Flower:

Erect, densely packed spike-like racemes ½ to 2 inches long on smooth stalks at the tips of branches and the upper leaf axils. Clusters are mostly uninterrupted though occasionally a few flowers are separated at the base. Flowers are pink to deep purple, 1/8 inch long, usually with 4 (sometimes 5) tepals (petals and similar sepals) that barely open and are nearly stalkless.

Leaves and stems:

Leaves are alternate, narrowly or broadly lance-shaped, usually with a dark blotch on upper surface, pointed at the tip, tapered or wedge-shaped at the base, typically 2 to 4 inches long (up to 7), and 1/3 to 1 inch wide, the stalk not more than 1/3 inch long, becoming stalkless in the upper plant. Leaf edges are toothless but with fine cilia like hairs or often rough when stroked towards the base. The surfaces are mostly smooth or with straight appressed hairs.

At the base of the leaf stalk is a brown, membranous sheath (ocrea) with pale ribbing that extends up around the branch stem, tearing away with age, its upper edge with distinct bristly hairs about 1/8 inch long. Stems are green or red, erect to spreading or occasionally sprawling, simple or branching, smooth throughout or with soft appressed hairs on the upper branches. When sprawling, it may root at the nodes in the lower plant.

Fruit:

Fruit is a dry seed (achene), disc shaped to 3-sided, brown or black with a smooth, shiny surface.

Notes:

Non-native Lady's-thumb, formerly Polygonum persicaria, has become widely established across North America and is one of the most common Smartweeds found in Minnesota. It can grow explosively in seasonal water basins, receding waterlines of lakeshores or any moist disturbed area, and at one time was designated a county-level noxious weed. It is similar to and can be difficult to distinguish from the native Nodding Smartweed (Persicaria lapathifolia) and more so Pennsylvania Smartweed (P. pensylvanica), both which share the same general habitats and have a number of overlapping characteristics. With the former, P. maculosa will have consistently shorter racemes that are mostly erect while P. lapathifolia will have longer and strongly nodding or dangling flower clusters. Like P. maculosa, P. pensylvanica also has shorter and more erect racemes but those of P. maculosa are typically a bit shorter and more slender. Also, the ocrea of P. maculosa has observable bristles or cilia fringe on the upper edge where the other two are usually fringeless, and P. maculosa will often have only 4 tepals and P. pensylvanica has 5. Lastly, both P. lapathifolia and P. pensylvanica occasionally have a darker spot on the upper leaf surface but P. maculosa almost always has a very dark spot present.