Plants: small or less frequently moderate-sized, slender, capitulum flat-topped and stellate; green or dark purplish red and green, rarely green throughout, often with a distinctive bluish cast when dry. Stems: red to green; superficial cortical cells aporose. Stem: leaves triangular-lingulate to lingulate, 1.1–1.4 mm, apex broad-rounded to narrowly truncate, border very broad at base (more than 0.3 width); hyaline cells efibrillose, rhombic, mostly 1-septate but can be non-septate. Branches: long and tapering, usually strongly 5-ranked. Branch: fascicles with 2 spreading and 1–2 pendent branches. Branch: leaves ovate-lanceolate, 0.9–1.4 mm, concave, straight, apex involute; hyaline cells on convex surface with very small ringed pores (less than 0.25 cell width) along commissures near apex, changing abruptly to large elliptical pores (0.4 cell width or more) basally, concave surface with large round pores in proximal margins and leaf base. Sexual: condition dioicous. Spores: 17–26 µm, finely papillose on both surfaces; proximal laesura less than 0.5 spore radius. Phenology: Capsules mature late summer to early autumn.
Sporophytes are uncommon in Sphagnum warnstorfii. This is one of the most minerotrophic species of the flora, is hygrophytic, and has a very broad niche. The most commonly associated vascular plants are Thuja occidentalis, Abies balsamea, and Picea rubens. Bryophytes typically associated with it are S. centrale, S. squarrosum, S. teres, Calliergonella cuspidata, and Campylium stellatum. This species is perhaps most similar to S. bartlettianum, with which it has small range overlap. Sphagnum warnstorfii has a shorter and less sharply pointed stem leaf and the red color with a characteristic bluish caste compared to the crimson red of S. bartlettianum. See also discussion under 81. S. russowii and 86. S. talbotianum.