Know Your Natives – Sensitive Fern

Sensitive fern (Onoclea sensibilis) of the Sensitive Fern (Onocleaceae) family, formerly of the Polypody Fern (Polypodiaceae) family, is a widespread fern with dimorphic fronds (leaves): deciduous, sterile, photosynthetic (green) fronds and persistent, non-green, fertile fronds. The genus name is from the Greek for “closed cup,” from the rolled pinnules (divisions of pinnae) that conceal the sori (fruit dots). The specific epithet is Latin for “sensitive.” This monotypic fern (only species in the genus) occurs across the eastern half of the U.S. and Canada, as well as in eastern Asia. In Arkansas, it occurs statewide. Natural habitat is shaded to sunny areas that are consistently damp to wet, such as swamps, marshes, seeps, ditches, bottomland forests, and margins of streams and lakes. The sterile fronds are “sensitive” to cold temperatures, dying with the first light frost. It is also called “bead fern” from the bead-like pinnules of fertile fronds.

The rootstock consists of shallow dark brown to black creeping rhizomes with occasional branches. With the rhizomes overlapping, thick colonies develop in favorable sites. New fronds grow from these rhizomes. The thickened bases of fronds parallel the rhizome, but stipes (leaf stalks) quickly turn to exit the soil vertically. Dead infertile fronds decay quickly, while their bases remain attached to the rhizomes for several years. Fertile fronds persist for over a year, with their bases persisting for several additional years. 

Photo 1: As shown, actively growing rhizome tips are growing toward the viewer; previous years’ pinnae bases extending away. A segment of a dead rhizomes (⅜ inch in diameter) is positioned at left. Photo – November 27.

Sensitive fern bears once-divided fronds of two distinct forms. Sterile fronds appear in early spring as reddish fiddleheads (also called crosiers), while fertile fronds appear in fall. Green sterile fronds are killed by light frost; fertile fronds remain viable overwinter and release spores the following spring.

Photo 2: In late March, sterile fiddleheads unfurl in this wetland. The beaded fertile fronds (from the previous year) have not yet released spores. Older fertile fronds decline on left.

The yellowish to pale green sterile fronds, 2-3 feet long and 1 foot wide on slender stipes to 1½ feet long, are broad and somewhat triangular in outline, with 6 to 12 opposite to nearly opposite pairs of pinnae (leaflets). Pinnae are lance-elliptic, to 8 inches long and 1½ inches wide, becoming more closely spaced and joined toward frond apex to form blunt lobes. Pinnae margins of smaller plants may be entire or wavy while those of larger plants vary from broadly serrate proximally to more entire distally. Fronds are generally hairless except for short hairs on the veins beneath and a few brown scales along the stipe. Wings appear along the lower rachis and become continuous toward the rounded tip. 

Photo 3: Infertile fronds, with winged rachises, have wavy to broadly serrate margins. A fertile frond, spores still retained, is shown at lower center. Photo – April 24.

Photo 4: Pinnae and wings have a similar appearance. Upper tertiary veins can be seen near lower left corner.

Fertile fronds have few similarities to sterile fronds. A fertile frond may be up to 29 inches tall, including a 19 inch stipe. The strong stipes have a furrowed to flattened upper side and a rounded lower side. Fertile fronds have sharply ascending, narrow pinnae to 2 inches long that bear a dozen or so opposite to near-opposite pairs of green pinnules. The pinnules, on short stalks, have five inrolled, tightly overlapping lobes which enclose firm bead-like clusters of sporangia. Fertile fronds are green in the fall, but stipes become tan in winter as the fertile portions the frond become dark brown. In spring, lobes of pinnules unfold as sporangia release spores. (Occasionally, plants may have pinnae with an appearance that is intermediate between sterile and fertile leaves, forma obtusilobata.)

Photo 5: Pinnae margins of sterile fronds may be entire, wavy, crenulated or broadly serrated. In this mid-October photo, fertile pinnae are recently emerged.

With spores dispersed, the reproductive activity of the “sporophyte phase” of the fern’s life cycle concludes. In the soil, spores germinate to produce a prothallus, the “gametophyte phase.” The tiny prothallus produces sperm and egg. Sperm swim through ground moisture to fertilize eggs that have remained attached to their prothallus. Fertilization produces a zygote that, in turn, develops into a new sporophyte plant.

Photo 6: In this mid-October photo, the new fertile frond on top has pinnules with overlapping lobes that form bead-like clusters. The year-old frond on bottom has open pinnules.

Sensitive fern would be a nice addition to a garden or natural area which has shaded moist areas or sunny wet to mucky sites. In a wetter site, the plant may need a barrier so that it does not spread too aggressively. This medium-sized fern, with plentiful light green sterile fronds during the growing season and prominent winter-time fertile fronds, has a strong structural character. Green or dried fertile fronds work well in floral arrangements.

The characteristics of sensitive fern are unique enough so that it should not be confused with any other native Arkansas ferns. A possible exception is netted chain fern (Woodwardia areolata) which also has separate sterile and fertile fronds and may grow in similar habitats. Sterile fronds are similar, however, alternate pinnae of netted chain fern have minutely serrated margins and acute apexes. Fertile fronds are distinct: sporangium clusters of netted chain fern are oblong and sited along narrow pinnae.

Article and photographs by ANPS member Sid Vogelpohl

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