Shape: Cinnamon Fern is an erect, statuesque, clump forming fern with arching fronds rising 1 to 3 feet in a vase shape circle from a crown-forming rhizome.
Fronds - Sterile: Fronds are either in sterile form or fertile in an entirely different shape. Sterile fronds have pinnate-pinnatifid blades and a tuft of cinnamon color hair at the underside base of the pinnae, with a few hair along the segment margins in the spring. There can be 20+ pairs of pinnae that taper in size toward the tip and are cut deeply into lobes (hence - pinnate-pinnatifid). The lower pinnae are shorter and tend to point downward in this genus. Veins are forked 1 to 3 times. Both the rachis and the stipe are smooth and green except that the stipe is covered with cinnamon colored woolly scales during the spring. The rachis has a groove on the front.
Fronds - Fertile: Fertile fronds are much shorter and narrower, changing from green to orange-brown in late spring to early summer. Some references state they appear before the sterile fronds, but the difference in timing appears to be very short and I have not observed it. The pinnae of the fertile fronds are very short, stiff and growing upward very close to the central rachis. These also are hairy.
Fertility: Instead of sori, the fertile fronds of this and other Osmundas have bare sporangia on short stalks, arranged in clusters. These are deep green at maturity changing orange-brown when they open and then to the dark orange-brown after the spores are released in early summer and then these fronds die back.
Fiddleheads: These are large and covered with silver-white hair which changes color to cinnamon-brown as they expand, ending up with just a tuft of the hair at the base of each pinna. Fiddleheads should not be eaten as they are known to be carcinogenic.
Habitat: Plant this fern in acidic, rich soil, sun or shade, but the soil must be consistently moist or wet. Plants go dormant if too dry and die if consistently dry. Not a plant for the first time fern grower. Old plants do not transplant easily due to the large clumpy rhizome. Spores need to be sown as soon as they mature.
Names: The genus Osmunda has an obscure derivation. Some believe it is named after an early Scandinavian writer Osmundus and some say it is named for Osmunder, a Saxon name for a Celtic god, Thor. The species cinnamomea means 'brown, like cinnamon' referring to the color of the mature sporangia. The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. Classification: There has been an attempt to reclassify Cinnamon Fern to Osmundastrum cinnamomeum but this is not universally accepted yet.
Comparison ferns: There are three that could be confusing - Royal fern, Osmunda regalis L., (The fertile pinnae have the sori at the tips of the green fronds), Interrupted Fern, Osmunda claytoniana L., (fertile parts are on a mid-section of the frond between [interrupts] the green pinnae) and Ostrich Fern, Matteuccia struthiopteris (L.) Todaro, (fertile frond is also separate but the fertile pinnae arch outward from the rachis, not appressed like the Cinnamon).
Above: 1st photo - The shape of Cinnamon Fern with the erect fertile fronds in the center. 2nd photo - The sterile fronds have pinnae that taper in size toward the tip.
Below: Fiddleheads - 1st photo - the first to rise are said to be those of the fertile fronds. Young fiddleheads have cinnamon-brown hair. 2nd photo - these have lost most of the cinnamon-brown hair. 3rd photo - The stipes of the fertile fronds showing the cinnamon brown scales.
Below: The fertile fronds have small pinnae appressed toward the rachis. 1st photo - as the fiddlehead unfurls. 2nd photo - at maturity after spore release. 3rd photo - Detail of the sporangia on the fertile frond pinnae at the point of the sporangia opening.
Below: Detail of the pinnate-pinnatifid blades where each pinnae is deeply cut toward, but not to, (the pinnatifid part) the costa of the pinnae forming lobes that do not overlap. Note the grooved top of the smooth green central rachis. (Underside photo below.)
Below: The underside of the central rachis is not grooved. Sterile fronds have a tuft of cinnamon color hair at the underside base of the pinnae. Note the forking of the veins.
Notes: Cinnamon Fern is indigenous to the Garden, Eloise Butler catalogued it on May 25, 1907. Martha Crone planted 112 of them in 1956 when she developed the Fern Glen. It has been planted as recently as 2006, by Curator Susan Wilkins. Cinnamon Fern is found throughout the eastern half of North America. Minnesota and Ontario are on the western edge of the range. Within Minnesota, it is found in the NE section of the state down as far as the north metro and then south along the Mississippi River. Absent in the western section of the state and all of the SW.
In North America there are only 4 main species of the Osmunda genus: O. cinnamomea, the Cinnamon Fern; O. claytoniana, the Interrupted Fern; O. ruggii, a sterile hybrid found only in Virginia; and O. regalis, the Royal Fern. The latter has two varieties but only one is found in North America - var. spectabilis. O. cinnamomea has had four varieties described in North America but current authorities such as Flora of North America do not distinguish them. All three main species are represented in the Garden. The hybrid in Virginia is described as - Osmunda ×ruggii R. Tryon [claytoniana × regalis].
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"