Botanically curious people often ask me, “How do you tell a sedge from a grass?” Many have heard that “sedges have edges,” but distinguishing grasses from sedges is often more complicated than any one-liner can capture. The sedge family (Cyperaceae) can be a daunting group of plants to learn, with nearly 300 species in our region. Subtle differences between some species can frustrate and discourage even seasoned experts. Yet, even casual observers may learn and appreciate more approachable groups of plants among the sedges, which might whet an appetite for learning the more difficult taxa.
My favorite sedge genus is Eriophorum (EH-ree-AH-for-um), known as the cottongrasses. Six species of cottongrasses are widespread in our region, and they are relatively easy to tell apart. Once you know you have a cottongrass, noting just a few more details can get you to a species.
Despite the common name, cottongrasses actually belong to the sedge family and have no close relation to cotton. The “cotton” refers to the fluffy heads of seeds, which have long, whitish bristles attached. When fruiting, the heads bear a strong resemblance to tufts of cotton held up high on long, slender stalks. The bristles act like parachutes, carrying seeds far on the wind. Further complicating the common name here, Eriophorum means “wool-bearer.” Still, we don’t call these plants “woolsedges.” Some have suggested “cottonsedges” as slightly more accurate, but I still prefer “cottongrasses” because the name succinctly captures the general impression these plants give on sight. Cottongrass is also a more familiar and approachable name for nonexperts, without the need for a taxonomy lesson.
Besides the cotton-white bristles, cottongrass species also share some habitat preferences. They grow in open, sunny wetlands, often in deep layers of peat. Most species prefer acidic wetlands, but both acidic bogs and calcareous fens can host a cottongrass species or two (or three!). Cottongrasses are generally northern species of higher latitude and/or elevation, and some species grow in tundra and alpine habitats. Many of the species in the Northeast have a circumboreal distribution, occurring across similar northern latitudes in Europe and Asia.
The first clue to distinguish one cottongrass species from another is the orientation of the cottony heads. Are they held upright, or are they nodding on long, slender stems? Two of our region’s species have upright fruiting spikes. These species are easy to tell apart by their growth habit and number of spikes. Tussock cottongrass, Eriophorum vaginatum, is unique among all our cottongrasses, growing in dense clumps with many stems close together. It is also the only one with a solitary spike (cluster of seeds). Conversely, tawny cottongrass, Eriophorum virginicum, grows in rhizomatous colonies like a diffuse forest of Seussian Truffula trees. It has several spikes held close together, appearing as one dense cottony mass of bristles. Both of these species prefer strongly acidic peatlands, such as Sphagnum bogs.
As its name suggests, tawny cottongrass has tan-colored bristles, whereas all other species noted here have bright white bristles. Sometimes the tawniness can seem convincingly white, especially under direct sunlight, but laying the spikes against a white sheet of paper can reveal the distinctive coloration. Tawny cottongrass is our most common and widespread species. It is also the last to flower, with seeds dispersing well into autumn. Any cottongrass still conspicuous in October is likely this species. It is also the most likely to tolerate a bit of shade, sometimes growing under a partial canopy of trees or persisting under woody shrubs.
The remaining four species all have nodding or drooping spikes. These are among the most elegant in the genus. They can be split into taxonomic pairs by looking at the slender, leaflike bracts that emerge from the base of the inflorescence, forming something like a leafy collar below the cotton-bearing portion of the plant. You will see either a single, more or less upright bract, or several (usually three) spreading bracts. Make sure you’re looking only at the base of the fruiting portion, as sometimes other small bracts can appear from within the spikes themselves.
The two species with several bracts have both visual and habitat clues to help distinguish them. The green-keeled cottongrass, Eriophorum viridicarinatum, is a reliable calciphile, characteristic of mineral-rich fens in areas with rich bedrock such as limestone. It flowers relatively early, shedding most of its seeds by early summer. This plant is deep green with brilliantly white bristles and subtle black or gray-brown accents on the scales tucked at the base of each cottony spike. While green-keeled cottongrass grows throughout our region, tall cottongrass, Eriophorum angustifolium, is more strictly northern in its distribution. Tall cottongrass grows in Maine and across southeastern Canada. Compared with green-keeled cottongrass, the most striking difference is the coloration: tall cottongrass has accents of red on the leaf sheaths, at the base of the bracts, and in the scales of its spikes.
The other pair of species, with solitary bracts and drooping spikes, can also be split by coloration and, to some extent, habitat. Few-nerved cottongrass, Eriophorum tenellum, has reddish brown scales. It is most common in strongly acidic bog-like habitats. Slender cottongrass, Eriophorum gracile, has gray or black-tinged
scales similar to green-keeled cottongrass. It most often grows in calcareous fens, although it also occurs in acidic habitats.
You aren’t likely to see cottongrasses everywhere you go. Several of these species are locally rare or uncommon. Or they grow in habitats that are scarce or difficult to access. Nevertheless, if you find what appears to be a cotton ball on a stem, you will now be prepared to make its acquaintance.