Sandy and gravelly disturbed ground (roadsides, clearings, fields, gravel pits, dunes, railroads); shores; spreading to dry open forests (oak, aspen, jack pine). The earliest Michigan collection dates from 1861 in Washtenaw Co., but this was documented from Fort Gratiot (St. Clair Co.) by Zina Pitcher in 1829 (see Torrey & Gray 1838–1840). By some authors considered native in eastern North America but introduced in the west; by others thought to be native primarily in the west; others suggest it to be introduced from Eurasia, and by others to be introduced into Europe. It is now a widespread weed, but is here tentatively considered to have been originally native to Michigan. 

Often misidentified as L. virginicum. The mature fruit of the latter is scarcely if at all longer than broad and tends to be broadest just below the middle. The fruit of L. densiflorum tends to be slightly longer than broad (oblong-elliptic to obovate), often very slightly broader just above the middle, but the shape differences are subtle. A better character is in the seed: in L. virginicum (at least in our region), the cotyledons are accumbent, so that in a cross-section one sees the round embryo at the end of the flattened, parallel cotyledons, diagrammatically shown as =0; in L. densiflorum, the cotyledons are incumbent, so that a cross-section of the seed appears to have 3 structures in a row: 000.