The morphology of the achene, and the commonly associated hypogynium, are probably the most important diagnostic characteristics in species identification.
Achenes roundly trigonous to terete, globose or ellipsoid; surface smooth, tuberculate (warty), pitted, or lobed, rarely pubescent; hypogynium disk-like, lobed or entire, sometimes absent.
Inflorescence an erect, interrupted spike of several sessile glomerules; hypogynium, a short, cylindrical disk, or absent 5.
Inflorescence of one or more terminal and lateral fascicles, paniculate; hypogynium well-developed.
Hypogynium obscurely trigonous, with a whitish papillose crust, lacking tubercles 4.
Achenes pitted, pubescent with tufts or lines of small hairs; hypogynium 3-lobed, the lobes oblong to lanceolate 1.
Achene tuberculate and papillate; glabrous; hypogynium with 6 globose tubercles in distinct pairs 3.
Achenes (1.0-) 1.5-2.5 (-3.5) mm long, globose to ovoid, with obtuse, apiculate apex, conspicuously pitted, white or gray, with tufts or lines of minute, yellowish to purplish hairs; hypogynium distinctly 3-lobed, the lobes oblong to lanceolate, apex obtuse to subacute, appressed to achene base.
Achenes 3-4 mm long, ovoid to subglobose, with obtuse, usually apiculate apex, smooth, glossy, white; hypogynium base obtuse trigonous, with 8-9 small, rounded or elongated , granular tubercles.