The Lady Slipper newsletter of the Kentucky Native Plant Society has been published since the Society’s founding in 1986. This is one of a series of reprints from past issues. This article, about sweet fern (Comptonia peregrina), first appeared in Vol. 26, No 1, Spring 2011. If you would like to see other past issues, visit the Lady Slipper Archives, where all issues from Vol. 1, No. 1, February 1986 to Vol. 35, 2020.

Sweet fern—A rare Kentucky shrub with an interesting history

By Tara Littlefield, Botanist, Kentucky State Nature Preserves Commission

The wax myrtle or bayberry family (Myricaceae) is known for its odor. These plants have resinous dots on their leaves, making their leaves aromatic. Plants in this family have a wide distribution, including Africa, Asia, Europe, North America and South America, missing only from Australasia. Myricaceae members are mostly shrubs to small trees and often grow in xeric or swampy acidic soils. More familiar members of the wax myrtle family include many in the Genus Myrica (sweet gale, wax myrtle), some of which are used as ornamentals and are economically important. In addition, the wax coating on the fruit of several species of Myrica, has been used traditionally to make candles.

So what does this interesting family have in common with Kentucky’s flora? We are lucky to have just one species in the wax myrtle family, Sweet fern (Comptonia peregrina). In addition, it is also a monotypic genus restricted to eastern North America. This means that the genus Comptonia has only one species (C. peregrina) worldwide, and just happens to be found here in KY! Of course the common name sweet fern is misleading. This woody shrub is certainly not a fern. However, the leaves have a similar shape to pinnules of a fern frond (leaf). But having sweet in the common name is no mistake. If you crush the leaves throughout the growing season, a lovely smell is emitted as the essential oils volatilize into the air.

Sweet fern is a clonal shrub that grows up to one meter high and spreads through rhizomes. The leaves are alternate and simple, linear and coarsely irregularly toothed, dark green above and a bit paler below. It is monoecious (meaning male and female flowers on different plants). The female flowers are not showy— short rounded catkins [dense cluster of apetalous flowers, usually associated with oaks, birches and willows] with reddish bracts. The male flowers are elongated yellow-green catkins clustered at the branch tips, the pollen being adapted to wind dispersal. The fruit is a round, bur-like cluster of ovoid nutlets that turn brown when mature in late summer. The bark is reddish and highly lenticeled (small corky pores or narrow lines on the bark that allow for gas exchange).

While very common in the northern part of its range (northeastern United States and Canada), sweet fern is state listed endangered in Kentucky, along with being state listed as rare inOhio, Tennessee, South Carolina, West Virginia, Georgia, and North Carolina. The populations of sweet fern in the southern part of its range are isolated and disjunct from the common habitats up north. There seems to be a close association of these remnant populations with the Appalachian Mountains, which suggests that the populations in the southern ranges remained in protected “refugia” during periods of great plant migrations, such as during glaciations.

Sweet fern is typically found in openings in coniferous forests with well drained dry, acidic sandy or gravely soils with periodic disturbances. In the north, it can be found in pine-oak barrens or jack pine and spruce forests that are maintained by fire, creating openings and decreasing competition. It has also been noted to colonize road banks and even highly disturbed soils such as mined areas. Contrary to these open coniferous habitats with periodic fire, the remnant populations of sweet fern in Kentucky and Tennessee are found on sandstone cobble bars, which are maintained by annual floods. Despite being found on habitats that are maintained by different disturbance regimes, these two communities share a few things in common—they are both dry, acidic, sandy and nutrient poor. Disturbances are a natural occurring impact in these communities that removes shrubs and saplings, thus decreasing competition so that sweet fern can thrive.

Sweet fern has adapted to these specialized habitats. It is a fires adapted species; it will resprout after a fire and increase its clonal sprouts through underground rhizomes. It is also a xerophyte, a plant adapted to dry conditions. And since it is adapted to living in nutrient poor, acidic soils, it has evolved with the bacteria Frankia that fixes nitrogen, somewhat like the more famous nitrogen fixing legumes who have partnered with the bacteria Rhizobium. Did you know that there are over 160 species of nonleguminous plants that fix nitrogen? It is also the host of the sweet fern blister rust (Cronartium comptoniae) which reduces the growth of pines, particularly Jack pine. What interesting relationships this shrub has with bacteria and fungi! In addition, sweet fern is the food plant to larvae of many species of Lepidoptera (moths and butterflies). These include the Io moth (Automeris io), and several Coleophora case-bearers (some of which are found exclusively on sweet fern).

But perhaps the most fascinating facts about the rare shrub sweet fern is what it can tell us about the evolution of plants, the history of the earth, and the paleovegetational past of Kentucky. Geologically speaking, sweet fern is an old plant. In Kentucky, it was most likely more common some 20,000 years ago during the last glaciation, as Kentucky used to look like Canada. Analysis of pollen in sediment cores taken from natural ponds in Kentucky confirms this, spruce and jack pine was common in the uplands in the bluegrass. Sometimes it is difficult to think of plants migrating north and south in order to adapt to a changing climate. But what is even more mind blowing is that the genus Comptonia is perhaps millions of years old. Numerous fossils of dozens of extinct species of Comptonia have been found all across the Northern hemisphere, and the earliest of the fossils have been dated back to the Cretaceous period (the age of the Dinosaurs) over 65 million years ago. The first flowering plants (angiosperms) evolved only 135 million years ago, so Comptonia is one of the oldest living plants in the world—a true living fossil!

So when April comes around, and all of the spring wildflowers are emerging, think of sweet fern tucked deep into the gorges of Big South Fork and Rockcastle, its catkins releasing pollen in the wind, using the nitrogen fixed from its bacterial friends, withstanding the massive floods of two of Kentucky’s last wild rivers. And if you use your imagination, you may be able to see dinosaurs and tree ferns in the distance.

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2. Darlington, Emlen. 1948. Notes on some North American Lepidoptera reared on Sweet Fern (Compontia asplenifolia Linnaeus) with Description of new species. Transactions of the American Entomological Society (1890-). Vol. 74, No. 3, pp. 173-185.
3. Liag, X., Wilde, V., Ferguon, D., Kvacek, Z, Ablaev, A., Wang, Y., and Li, C. 2010. Comptonia naumannii (Myricaceae) from the early Miocene of Weichang, China, and the paleobiogeographical implication of the genus. Review of Paleobotany and Palynology. Vol. 163, p. 52-63.
4. Medley, Max and Eugene Wofford. 1980. Thuja occidentalis L. and other noteworthy collections from the Big South Fork of the Cumberland River in McCreary County, Kentucky. Castanea. Vol. 45, No. 3, pp. 213-215.
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   https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.134920/Comptonia_peregrina
6. Wilkins, Gary, Delcourt, Paul, Delcourt, Hazel, Harrison, Frederick, and Turner, Manson. 1991. Paleoecology of central Kentucky sicne the last glacial maximum. Quaternary Research. Vol. 36, Issue 2.
7. Virginia Tech Woody Database
   http://dendro.cnre.vt.edu/dendrology/syllabus/factsheet.cfm?ID=869
8. Zomlefer, W. 1994. Guide to Flowering Plant Families. University of NC Press, Chapel Hill.