Flower:

One to 3 flowers, rarely 4 or 5, clustered in leaf axils all along the stem. Clusters and individual flowers are stalkless or nearly so, the stalks less than .5 mm long. Each flower is ¼ inch across or less, has 4 (rarely 8) pink to lavender, wavy petals 2 to 4 mm long with 4 to 8 light yellow tipped stamens surrounding a stout green style in the center. The calyx cupping the flower is cylindrical, less than ¼ inch (3 to 5 mm) long, the sepal edges fused and broadly triangular at the tip.

Leaves and stems:

Leaves are opposite, the pairs set at right angles to the ones above and below, ¾ to 4 inches (to 10 cm) long, ¼ to about ½ inch (to 15 mm) wide, pointed to blunt at the tip, toothless, hairless, stalkless, mostly green or tinged red, the lowest leaves more oblong-elliptic becoming lance-linear up the stem. The leaf base is typically heart-shaped with a pair of broad but shallow lobes, but is sometimes merely rounded. Stems are smooth, 4-sided, usually pinkish to bright red, unbranched or branched mostly in the lower plant, the branches often prostrate from the base before rising up (decumbent).

Fruit:

Fruit is a round capsule 4 to 6 mm (to ¼ inch) in diameter, as long as or slightly longer than the persistent calyx at maturity. Capsules have 4 chambers and turn red as seed ripens.

Notes:

Grand Redstem inhabits the muddy banks and mudflats of the Red River and its tributaries, the entire reach of the Minnesota River, flowing to the Mississippi River just past where it joins the Cannon River. Mostly terrestrial and appearing after seasonal high waters have receded, it will persist in standing water if water levels rise again. While not found in the same habitat, it may perhaps be confused with either Toothcup (Rotala ramosior) or Water Purslane (Ludwigia palustris), especially when petals are absent. In Rotala, the flowers are always single in the axils, the leaves tapering to a short stalk. Ludwigia is always creeping, rooting at the nodes, and also has only single flowers in the axils with more broadly oval leaves on a longer tapered stalk.

It was long thought the species in Minnesota was Ammannia coccinea, but we all now realize it's been A. robusta all along. A. coccinea is distinguished by usually having 5 or more flowers in a cluster, the cluster being on a stout stalk (peduncle) up to 9 mm (~1/3 inch) long, flowers are a deeper pink-purple with brighter yellow anthers, and its geographical range is primarily south of Minnesota.