Flower:

Erect to slightly nodding spike-like racemes, ½ to 2 inches long, dense and uninterrupted with a tip blunt, at the tips of branches and upper leaf axils. The stalk is smooth, or with soft appressed hairs, or short, stiff glandular hairs. Flowers are greenish white to light or rosy pink, 1/8 inch long with 5 tepals (petals and similar sepals) that barely open and are nearly stalkless.

Leaves and stems:

Leaves are alternate, narrowly or broadly lance-shaped, sometimes with a dark blotch on the upper surface but usually not. Leaves are pointed at the tip, tapered or wedge-shaped at the base, 4 to 6½ inches long and ½ to 1¾ inches wide, the stalk 1/3 to ¾ inch long, becoming stalkless in the upper plant. Leaf edges are toothless but with short cilia like hairs, and often rough when stroked towards base. The surfaces are mostly smooth or with fine 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 typically smooth and without bristles or cilia-like hairs. Stems may be ribbed, are erect to spreading or occasionally sprawling, simple or branching, smooth throughout or with soft appressed hairs or stiff glandular hairs on the upper branches. When sprawling, it may root at the nodes in the lower plant.

Fruit:

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

Notes:

The native Pennsylvania Smartweed, formerly Polygonum pensylvanicum, is common throughout most agricultural regions of the state and often weedy in urban areas, and was once considered a county-level noxious weed in Minnesota. It can grow explosively in seasonal water basins, receding waterlines of lakeshores or any moist disturbed area. While not considered especially beautiful from a horticultural standpoint, the Smartweeds are an important food source for ducks and other birds as well as insects. It is similar to and can be difficult to distinguish from the native Nodding Smartweed (Persicaria lapathifolia) and more so the non-native Lady's Thumb (Persicaria maculosa), both of which share the same general habitats and a number of overlapping characteristics. With the former, P. pensylvanica will have consistently shorter racemes that are mostly erect to slightly nodding while P. lapathifolia will have longer and strongly nodding or dangling flower clusters. P. maculosa also has shorter and more erect racemes than P. lapathifolia but they are typically shorter and more slender than P. pensylvanica, its ocrea consistently has an observable bristly fringe on the upper edge where P. pensylvanica is usually fringeless, and P. maculosa flowers typically have 4 tepals where P. pensylvanica has 5. Also while both P. lapathifolia and P. pensylvanica occasionally have a darker spot on the upper leaf surface, P. maculosa almost always has a distinct dark spot present. While some references put the maximum height of P. pensylvanica at 6 feet, it's more common to see it under 3 feet tall. It exploded in my own back yard after bobcatting a sunny, moist corner and it never got much over 18 inches tall.