Marsh seedbox is a mat‑forming, herbaceous plant native to much of North America and widely distributed across other continents. It grows in muddy soils, marshes, shorelines, riverbanks, and sandbars, often forming low, creeping mats 3–12 inches tall. Its red stems and tiny green, petalless flowers are easily overlooked. Depending on water levels, it may grow submerged with only the stem tips visible, or spread across exposed mudflats where it is often mistaken for a weed.

The ⅛ inch greenish-cream-colored flowers are almost unseen, with only one flower growing at the axis of every leaf, and 4 pointed lobes flaring outward from the cup-shaped calyx.

Leaves are opposite, green, and round to oval, often developing pinkish or reddish tones in strong sunlight; the roots share this pinkish cast.

Although its tiny, petal‑less flowers attract few pollinators, marsh seedbox plays an important ecological role in aquatic habitats. Its dense mats provide shelter for midges, mayfly nymphs, small beetles, freshwater shrimp, and amphipods, and offer cover for tadpoles, frogs, and salamanders. Because of its red stems and ability to grow submerged, it is also widely used in aquariums.