Flower:

Branching clusters at the top of the plant, each branch with several to many whorls of 10 to 25 long-stalked flowers, the whorls usually not crowded along the branch. Flowers are less than ¼ inch long (3.5 to 5 mm), green to pinkish, 3-sided with 2 series of tepals (petals and similar sepals) and the outer tepals much smaller than the inner.

The 3 inner tepals, also called valves, are broadly egg-shaped to triangular to 3-lobed, usually longer than wide, distinctly veined across the surface, the edges flat and mostly smooth. At the base of each inner tepal is a projection called a grain, at least half as long as the tepal at maturity, lance to narrowly egg-shaped typically tapering to a blunt tip, with the grains of the 3 tepals all about the same size. Tepals are spreading at flowering time, revealing stamens and styles, and close up after pollination. Flower stalks are smooth, very slender at the base and thickening near the flower, 3 to 5 times as long as the inner tepals, with a joint near the base.

Leaves and stems:

Leaves are all alternate, 2 to 12 inches long, 3/8 to 2 inches wide (5 to 7 times as long as wide), lance-linear, hairless, toothless, pointed at the tip, tapering at the base, with a faint network of veins and flat blades, the edges never crinkled and only weakly wavy, if at all. The lowest leaves are long-stalked, becoming shorter stalked as they ascend the stem.

At the base of the stalk is a brownish papery sheath (ocrea) that surrounds the stem and mostly disintegrates with maturity. Stems are erect, hairless, faintly ribbed, and unbranched except in the flower clusters.

Fruit:

A flower produces a single seed, wrapped in the persistent tepals, which form a capsule-like structure and dry to rusty brown.

Seeds are 3-sided, egg to teardrop-shaped with a short taper at the tip, dark brown to reddish brown, and 2.3 to 3.1 mm long.

Notes:

Swamp Dock is an uncommon native species of moist soils that reaches the northern edge of its range in Minnesota. There are at least 11 similar Rumex species in the state, 5 of which are native. Some of the distinguishing characteristics are whether the leaves are crinkly-wavy or relatively flat, the shape of the inner tepals at maturity, size and shape of the grains, whether the grains on all 3 inner tepals are about the same size, sometimes the length of the flower stalk or where the stalk is jointed or the vein pattern on the leaves. Swamp Dock has flat leaves that may be slightly wavy but never crinkled along the edge, tepals up to 5 mm long that are broadly egg-shaped to triangular or 3-lobed, all 3 grains about the same, flower stalks 3 to 5 times as long as the inner tepals and thickened near the flower, and faint lateral leaf veins.

The narrow, flat leaf blades and lack of a rosette of basal leaves distinguish it from all of the Rumex species with more strongly crinkled-wavy leaves, Curly Dock (Rumex crispus) being the most common. The most similar species are Pale Dock (Rumex altissimus) and Mexican Dock (Rumex triangulivalvis), both of which also have flat leaves but usually denser flower clusters and much shorter flower stalks. R. altissimus also has proportionately broader leaves (2.5 to 4 times as long as wide) and tepals to 6mm long. R. triangulivalvis has smaller, more consistently triangular tepals up to 3 mm long. Patience Dock (Rumex patientia) may have relatively flat leaves, but the tepals are larger (to 8 mm long), round to kidney-shaped, and typically have a single, tiny grain on each flower.