2. Thaspium trifoliatum (L.) A. Gray

Pl. 213 i–k; Map 888

Stems 20–80 cm long, usually pubescent with a band of minute hairs (visible only with magnification) at the base of at least the uppermost leaf sheaths, otherwise glabrous. Basal leaves with the blades 3–9 cm long, simple or ternately 1 time lobed or compound, the leaflets (when present) 1–7 cm long, broadly ovate to oblong-obovate or lanceolate, narrowed to rounded (often unequally) or cordate at the base, sometimes with 1 or 2 lobes toward the base, the margins otherwise finely toothed, glabrous, with a narrow, white border, the surfaces glabrous or slightly roughened along the main veins. Stem leaves similar to the basal leaves, mostly ternately 1 time lobed or compound, less commonly simple or ternately 2 times lobed or compound, those just above the stem base usually somewhat larger than the basal leaves, the median and upper leaves somewhat reduced in size. Rays 6–10(–18), 0.5–3.0 cm long, unequal in length. Involucel of linear to ovate-triangular bractlets. Flower stalks 1–4 mm long. Petals bright yellow or dark purple. Fruits 3–4 mm long. 2n=22. April–June.

Scattered nearly throughout the state, but more common south of the Missouri River (eastern U.S. west to Minnesota and Texas; Canada). Mesic to dry upland forests, upland prairies, savannas, glades, ledges and tops of bluffs, and less commonly banks of streams; also old fields, roadsides, and railroads.

Steyermark (1963) maintained that T. trifoliatum is totally glabrous, but examination of specimens reveals that on many plants the bases of the leaf sheaths have a ring of pubescence similar to that described for T. barbinode. In T. barbinode, the hairs are slightly longer and white, and are thus usually visible to the naked eye, whereas in T. trifoliatum the hairs are minute and can only be discerned with magnification. Some authors (McGregor, 1986d) refer to the nodes of T. barbinode as having a beard of short hairs and those of the latter species as merely roughened. A few specimens from Adair and Montgomery Counties are anomalous for either of the two species and may represent hybrids between them. These plants have inflorescences with too many rays and leaves that are too divided to be comfortably placed in T. trifoliatum, but the rays are somewhat unequal in length and the leaflets have white margins that are glabrous or only slightly hairy, and thus are atypical for T. barbinode. Flower color also seems to vary from pale to bright yellow in such plants. Lindsey (1982) documented that species of both Thaspium and Zizia are highly outcrossing in nature. Further studies are needed to evaluate the potential for hybridization between T. barbinode and T. trifoliatum.

Two varieties, differing only in the amount of anthocyanin production, are accepted within T. trifoliatum by most authors. The two varieties differ somewhat in overall distribution, with var. trifoliatum occupying the southeastern portion of the range of the more widespread var. flavum.