This is known only from s. Ill., s. Ind., west/central Ky. and west/central Tenn. (Schwegman 1982; Heikens 2003b; Nesom 2009; Ch+.). In Ky. rather few colls. have been identified and accessed, but Oxalis in general needs more revision. It has recently been found by V. Voelker (NP) as far east as POWE (inaturalist.org/observations/287189432). And there are also records from the edge of the Cumberland Plateau in Tenn. (SERNEC). There may be intergradation with grandis, but no clear hybrids have been documented. There has also been some initial confusion with macrantha (= priceae), but that species now appears clearly distinct. O. illinoensis differs from grandis in its terminal, fleshy fusiform tubers connected by relatively slender herbaceous rhizomes (versus more continuous lignescent rhizomes); however, tubers sometimes cannot be found in the woods. Its leaflets are usually yellowish-green, lacking purplish margins (versus usually deeper green, purple-margined), more or less straight-sided above the base (versus clearly concave), and with relatively shallow, less acute terminal notches. Its peduncles are mostly shorter, with flowers usually among the leaves (versus clearly extending above). Flowers are often larger (petals mostly 12-18 mm long versus 10-14 mm), and more strongly red-lined within.O. illinoensis may tend to occur on drier sites than grandis on average, and may be more restricted to calcareous soils. However, we need more detailed study of localities where both taxa are reported.