Species commonly cultivated
Rotala macrandra Koehne R. mexicana Cham. & Schltdl.
R. rotundifolia (Buch.-Ham. ex Roxb.) Koehne
R. wallichii (Hook. f.) Koehne Rotala sp. ’Nanjenshan’ (Taiwan)
Rotala rotundifolia is introduced in Alabama and Florida (United States), while R. indica (Willdenow) is recorded in rice fields in California and Louisiana (United States).
Members of this genus are sometimes recorded as weedy in rice fields and irrigation ditches.
Stem erect, creeping or floating. Leaves opposite, decussate, sometimes whorled, usually sessile; leaf blade shape variable: capillary, oblong to ovate, green to dark red; distinctly heterophyllous: submersed leaves typically narrower and thinner than emergent leaves. Inflorescence axillary or a spike or raceme. Flowers actinomorphic; sepals fused, calyx tube variously shaped, lobes usually 4; petals present, pink or white or absent. Dispersal by seed.
still, shallow waters and water margins, also wet soil
Rotala is a large genus of ca. 46 species, but only four or five are commonly traded in the industry. Many species are wetland herbs or shrubs and are not strictly aquatic. Rotala wallichii is easily differentiated by its needle-shaped submersed leaves and white flowers. Rotala rotundifolia has ovate to elongate submersed leaves, ovate emergent leaves, and pink flowers. Rotala macrandra has very broad ovate leaves on submersed and emergent stems (wavy in submersed form) and lacks flower petals. Rotala mexicana has ovate to narrow elongate leaves in both submersed and emergent forms and also lacks flower petals. Rotala rotundifolia is often sold as R. indica. Rotala sp. 'Nanjenshan' appears to be a possible natural hybrid of R. wallichii and R. rotundifolia, known only from a single location in Taiwan.