This variable winter annual occurs mostly in the Ozark-Ouachita region and the Interior Low Plateaus, on various types of rock; 2n = 22, 44, 66 (Baldwin 1943, Smith 1946; Cr). In Ky. it is largely restricted to limestone outcrops in three regions: (1) clifftops and rocky pastures in the central Bluegrass region (especially near the Kentucky River Palisades); (2) similar sites along the Cumberland Rv. and its tributaries downstream from the Appalachian Plateaus; (3) flat-rock glades or similar nearby sites in the Pennyrhile Karst Plain around the Shawnee Hills, sometimes on sandstone with loess or middens nearby (e.g. at Indian Hill in EDMO). Curiously, it is unknown along the locally extensive limestone cliffs of the Knobs region. There may be considerable genetic differentiation among these populations and others across the species' range. Although pulchellum is largely winter annual, a few plants in some populations appear to behave as biennials (perhaps germinating late), remaining vegetative through the summer until the subsequent spriing. Plants of flat-rock glades in c. Tenn. appear to be distinct in several characters (D. Estes, pers. comm.). With some biogeographic parallels, cryptic variants may also exist among associated xerophytic calciphiles in Cerastium, Sabulina [Mononeuria], Houstonia, Ruellia, Rudbeckia, Allium and Sporobolus.