Two Wetland Groups, Often Confused

In a British fen, bog or wet meadow, the eye is drawn to movement: sedges shifting in the breeze, cotton grasses holding their white heads above dark peat. Both belong to the sedge family, Cyperaceae, and both are flowering plants rather than grasses. But they are not the same thing, and the difference matters if you want to name what you see. In field guides, “true sedges” usually means the genus Carex. Cotton grasses are a separate genus, Eriophorum. They share a family, a love of wet ground and a habit of growing in crowds, yet their stems, flower spikes and seed heads are reassuringly distinct once you know what to look for.

The Stem Test: Triangles and Rounded Stalks

The quickest check is the stem. Roll a true sedge stem gently between thumb and forefinger. It should feel triangular, with three distinct edges, like a slender prism. You can also cut a thin slice and look at the cross-section: a triangle, sometimes softly rounded but never a perfect circle. This is the classic “sedges have edges” rule, and it works for most British Carex species, from the tussocky greater tussock-sedge to the slender common sedge.

Cotton grasses break that rule. Their stems are rounded and smooth, without the three-cornered feel. Run a finger up a flowering stem of common cottongrass or hare’s-tail cottongrass and it rolls easily between thumb and finger. The difference is practical in the field: if your stem has edges, think sedge; if it is round, think cotton grass, especially when the white heads are present. A hand lens helps with young shoots, but the finger test is often enough.

Bottlebrush Spikes Versus Cotton Heads

True sedges carry their flowers in spikes. These are made of overlapping scales, packed tightly along a stem or clustered at the top. Some look like tiny bottlebrushes, especially the cylindrical spikes of bottle sedge, which stand stiffly among the leaves. Others are darker, greenish or brown, and some nod as they age. The important point is that sedge spikes are structured, scaly and usually green-brown at flowering time. They do not dissolve into white fluff.

Cotton grasses are unmistakable when in seed. Each stem carries a head of long, silky white bristles, the “cotton” that gives the genus its name. Common cottongrass usually has several nodding white heads on each stem, while hare’s-tail cottongrass produces a single, erect, brush-like head. These are seed heads, not flowers, and they can last for weeks in summer and early autumn, catching low sunlight and making a bog look as if it has been dusted with snow. If you squeeze a cotton grass head, it feels soft and airy, not scaly. That texture is the giveaway.

Leaves, Tussocks and Wetland Homes

Leaves add another clue. True sedges often have flat, keeled or channelled leaves with sharp edges; some, like the greater tussock-sedge, build great raised tussocks in fens and wet woods. Others form loose tufts or creeping mats at the edge of ponds and rivers. Their leaves are usually tough and ribbed, and they may feel rough if you draw them through your hand. Cotton grasses have narrower, more grass-like leaves, often grey-green and upright. They are at home on acid ground: blanket bog, wet heath, moorland flushes and peatland pools. True sedges are more varied, filling alkaline fens, marshes, wet meadows, ditches and lake margins as well as acid bogs.

• True sedge habitats: fens, wet woodland, marshes, pond margins, upland flushes, some bogs.
• Cotton grass habitats: blanket bog, raised bog, wet heath, acid moorland, peat cuttings and bog pools.
• Seasonal clue: cotton grass is most obvious from late spring to late summer; sedge spikes are often best identified in early summer, before the leaves collapse.

Writing the Wetland with Precision

Wetland nature writing becomes richer when it is exact. Instead of describing a “sea of grass” or a “reedy marsh”, name the plants doing the work. A writer who knows that the white heads belong to cotton grass, and that the stiff triangular stems nearby are sedges, can build a scene with real texture. You can write about the wind moving through the cotton grass, each head turning like a small flag, while the sedges below stand in their own green order. You can notice how hare’s-tail cottongrass holds a single brush above the peat, or how bottle sedge spikes stand like dark bottlebrushes against the pale water.

Precision also helps with conservation and memory. Many British wetlands are fragile, drained, grazed or restored, and the presence of true sedges and cotton grasses tells a story about water, peat and soil chemistry. Keep a notebook. Note the date, the habitat, the stem shape and the seed heads. Draw a simple cross-section. Write down the weather and the way the light falls. Over a season, you will build a local vocabulary that no general guide can give you. And the next time you sink into a boggy corner of Britain, you will know which plants are the sedges and which are the cotton grasses, and why the difference is worth writing about.