This vine occurs largely in or near the lower Mississippi Valley (broadly defined), typically in floodplain forests but sometimes on adjacent rocky bluffs as well. Several records have tentative identification, or may be escapes from cultivation. I. tomentosa is easily confused with macrophylla (FNA 3, W), but can be distinguished by its calyx color (yellow with purple mouth versus uniform purple or brown), and its more or less tomentose stems (versus essentially glabrous). Leaves are relatively blunt (versus abuptly pointed) and tend to be more pubescent, but these differences are often not clearcut in Ky. Both species are sometimes cultivated and may occasionally escape. Caterpillars of the Eastern Pipe-vine Swallowtail butterfly feed on Isotrema as well as Endodeca.