Perennial aquatic herb to 0.6 m tall Leaves: emersed (rarely floating), triangular-stalked, to 18 cm long and 12 cm wide, egg-shaped to elliptic. Inflorescence: loosely branched (panicle), terminating a stalk to 1 m long. Flowers: bisexual, with 1.5 - 2.5 mm long sepals. The three white petals are 1.3 - 2.5 mm long with a long pointed tip and slightly unevenly cut margins. Fruit: a cluster of achenes 2 - 4 mm in diameter. Each achene has an ascending beak, is 1.5 - 2.2 mm long, and has a rounded outer edge with a single groove.
Similar species: The similar Alisma triviale has flowers greater than 4 mm across and achenes greater than 2 mm long.
Flowering: early July to early September
Habitat and ecology: Frequent in marshes, shallow water, and ditches.
Occurence in the Chicago region: native
Etymology: Alisma is the classical Greek name for the water plantain. Subcordatum means "somewhat heart-shaped."
Author: The Morton Arboretum
From Flora of Indiana (1940) by Charles C. Deam
Infrequent to frequent throughout the state, being more common in the lake area where dredged ditches are more frequent. It is found in muddy or mucky soil in ditches, ponds, and sloughs and about lakes.
Indiana Coefficient of Conservatism: C = 2
Wetland Indicator Status: OBL
Diagnostic Traits: Rosette emergent leaves +/-ovate with netted venation; inflorescence of whorled branches and branchlets; flowers perfect, 3-4 mm broad; sepals to 2.5 mm long; petals 3; stamens 6-9; pistils in a ring on a flat receptacle; achenes to 2.2 mm.
Much like no. 1 [Alisma triviale Pursh], but with smaller fls; sep 1.5-2.3 mm; pet 1.8-2.5 נ1.4-2.0 mm, barely or scarcely longer than the sep; anthers 0.4-0.6 mm; style very short, 0.2-0.4 mm; achenes 1.5-2.2 mm; 2n=14. Usually in shallow water; Vt. and Mass. to N.D., s. to Fla. and Tex. (A. plantago-aquatica var. parviflorum) Variously reported to intergrade with no. 1 or to be genetically isolated from it.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.