The species of this genus tend to be so variable that overlapping measurements in the keys may appear discouraging. In fact, the species are rather easily distinguished, and specimens in the range of overlap for one character will usually be clear-cut on the basis of others (or of the same structures elsewhere on the plant; e.g., calyx lobes may differ at different nodes). The corky borders of the nutlets doubtless contribute to their buoyancy, and dense rows of seedlings are a common sight on shores and banks where the fruits have been washed. Measurements of leaf length used in length/width ratios include the petiole (if any).
Lycopus amplectens Raf., a Coastal Plain disjunct, is known from northern Indiana and may occur in southwestern Michigan. It has sessile leaves like L. asper, but with only 4–6 teeth per side and is a more slender plant with glabrous or finely puberulent stems. Lycopus asper has leaves with usually more than 6 teeth per side and is a coarse species with the stem spreading hairy.
1. Calyx lobes less than 1 mm long, ± deltoid, at most barely as long as the mature nutlets.
2. Plant arising from a soft corm-like tuber barely underground; stamens conspicuously exserted beyond corolla tube; nutlets not over 1.5 mm long, with tubercles or teeth only on or near outer rim of apex, the group of 4 nutlets thus concave or bowl-like across the tops; common throughout Michigan.
2. Plant without a conspicuous tuberous base; stamens included; nutlets ca. 1.3–2.1 mm long, with teeth all across the apex, the group of 4 nutlets thus square or even convex across the tops of the teeth; rare, mostly in southern Michigan.
1. Calyx lobes (1–) 1.2–2.8 mm long, acuminate to subulate at tip, distinctly exceeding the nutlets.
3. Nutlets ± tuberculate, toothed, or knobby on the apex; leaves (at least the middle ones) either clearly sessile with sharply and regularly serrate margin or tapered to petiolate base with shallowly and remotely serrate margin.
4. Leaves with ± obtuse, sessile base; bracts of inflorescence lanceolate, the larger ones equaling or exceeding the adjacent calyces; nutlets 1.7–2.1 mm long.
4. Leaves strongly tapering to base, ± petiolate; bracts of inflorescence minute, linear, much shorter than calyces; nutlets not over 1.5 mm long.
3. Nutlets with smooth corky rim along outer margin of apex, but not tuberculate or knobby; leaves tapered to petiolate or subpetiolate base, prominently dentate (or often narrowly lobed toward base).
5. Principal cauline leaves (1.5–) 2.3–5.5 (–8) times as long as broad, the lower ones often deeply pinnatifid into linear lobes, the teeth (or lobes) mostly with forward margin at an acute angle to the midrib; calyx lobes (1–) 1.3–1.8 (–2) mm long; leaves usually glabrous or merely scabrous above.
5. Principal cauline leaves (2.1–) 2.5–3.1 times as long as broad, coarsely dentate with forward margin of middle teeth at ± a right angle to midrib; calyx lobes (1.5–) 1.8–2.8 mm long; leaves at least sparsely strigose above.