From Flora of Indiana (1940) by Charles C. Deam

Oxalis illinoensis occurs in the western portion of the range of O. grandis, especially Indiana. The two can be subtly different. Oxalis illinoensis flowers have prominent red lines proximally; herbaceous rhizomes with tubers or tuberlike thickenings; leaflet lobes apically truncate, margins green; and flowers mostly at level of leaves.

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Indiana Coefficient of Conservatism: C = 9

Wetland Indicator Status: N/A

Diagnostic Traits: Stems erect from herbaceous, stoloniform rhizomes with fusiform tubers or thickenings; leaves trifoliate, the obcordate leaflets somewhat flattened distally and lacking purple margins; flowers produced within the level of the leaves, yellow with strong red lines in the throat; petals 12-18 mm.