Shape: Eastern Marsh Fern does not form clumps but instead raises single erect fronds along an underground rhizome.
Fronds - Fronds are either in sterile form or fertile form with the shorter sterile fronds appearing first in Spring. The blades are lanceolate shaped, widest just below the middle. Fronds are 2 to 8 inches wide and 7 to 36 inches long and are produced through the Summer. There are no aromatic glands. The stipe is long - as long or longer than the blade, smooth, straw color above the base, sometimes with a few tan scales near the base. The upper side of the central rachis is grooved. Sterile fronds are shorter with shorter stipes. Blades are medium green to dark green, and vary from pinnatifid to pinnate - pinnatifid. The fertile fronds frequently appear twisted as the lower pinnae are arranged in a different plane than the uppers.
Pinnae: The pinnae are lance shaped, usually 12 or more pairs, perpendicular to the rachis, opposite each other, or sub-opposite, and perpendicular to the ground. The lower pinnae are slightly shorter. The fertile pinnae appear to be more narrow, due to the margins of the pinnules rolling over toward the underside.
Pinnules: The lower pinnae will be cut into pinnules almost to the costa. The back side of the costa can have sparse to dense fine whitish hairs. Veins on sterile pinnules forked. Pinnule margins are entire, not lobed or toothed but with some fine edge hair.
Fertility: The sori are placed on the back of the pinnule in close rows near the midvein. Sori are round with a thin, kidney shaped indusia covering them, usually hairy and without glands. The indusia drops away as the sori mature.
Habitat: Marsh Fern grows from a creeping rhizome. Fronds emerge along the rhizome. It is usually found in the moist ground of swamps, bogs, marshes and other wet areas but can also show up in wet prairie and wooded areas.
Names: The genus Thelypteris is derived from two Greek words - thelus, meaning a 'female' or 'a nipple' and pteris, meaning 'fern', in particular a fern with a round sori on the fertile fronts. The species palustris means 'marsh loving' or 'found in wet places'. The variety name pubescens means 'with fine hair' referring to the hair usually found on the underside of the costa. The author names for the plant classification are: First to classify was ‘G.Lawson’ which is for George Lawson (1827-1895) Scottish-Canadian botanist, considered the Father of Canadian botany. He helped create Canada’s first botanical garden. His work was amended by ‘Fernald’ is for Merritt Lyndon Fernald (1873-1950) American botanist, Harvard Professor, scholar of taxonomy, author of over 850 papers, editor of the 7th & 8th editions of Gray’s Manual of Botany.
There are two varieties of Marsh Fern but only var. pubescens is found in North America. The second variety, var. palustris is found in Asia. The older name for this species Aspidium thelypteris has been folded into C. fragilis also.
Comparison ferns: Marsh Fern may be confused with Hay-scented Fern, Dennstaedtia punctilobula, which also has tall single fronds, but there the stipe is always shorter than the blade and the stipes are hairy.
Above: 1st photo - The shape of Marsh Fern frond, widest just below the middle. 2nd photo - Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: 1st photo - a young frond just unfolding. 2nd photo - The lower pinnae have pinnules not quite cut through to the costa. Both the costa and the central rachis are grooved on the upper side, the rachis with dense fine hair. Pinnules have entire margins and a central mid-vein.
Below: 1st photo - the pinna on this plant are slightly offset from each other (sub-opposite). In both photos the curled edges of the pinnules of a fertile frond are evident. 2nd photo - mature sori.
Below: 1st photo - the sori are covered with a kidney or horseshoe shaped hairy indusia prior to maturity. This drops away at maturity - end photo.
Notes: Eastern Marsh Fern is indigenous to the Garden, Eloise Butler catalogued it on May 25, 1907 under the older name of Aspidium thelypteris. She planted it in 1932 near the new Mallard Pool. Martha Crone planted the species in 1946 and listed it on her 1951 census. It has been continually listed on later census reports.
Within Minnesota, it is found in most of the state except for a number of counties in the southwest section of the state.
In North America Eastern Marsh Fern is found in the eastern half from the lower Canadian Provinces southward but not the deep south. There are 21 species of the Thelypteris genus in North America but only this one is found in Minnesota.
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"