Bull. Torrey Bot. Club 24: 172. 1897
Phenology: Flowering mid winter–spring (late Feb–mid Apr).
Habitat: Rich to dryish deciduous forests of mature or second-growth timber, dominated by oaks or with beech-oaks, occasionally with scattered pines present, flat ground along streams where soil can be fairly moist, rich clay or sand
Elevation: 100–200 m
Trillium underwoodii can cause considerable confusion for the beginning student of the genus. In some ways it closely resembles a small-statured T. cuneatum. As T. underwoodii continues to expand after flowering, it may become somewhat taller and less “ground hugging,” and it then appears similar to a smaller plant of T. decipiens, which grows in parts of the same range, especially when its bracts do not touch the ground at anthesis. It also has vague similarities to T. reliquum, with which it grows sympatrically in parts of its range. Care should be taken to observe the details of floral structure accurately.
Distribution of Trillium underwoodii is much more restricted than that given by J. K. Small (1933), who indicated a range extending from Alabama to North Carolina and Arkansas.