Plants: (1–)2–5(–10) cm, dense or open mats. Stems: green or yellow-green, usually brownish with age; stems of two types, fertile stems erect, branching distally or not, dendroid or not, sterile stems plagiotropic or arching, rarely erect, to 20 cm; rhizoids brown, macronemata mainly proximal, occasionally along underside of sterile stems, micronemata present. Leaves: green or yellow-green, rarely black with age, variously crisped or contorted when dry, erect-spreading, usually flat, occasionally irregularly wavy or transversely undulate when moist, elliptic, obovate, oblong, oblong-lingulate, oblong-elliptic, or rarely orbicular or diamond-shaped, 1–14 mm; base decurrent or not; margins plane, green or yellow-green, 1-stratose, toothed, often to near base, sometimes only distally, rarely entire, teeth single, sharp or blunt, of 1–2(–4) cells, rarely hooked; apex acute, acuminate, obtuse, rounded, truncate, retuse, or emarginate, mucronate, apiculate, or cuspidate, cusp toothed or not, rarely bent to one side; costa percurrent, excurrent, or rarely subpercurrent, distal abaxial surface smooth; medial laminal cells short-elongate, elongate, or ± isodiametric, (15–)30–70(–85) µm, sometimes in diagonal or longitudinal rows, collenchymatous or not, walls pitted or not; marginal cells differentiated, linear or rhomboidal, in 2–4(–5) rows. : Specialized asexual reproduction usually absent (by stolonlike stems in P. undulatum). Sexual: condition synoicous or dioicous. Seta: single or multiple, yellow, yellow-green, brown, sometimes reddish or greenish, rarely orange, dark red, or blackish with age, 1.2–5 cm, straight to flexuose. Capsule: horizontal to pendent, yellow or yellowish brown, cylindric, oblong, oblong-cylindric, obovoid, or ovoid, 1.5–5 mm; operculum conic-apiculate or rostrate; exostome yellow or brown; endostome yellow to yellowish brown. Spores: 18–40 µm. Nearly worldwide.

Species 26 (11 in the flora). Plagiomnium is characterized by singly serrate, 1-stratose leaf margins, the absence of red stem tissue, and, in most species, the production of long sterile plagiotropic or arching stems. Plagiotropic stems are absent in P. venustum and some fertile patches of a few other species, in particular P. insigne. In many Plagiomnium populations, fertile stems are absent with plagiotropic stems producing vegetative mats over and among litter and other mosses. Morphological variation is common in Plagiomnium and has led to misidentifications in regional herbaria. Many characters that have been used in extant keys, such as sharpness, size, and cell composition of marginal teeth, presence and length of leaf decurrencies, and degree of laminal cell pitting, can vary within and between populations. Extant keys often use sexuality to separate species; unfortunately, many collections are sterile, and this character is of little use. As with Brachythecium, sterile collections of Plagiomnium may be unidentifiable, especially weakly developed plants from wetland habitats. Based on the review of the collections examined for this treatment, more species are likely present within the range of the flora, and a thorough review of North American Plagiomnium is recommended. Three species, Plagiomnium carolinianum, P. rostratum, and P. undulatum, have rostrate opercula; the remaining species have conic-apiculate opercula. Plagiomnium includes four species endemic to the flora area, P. carolinianum, P. ciliare, P. floridanum, and P. venustum, and one apparently introduced species, P. undulatum.