Annual herb with a taproot 20 cm - 0.8 m tall Stem: prostrate to upright, four-angled, branched, sparsely hairy. Leaves: opposite, stalkless, 2 - 5 cm long, to 6 mm wide, linear to narrowly lance-shaped with a pointed to bristle-like tip, sometimes minutely toothed, rough, sparsely hairy. Stipules with five to eight upright, reddish, hair-like bristles. Inflorescence: a small cluster of one to three flowers in the leaf axils. Flowers: stalkless, pale purple to white, 4 - 6 mm long, funnel-shaped, hairy externally, with four short lobes. Lobes pointed to bristle-like. Stamens four, slightly exserted. Anthers whitish to pale yellow. Fruit: a dry nutlet, indehiscent, 2.5 - 6 mm wide, egg-shaped to spherical, paired, separating when ripe, hairy, crowned with persistent sepals.
Similar species: The pale purple flowers and hair-like stipular bristles are together a diagnostic characteristic of this plant.
Flowering: mid-July to late September
Habitat and ecology: Probably introduced from states in the south. Found along railroads in sandy soil and cinder ballast. It is also found in sandy clearings.
Occurence in the Chicago region: non-native
Etymology: Diodia is the Greek word for toll or thoroughfare. Teres means cylindrical (circular in cross section).