Triosteum angustifolium L.
Yellow Flowered Horse Gentian
Family - Caprifoliaceae
Stem - Ascending to erect, to 70 cm, pubescent with bristly spreading hairs, sometimes also with shorter minutely glandular hairs.
Stem and node.
Leaf - Leaves opposite and typically decussate, narrowly ellpitic or oblanceolate, narrow at base and not perfoliate, variously hairy.
Decussate leaf pattern.
Inflorescence - Flowers solitary in leaf axils (2 per node).
Calyx - Sepals 5, 9-12 mm long, surfaces glabrous or nearly so, margins with long, bristly hairs.
Flowers - Corollas usually yellow but occasionally orange or red, 13-17 mm long, mouth noticeably oblique, outer surface with glandular hairs. Style exserted from corolla <2 mm.
Flowers (red form).
Flowers (yellow form).
Fruits - Fruits 5-7 mm in diameter, yellow-orange, moderately to densely hairy at maturity..
Flowering - April - May.
Habitat - Forests, bluffs, streambanks.
Origin - Native to North America.
Other info. Of the three horse gentians found in Missouri, this is the least common. Yellow-flowered plants are always T. angustifolium; however, red flowers do not eliminate the possibility. The leaves of this species are narrower than those of the other horse gentians, and the hairs of the stems and sepal margins longer. The red-flowered plant shown in the pictures above was found in a gladelike habitat, which may be responsible for the reddish cast of the leaves. The red-flowered form has been called fo. rubrum F. Lane. Some authors have also split the species into two varieties based upon the pubescence of the leaf abaxial surface.
Photographs taken at Valley View Glade Natural Area, Jefferson County, MO, 4-28-2010 and 5-8-2017, and at Young Conservation Area, Jefferson County, MO, 4-22-2015 (SRTurner).