Pignut hickory is widely distributed, ranging from southwestern New Hampshire west to eastern Illinois, south to Louisiana and from there, east to central Florida. Distribution maps of C. glabra have included the distribution of C. ovalis, especially in maps prepared by Little (1971) since the reduction of C. ovalis to synonomy with C. glabra (Little 1969). Distribution reported here ( distribution map) is based on maps prepared prior to that reduction (Little 1949), as well as subsequent reports which continue to distinguish the two taxa. Proponents of recognizing both species associate C. glabra with sites in valleys, along streams or less exposed hillsides, while C. ovalis is more common on dry exposed upland hillsides (Manning 1950). C. glabra is a minor component of two forest types: Post Oak-Black Oak; and White Oak-Red Oak-Hickory (Fowells, 1965).

C. glabra is distinguished by its glabrous leaves, usually with 5 leaflets, its pyriform fruit with smooth husks which are only partially dehiscent, and by the tight bark of mature trees. C. ovalis shares the feature of glabrous leaves, but usually has 7 leaflets. C. ovalis has warty husks which dehisce to the base along sutures which are often somewhat winged. The bark of mature trees sheds in long thin strips. It is interesting that the only named cultivar of C. glabra is evidently wrongly attributed to that species. 'Brackett' was named in 1896, from a specimen received from G. B. Brackett, Denmark, Iowa (Heiges 1896). The figure of the fruit (Heiges 1896, Pl. 12, fig. 4, 4a, 4b.) is clearly not C. glabra, but is evidently C. X laneyi, the hybrid between C. ovata and C. cordiformis. This determination is based on the presence of prominent lacunae, which are lacking in both C. glabra and C. ovata, but are present in C. cordiformis and its hybrids; by the general appearance of the nut, with the prominent apex characteristic of C. cordiformis and its hybrids but atypical of C. glabra; by the dehiscent husks, apparently partially winged as in C. cordiformis, but atypical of C. glabra; and by the kernel which lacks the convolutions of C. cordiformis but is consistent with kernels of C. X laneyi. Heiges (1896) noted that "the kernel is large and full, and in flavor resembles closely H. ovata, but in color of surface is more like the bitternut, H. minima" (p 69).

Manning (1950) noted that C. glabra readily hybridized with C. ovalis when the two occurred together, with hybrids confusing the distinctions between the species.