Flower:

Spike clusters 2 to 4-inches long; individual flowers are 1/8-inch across, deep pink with 5 petal-like sepals and long stamens. Each plant has 1 or 2 spikes.

Leaves and stem:

Leaves are up to 8 inches long and 3 inches wide with smooth but wavy edges tapering to a point at the tip, and may have fine hairs. A brown paper-like sheath is attached to the stem at the base of each leaf (not a great picture of this, but at least you should see the color difference). Leaves are alternately attached up the stem. Plants may be erect or sprawling, branched or not, sometimes finely hairy.

Notes:

Swamp Smartweed is a variable species, with both terrestrial and aquatic versions, adapting as needed to variations in water level. The leaves of the aquatic form usually aren't as wavy. In most field guides it's called Polygonum amphibium, but the accepted name in Minnesota is Persicaria amphibia. While there are a number of plants in this family that have pink(ish) flower spikes, Swamp Smartweed has the most shocking pink flowers of the lot and is the most easily recognizable species in this group.