickly go dormant when the rains cease. Both the polypody and goldback fern favor north-facing surfaces such as cliff faces and thin, rocky soils. The goldback finds cracks in roadcuts or trailcuts in which to insert its thread-like roots. The polypody (=many feet) creeps indefinitely outward with its rhizomes, sometimes forming massive sheets. These vegetative masses in turn provide lodging places for seed of other plants such as buckwheat or succulents like dudleya and stonecrop (Sedum ), thus paving the way for plant succession. The other native polypody is leather fern, whose thick, leathery leaves are held by the plant year-round. It survives dessication by a shiny, cuticle-covered upper frond surface and cartilaginous, in-rolled margins protecting the spores on the underside. Surviving more than six months of dryness is tough for an evergreen fern, so the haunts of this plant are where it is regularly kissed by coastal fog. In such places as the eucalyptus-cypress plantation on Mt. Davidson, where it is not only bathed by refreshing fogs but also dripped on by the tree canopies, it forms massive sheets on the tree boles and limbs. In heavy winter gales such as we experiences last winter, some of these sheets are dislodged and fall to earth.

The cheerful lady fern embodies the very essence of rebirth of hope in late winter when the unfurling fresh light green fronds contrast with the deeper green of other plants and the dark brown of last year’s fronds and leaf litter. It needs constant moisture in the growing season and doesn’t want to ever dry completely. Streamside or poolside is ideal, and it brightens up dark, shady places.

Speaking of moisture, the reason why Woodwardia was extirpated in San Francisco is because of its high moisture demands. It likes its roots to be in contact with copious water at all times of the year, a kind of habitat that was preempted by invasive weeds. This big, bold, dramatic fern ought to be reintroduced to its rightful place, along the shores of Lake Merced, Lobos Creek, and Islais Creek in Glen Canyon.

The evergreen wood fern somehow manages to survive the long dry season without the special equipment of leather fern. This is all the more surprising when you consider its habitat –dry slopes, usually in the dappled shade of oaks or shrubs. Its fronds resemble those of lady fern, although smaller. Although not as appealing as lady fern, its drought tolerance and evergreen character may recommend it to gardeners. The same can be said of our western sword fern, the very same fern so prominent a part of redwood forests. It thrives along the coast as a component of scrub and coastal prairie. It is tough and reliable and makes a nice companion for columbine (Aquilegia), irises, California fescue (Festuca californica), and Nootka reed grass (Calamagrostis nutkaensis). Large numbers have survived the heavy-duty competition of English ivy and German ivy in Glen Canyon, on Mt. Davidson, around Lake Merced, and in the Presidio.

What does one say about the ubiquitous, cosmopolitan bracken fern? Wherever it occurs in the world it takes a geographically distinct form. The rhizome ranges over wide expanses, sometimes covering several acres. A single frond arises at intervals along this rhizome, the yellow stem being tough and wiry. It can become invasive in the garden, but is easily controlled by pulling up unwanted fronds on occasion.



SF = A Flora of San Francisco, California; P = Presidio; BH = Bernal Heights; BV = Bayview Hill; GC = Glen Canyon; GP = Grandview Park; LC = Lobos Creek; LH = Laguna Honda; MD = Mount Davidson; SH = Strawberry Hill (Golden Gate Park); TP = Twin Peaks; YBI = Yerba Buena Island

Azollaceae – Mosquito Fern FamilyAzolla filiculoides (mosquito fern): LC,P

Blechnaceae – Deer Fern FamilyWoodwardia fimbriata (giant chain fern): SF,P? (believed extirpated), GC (reintroduced)

Dennstaedtiaceae – Bracken FamilyPteridium aquilinum var. pubescens (western brackenfern): GP,LC,LH,MD,P,SF,SH,TP,YBI

Dryopteridaceae – Fern FamilyAthyrium filix-femina var. cyclosorum (lady fern): P,GC,MD,SF,SH
Dryopteris arguta (wood fern): BV,LH,MD,P,SF SH,YBI
Dryopteris expansa (wood fern): LC (not listed in flora)
Polystichum munitum (western sword fern): BH,GC,LH,MD,P,SF,TP,YBI

Isoetaceae – Quillwort FamilyIsoetes nuttallii (Nuttall’s quillwort): P,SF

Marsileaceae – Marsilea FamilyMarsilea vestita ssp. vestita (clover fern): SF (extirpated)

Polypodiaceae – Fern FamilyPolypodium californicum (California polypody): BH,BV,GC,GP,LH,MD,P,SF,SH,TP,YBI
Polypodium scouleri (coast polypody): GP,LH,MD,P,SF

Pteridaceae – Fern FamilyAdiantum jordanii (maidenhair fern): P,SF,YBI
Pellaea andromedaefolia (coffee fern): LH,SF,YBI
Pellaea mucronata var. mucronata (bird’s-foot fern): SF
Pentagramma triangularis ssp. triangularis (goldback fern): BH,BV,P,SF,TP,YBI