Flower:

Flowers are stalkless and single in the leaf axils all along the stem, the 4 white to pinkish petals minute, often absent altogether. The base of the flower is a thickened cup 1/8 to 1/6 inch (2 to 4 mm) long, greenish to often pinkish or even deep red in color, with a blocky or toothy rim around the top. Within the floral opening are four stamens and a single central style, mostly indiscernible without a hand lens peering straight into the flower top.

Leaves and stems:

Leaves are opposite, the pairs set at right angles to the ones above and below, oblong-elliptic, widest at or above the middle, 3/8 to 2 inches (1 to 5 cm) long and up to ½ inch wide, bright green, toothless, hairless, the central vein often bright red, and tapering at the base. Stems are erect, often heavily branched, the branches ascending, smooth and weakly 4 angled, also often pinkish to bright red.

Fruit:

Fruit is a round capsule 2 to 4.5 mm in diameter with four chambers.

Notes:

Toothcup enjoys a fairly broad range throughout North America down through Mexico and into Central America, even becoming invasive after introduction into Taiwan. Suggesting a more tropical origin, it should not be surprising it is typically rare throughout the northern portions of its range. In Minnesota it is a state listed Threatened species and, according to the DNR, its habitat is restricted to sandy margins of small ponds and lakes in the Anoka Sandplain, much of it already destroyed or under threat of destruction by development.

While somewhat unique in form and habit, it is not without similar species with which it could be confused. Most notably is Grand Redstem (Ammannia robusta), which is in the same family. Grand Redstem's flowers occur in tight clusters, usually 1 to 3 per leaf axil, have larger pink to lavender petals, its leaves are more lance linear that are mostly heart-shaped at the base, and its preferred habitats are the the muddy flats and river banks from the Red River, through the Minnesota River valley, and down the Mississippi River to just south of where it joins the Cannon River. Another species with marked similarities that can be found growing side by side with Rotala is Water Purslane (Ludwigia palustris), which is more spreading and creeping, frequently rooting down at the nodes. Its leaf blades are more spoon-shaped, and its flowers have four spreading, triangular sepal lobes. False Loosestrife (Ludwigia polycarpa) may also grow with Rotala but has alternate leaves where these others all have opposite leaves.