Flower:

Branching clusters at the top of the plant, each branch with numerous whorls of 15 to 25 slender-stalked flowers, usually crowded towards the branch tips and more widely separated below. Flowers are less than 1/4 inch long (3.5 to 5 mm), yellowish to light 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 nearly round to heart or kidney-shaped, distinctly veined across the surface, the edges mostly flat and smooth. The base of the midvein of at least one inner tepal is slightly swollen, occasionally producing a short, slender protrusion called a grain. Tepals are spreading at flowering time, revealing stamens and styles, and close up after pollination. Flower stalks are very slender and smooth, up to twice as long as the inner tepals at flowering time, with a distinct, swollen joint in the lower third. 

Leaves and stems:

Leaves are both basal and alternate, hairless though sometimes rough along the veins on the underside, mostly toothless around the edge and crinkly-wavy, pointed at the tip, the base wedge-shaped to straight across to somewhat heart-shaped. Basal leaves are lance-linear, 6 to 12 inches long and to 1½ inches wide, on a stalk about as long as the blade. Leaves become smaller, more tapering at both ends, and shorter stalked as they ascend the stem.

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

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, elliptic, reddish brown, and 2 to 2.5 mm long.

Notes:

Field Dock is an uncommon weed of disturbed soils but is likely under-reported 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. Field Dock has crinkly-wavy leaves, tepals up to 5 mm long that are kidney-shaped to nearly round and not ragged around the edge, usually just a slight swelling of the midvein and no grains (occasionally a single one), and the flower stalk has a swollen joint in the lower third. It has one of the smallest tepals of the Minnesota docks.

Of the other docks with crinkled-wavy leaves, the most common by far is Curly Dock (Rumex crispus), which has narrower and more egg-shaped tepals, and at least one tepal in each flower, usually all 3, has a distinct grain. Narrowleaf Dock (Rumex stenophyllus) has distinctly toothed inner tepals, each with a grain. Dooryard Dock (Rumex longifolius) has much broader leaves and larger, more heart-shaped tepals. Great Water Dock (Rumex britannica), a native wetland species, has basal leaves that are more or less flat, 3 grains that are all about the same size and shape. Patience Dock (Rumex patientia) has tepals much the same as Field Dock, but much larger (to 8 mm) and typically with a single distinct grain per flower, about ¼ the length of the tepal. Bitter Dock (Rumex obtusifolius) has broad, heart-shaped basal leaves and tepals with a few large teeth around the edge.

We suspect some of these non-native species are under-reported in the state, largely ignored because they're all assumed to be Curly Dock.