Series I: Confusing N.C. Ferns

 

Cinnamon Fern vs. Interrupted Fern

North Carolina is home to many diverse species of ferns ranging from minute filmy ferns to large ferns such as Royal fern, Osmunda regalis, which can easily top 100 centimeters (3.3 feet) in height¹. In the North Carolina mountains two of our more common larger fern species can be easily confused with each other if sori are not present. This article will focus on how to identify and distinguish between Cinnamon Fern, Osmundastrum cinnamomeum and Interrupted Fern, Osmunda claytoniana. (See Table 1, 2).

Table 1: Historical and Current Taxonomic Treatment

Common Name(s)	Radford & Bell 2 (1968)	Flora of North  America 3 (1993)	Weakley 4 (2015)
Cinnamon Fern	Osmunda cinnamomea L.	Osmunda cinnamomea L.	Osmundastrum cinnamomeum (L.) C. Presl
Interrupted Fern	Osmunda claytoniana L.	Osmunda claytoniana L.	Osmunda claytoniana L. var. claytoniana

Table 2: Table of general characteristics for Cinnamon Fern and Interrupted Fern. Note that shared traits between the species are in bold type.

Cinnamon Fern Osmundastrum cinnamomeum	Interrupted Fern Osmunda claytoniana L. var. claytoniana
Large coarse ferns up to 3 feet +	Large coarse ferns up to 3 feet +
Plants grow from a short, stout, rhizome resulting in a large clump of fronds	Plants grow from a short, stout, rhizome resulting in a large clump of fronds
Blades one pinnate-pinnatifid	Blades one pinnate-pinnatifid
Deciduous	Deciduous
Dimorphic – separate sterile and fertile fronds	Dimorphic – separate sterile and fertile pinnae
Pinnae margins entire	Pinnae margins entire
Common	Common
Wooly tufts of hairs (trichomes) underneath blade in axis of pinna and rachis	Wooly tufts absent

Cinnamon fern, Osmundastrum cinnamomeum, is commonly found across the state in a large variety of wet habitats such as wet ditches, swamps, bogs, and mesic woodlands. It is a large coarse fern with one pinnate-pinnatifid blades (Figure 1). The fronds form large clumps that grow out from short stocky rhizomes. Individual plants can range from to 90 up to 150 centimeters (3-5 feet) in height. The fern is dimorphic, meaning that the fertile fronds look different from the sterile fronds. Fertile fronds have highly modified pinna with large globose sporangia. The sporangia start off green but quickly turn a rusty brown color, hence part of the reason for the common name ‘Cinnamon’ fern (Figure 2, 3, 4 ). The fertile fronds form early in the spring and quickly wither. 

Figure 1: This is an example of a fern frond that is one pinnate pinnatifid. Notice the initial frond divisions or pinnae are subdivided (pinnate) but not  deep enough to produce distinct and completely separate pinnules (pinnatifid).

Figure 2: A nice example of Cinnamon Fern, Osmundastrum cinnamomeum, notice the large size, dimorphic fronds, and concentrated growth habit from the short thick rhizome.

 

 

 

 

 

 

 

 

Figure 3. Another example of a Cinnamon Fern showing the fertile frond surrounded by sterile fronds.

Interrupted ferns, Osmunda claytoniana, are commonly found in the mountains of North Carolina. They are also large coarse ferns with one pinnate-pinnatifid blades. They average about 90 cm. in height (3 feet). Like Cinnamon ferns they are also dimorphic. The fertile fronds resemble the sterile fronds except the center pinnae are modified and have clusters of large globose sporangia. When young the sporangia are green but turn brown-black at maturity. Similar to the Cinnamon Ferns the fertile pinnae quickly wither leaving a gap in the middle of the frond. No other fern has this unique arrangement of sporangia in the center of the blade hence the common name ‘Interrupted Fern’ (Figures 4, 5, 6). I also commonly find large populations of Interrupted Fern where no plants have produced fertile pinnae. I believe the short-lived nature of the Cinnamon Fern’s fertile frond and the lack of fertile pennae on numerous Interrupted Ferns is what leads many to confuse these two fern species with each other, especially later in the year.

Figure 4: A nice example of Interrupted Fern, Osmunda claytoniana. Notice the fertile pinnae in the center of the frond with sterile pinnae above and below.

Figure 5: A close up of Interrupted Fern fertile pinnae that are young and still green. Picture is courtesy of the National Park Service.

Figure 6: Fertile Interrupted Fern pinnae after they have matured and turned brown.

However, several factors can help to distinguish between them. Cinnamon fern will have wooly tufts of trichomes in the axils between the rachis and pinnae, interrupted fern does not (Figures 7, 8). But, be careful because late in the summer these trichomes can wear away so another feature to look at is the tip of the blade. Cinnamon ferns have tapered acute tips while interrupted ferns have very blunt tips (Figures 9,10, 11). Also, consider the habitat. Cinnamon Ferns prefer moister habitats than Interrupted Ferns. For example, in the South Mountain State Park there is a trail that transverses a slope. Interrupted Fern is abundant along the trail and upper regions of the slope where the soil is well drained. But, as one travels down the slope towards a small creek, Interrupted Fern becomes less prominent and is eventually replaced by a large population of Cinnamon Fern in the flatter, moister areas along the water. 

Figure7: Cinnamon Fern, Osmundastrum cinnamomeum, showing the distinct tufts of trichomes at the pinnae - rachis junction.

Figure 8: Interrupted Fern, Osmunda claytoniana, showing the lack of trichomes at the pinnae - rachis junction.

Figure 9: This is a close-up of Cinnamon Fern pinnae showing the accuminate-acute tips.

Figure 10. This is a close-up of Interrupted Fern pinnae showing the more obtuse-rounded tips.

Figure 11: Interrupted Fern is on the left and Cinnamon Fern is on the right. Notice the overall more blunt and rounded ends of the Interrupted Fern frond and pinnae compared to the more accuminate-acute ends of the Cinnamon Fern frond and pinnae.

Cinnamon and Interrupted ferns are the two most commonly seen large ferns in the North Carolina mountains. They can be easily identified by looking at the differences in fertile fronds (if present), differences in blade tips, and looking for the presence or absence of trichomes at the pinnae-rachis axis.

Resources:

1. Morin, Nancy R. Flora of North America North of Mexico Volume 2: Pteridophytes and Gymnosperms. New York, NY: Oxford U Press, 1993. Print.

2. Radford, Albert Ernest, Harry E. Ahles, and Clyde Ritchie Bell. Manual of the vascular flora of the Carolinas. Chapel Hill, NC: U of North Carolina Press, 1983. Print.

3. Weakley, Alan S. Flora of the Southern and Mid-Atlantic states. Chapel Hill, NC: U of North Carolina Herbarium, North Carolina Botanical Garden, U of North Carolina, Chapel Hill, 2015. Print.

4. Hardy Fern Library. N.p., n.d. Web. 14 July 2017. <http://hardyfernlibrary.com/ferns/home.cfm>.

5. All photos, unless otherwise noted, were taken by and are property of Tracie Jeffries.