The stout green stems are erect, typically unbranched, and hollow. On each side of the square stem is a single vertical groove (Glen 2012).
The leaves are sessile, unlobed, and usually unstalked (Glen 2012). Each pair of opposite leaves is at right angles to the leaf pairs above and below it (Valo 2012). They are narrowly oblong to inversely lance-shaped, 1.5 cm to 4 cm long, and 0.25 cm to 1.375 cm wide. The leaves decrease in size as they ascend the stem. Each leaf tapers to a point at the tip and expands a broad base at the point of attachment with the stem. The upper surface is often rough to the touch due to minute, stiff hairs; however, it can be hairless. The lower surface is hairless. The margins are serrate, coarsely and evenly toothed with sharp, forward-pointing teeth. When crushed the leaves do not smell of mint (Valo 2012). The inflorescence is a tight cluster of small, white, stalkless flowers (Glen 2012). Pairs of clusters in opposite leaf axils form false whorls on the upper 2/3 of the stem (Valo 2012). There are 5 white corolla (petals) that unite at the base into a tube, then separate at the tip into 4 lobes. The corolla is slightly longer than the calyx. There are 2 fertile stamens with brown anthers and 2 sterile stamens (staminodes). The stamens and the style are longer than corolla tube. The staminodes are club-like and are shorter than the corolla tube. The flowers not have a fragrance (Valo 2012).
Calyx lobes consist of 5 green narrow sepals (the outer whorl of protective leaves of a flower) are united for most of their 4mm length, bell-shaped tube with 5 teeth (Glen 2012). The calyx teeth are 1⁄16 cm to ? cm long, firm, and awl-shaped (Valo 2012).
The fruit is a set of 4 egg-shaped, brown, hairless, ridged nutlets with one seed each. The center of the nutlets is depressed. When the fruit is mature the calyx lobes distinctly surpass the mature nutlets. The ridges on the nutlets are corky, aiding dispersal by allowing the nut to float on water (Valo 2012).
Below ground L. asper has long, slender, horizontal rhizomes and fibrous roots. The rhizome has a tuber at the tip from which next year’s stem will rise. It does not produce stolons (above-ground runners) (Valo 2012).
Lycopus asper is very similar in appearance to the native L. virginicus (USDA NRCS 2012). As noted above, L. asper has narrow, acutely pointed lobes of its sepals (outer floral ring), while L. virginicus has sepals that are ovate and blunt (Glen 2012).