Plants small to moderate-sized, slender and soft, lax, moderately weak to moderately stiff-stemmed; green, pale yellowish green, yellowish brown, grayish brown or reddish brown; capitulum typically compact and twisted in the middle like a ball of yarn, spreading branches curved giving a pinwheel appearance. Stems pale green to pale brown, rarely with pinkish red patches, superficial cortex of undifferentiated to slightly differentiated. Stem leaves triangular-lingulate to lingulate, 0.7--1.3 mm, appressed to stem, apex obtuse to broadly obtuse and erose to somewhat lacerate, hyaline cells efibrillose and nonseptate. Branches curved, unranked to less commonly (in wet-grown forms) 5-ranked, leaves not much elongate at distal end. Branch fascicles with 2(--3) spreading and 2 pendent branches. Branch stems green$ but sometimes reddish at proximal end, with cortex enlarged with conspicuous retort cells. Branch leaves ovate-lanceolate to broadly ovate-lanceolate, 1.5--2.5 mm, strongly undulate and moderately recurved when dry, straight; margin entire; greater thanhyaline cells on convex surface with 1--2 pores per cell at cell apex, on concave surface with round wall thinnings in the cell ends and angles; chlorophyllous cells triangular in transverse section and typically just slightly exposed on the concave surface. Sexual condition dioicous. Spores 23--25 µm; moderately to coarsely papillose on both surfaces; proximal laesura approximately 0.5 spore radius..

Forming carpets in poor to medium fens, mostly sedge-fens and mire edge habitat; N.B., Nfld., N.S., Ont., P.E.I., Que., Conn., Ill., Ind., Maine., Md., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., Tenn., Vt., W.Va., Wis. Europe.`

Sporophytes uncommon, mature early to late summer. Sphagnum angustifolium and S. recurvum are the other Cuspidata with a similar range and ecology that have rounded stem leaves. In S. angustifolium the stem leaves are more triangular and rarely erose while the branch leaves are narrower and more strongly 5-ranked. Sphagnum recurvum also has narrower and more 5-ranked branch leaves than S. flexuosum as well as a much more strongly differentiated stem cortex. In S. flexuosum the branch leaves are only slightly recurved whereas in S. recurvum they are sharply recurved.