Orobanche riparia

Plants simple or few to many-branched, 5–35 cm, stout, sometimes slender, base enlarged in robust specimens. Roots inconspicuous or conspicuous (often forming an amorphous mass), slender, branched or unbranched. Leaves numerous, appressed; blade broadly ovate to ovate-triangular, narrower distally, 6–9 mm, margins entire, apex acute or obtuse, surfaces glabrous. Inflorescences spikelike racemes, purple, lavender, or pallid, sometimes branched, glandular-pubescent, often ± viscid; flowers numerous; bracts usually reflexed, lanceolate, 8–15 mm, apex acute, glandular-pubescent. Pedicels 0–10 (–12) mm, much shorter than plant axis; bracteoles 2. Flowers: calyx purple, rarely pale lavender externally, weakly bilaterally symmetric, 7–11 (–13) mm, deeply divided into 5 lobes, lobes lanceolate-linear to linear-subulate, densely glandular-pubescent; corolla (13–) 15–22 mm, tube white, distally often tinged with purple or pink, or with dark purple veins, constricted above ovary, slightly to moderately bent forward, glandular-puberulent to pubescent; palatal folds prominent, yellow, pubescent; lips internally ± purple or lavender, often with darker veins, abaxial lip erect to slightly spreading, 3–4 mm, lobes narrowly oblong-triangular, apex acute, adaxial lip erect to ± spreading, 4–6 mm, lobes triangular, apex acute; filaments glabrous or pubescent at base, anthers included, glabrous or sparsely pubescent. Capsules ovoid, 7–10 mm. Seeds 0.3–0.5 mm. 2n = 48.

Phenology: Flowering Aug–Sep.
Habitat: Stream banks, sand bars, flood plains.
Elevation: 100–1500 m.

Colo., D.C., Ill., Ind., Kans., Ky., Mo., Nebr., N.Mex., Ohio, Okla., Tenn., Tex., Va., W.Va.

Discussion

Orobanche riparia was formerly included in O. ludoviciana (P. A. Munz 1930). The two species can be distinguished on the basis of morphology, habitat, host associations, and phenology. Orobanche riparia appears to be more closely allied to O. cooperi than O. ludoviciana. The eastern and western populations show slight differences in color intensity and degree of pubescence.

Orobanche riparia is the only species of Orobanche that exclusively parasitizes annual hosts, including Ambrosia trifida, Dicoria canescens, and Xanthium strumarium (Asteraceae), and rarely Nicotiana tabacum (Solanaceae) (L. T. Collins et al. 2009). Its riparian distribution, entirely on sandbars, sandy banks, and silt deposits mainly of the Mississippi, Ohio, Platte, and Rio Grande rivers and tributaries, is likewise unique. Extant populations are most abundant along the Ohio and Platte rivers but are imperiled by habitat destruction along stream banks.

Selected References

None.

Lower Taxa