This western annual has spread east to become about as common as virginicum in Ohio (A. Cusick, pers. comm.) and some other northeastern states (K). It is easily confused with virginicum; despite much variation within each species, formal varieties are not generally recognized in eastern North America. Hybrids are not confirmed but have been suggested; 2n = 32 in both (Y, FNA 7). Some published keys have been difficult to apply, and colls. of both species need rechecking; see also Verloove (2016) for useful treatment. L. densiflorum differs superficially in its generally denser inflorescences, often developing several major branches that curve up from the plant base, and with ca. 5-8 bracteal leaves below each raceme. In virginicum, branching is more gradual, mostly in the upper half of the plant, and with only ca. 3-5 bracteal leaves below each raceme. Plants of densiflorum have straight, slender to subclavate hairs (versus usually curved-subappressed, cylindrical); fruiting pedicels are generally shorter (1.5-4 mm long versus 2-6 mm); petals are absent or rudimentary (0-0.9 mm long versus 1-2.5 mm); fruits are generally smaller and slightly more elongated (2-3.5 x 1.5-3 mm versus 2.5-4 x 2.5-3.5 mm). Although FNA 7 states that fruits are "widest beyond middle" this is not consistent, as shown by the figure in FNA. Its flowering in Ky. appears to be generally earlier, mostly during Apr-Jun or earler in mild winters (versus Apr-Sep). It is locally frequent in urban areas along roads on asphalt, often rooting in cracks or puddled areas that dry out during the summer. Most Ky. records date from 1920-1950 but the species appears to have become generally overlooked, and it is probably widespread although still much less abundant than virginicum.