Pontederia vaginalis Burman f., Fl. Indica, 80. 1768; Monochoria plantaginea (Roxburgh) Kunth; Monochoria vaginalis var. pauciflora (Blume) Merrill

Flowering stems 8–16.5 cm, distal internode 3–15 mm. Sessile leaves up to 5 cm, blade linear, apex acuminate. Petiolate leaves: stipule 3.5–5.7 cm; petiole 7–28 cm, rigid; blade cordate, 4–8 × 1.8–5.2 cm. Inflorescences paniculate, 3–8-flowered, all flowers opening same day; spathes 1.9–4 cm, sometimes with lanceolate to ovate extension. Perianth: limb lobes blue or white, narrowly ovate, 10–14 mm; larger stamen with hook, 4–6 mm, shorter stamens 3–4 mm; filaments linear, glabrous; style glabrous. Seeds 10–11-winged, 0.8–1.2 × 0.4–0.6 mm.

Flowering (Jul--)Sep--Oct. Rice fields, ephemeral ponds; 0--200 m; introduced; Calif.; Asia (India, China, Japan, and Malayan Islands).

By the 1970s, Monochoria vaginalis had become part of the rice-field flora of California (S. C. H. Barrett and D. E. Seaman 1980). C. D. K. Cook (1989) described this species as characteristic of rice fields; however, no records are known from the rice fields of Missouri to Louisiana.

As is typical of many aquatic annuals, plant size, leaf shape, and flower number are highly variable in relation to the amount of water (C. D. K. Cook 1989). Specimens from California typically have three to eight flowers, while descriptions of those from other parts of the world (H. Aston 1977; C. D. K. Cook 1989) suggest that the inflorescences of this species may have up to 20 flowers.