Helanthium tenellum (Mart. ex Schult. & Schult. f.) Britton
CC = 9
CW = -5
MOC = 3
Family - Alismataceae
Habit - Perennial forb with stolons.
Plants - Small, delicate perennials with stolons.
Leaves - Long petiolate, with entire leaf to 10 cm. Blades simple, entire, linear to elliptic, tapered at both ends, glabrous.
Inflorescences - Erect, 1-2 per plant, umbellate, with 3-6 flowers. Bracts to 3 mm, narrowly triangular.
Flowers - Perfect, actinomorphic, 3-merous. Sepals 1-2 mm long, ovate, appressed in fruit. Petals 1-3 mm long, white, obovate nearly orbicular. Stamens 6 or 9, the anthers 0.2-0.3 mm long, the filament attached at the anther base. Pistils 10 to many, arranged in a headlike cluster.
Fruits - Fruits 0.9-1.2 mm long, enclosed in persistent calyx, reddish brown to black, with 8 ribs, the beak a minute tooth, attached obliquely.
Flowering - July - October.
Habitat - Emergent aquatic on margins of receding ponds and wet swales, usually on sandy substrate.
Origin - Native to the U.S.
Lookalikes - None.
Other info. - This tiny species ranges across scattered locations in the southern and eastern U.S., but it is uncommon to rare nearly everywhere. In Missouri it is only known from three counties, and carries a state conservation ranking of S1 (critically imperiled). Although individual plants are very small, a large population can be conspicuous with hundreds of flowers.
Photographs taken southeast of Benton, Scott County, MO, 8-29-2011 and 8-28-2015 (SRTurner).