Both species treated here are Eurasian natives, sporadically established, perhaps originally as escapes from cultivation. The attractive flowers are sensitive to light, and mostly remain closed except during the middle of sunny days.
1. Flowers sessile or nearly so, the calyx immediately subtended by bracts; stems usually arising from a basal rosette; mature corolla lobes ca. (4–) 4.5–5.5 mm long.
1. Flowers all or mostly on short pedicels ca. (1–) 2–4.5 mm long; stems without a basal rosette; mature corolla lobes ca. 2–3.5 mm long.
All species found in Centaurium
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 21, 2018. https://www.michiganflora.net/genus.aspx?id=Centaurium.