Swamp Smartweed (Polygonum amphibium)


Buckwheat Family (Polygonaceae)

Other Names:

Polygonum coccineum, swamp knotweed, tanweed, devil's shoestring, tansy mustard.

Origin and Distribution:

A native of North America, swamp smartweed is a highly variable perennial. It exhibits two forms, terrestrial and aquatic. For this reason, the plant will invade shores, wet prairies, swamps, ponds, ditches, and quiet streams. However, it is quite common for plants to grow in drier soil. Today, the species can be found throughout much of North America.

Plant Description:

Swamp smartweed reproduces using rhizomes (rootstocks) and seeds. Plants normally grow from 2' to 3' tall. The stems are usually unbranched and thicken to form nodes at the leaf joints. This trait is partially responsible for the common name of knotweed. Swamp smartweed may also exhibit a red-striped stem. The leaves of the plants are alternate, oblong, and tapering at both ends. They are usually pointed at the tip and exhibit smooth margins. The leaves of land plants grow up to 8" long, while those of aquatic plants are somewhat smaller (up to 6). The leaves of the land plants are generally more oval than those of aquatic plants, which exhibits floating, arrow-shaped leaves. The leaf stalks of both plant forms encircle the stem to form nodes. The plants display extensive root systems. They are known to grow roots at the nodes of the stem. Swamp smartweeds exhibit tall, slender, and erect flower clusters. The spike-like flower clusters grow from 1.5cm to 7cm tall normally much taller than 1. The flowers may be red, pink, or white (very rarely) in color. The species produces seed like fruit, which may be dark brown or black in color.

Similar Species:

Swamp smartweed is frequently confused with Polygonum amphibium (or water smartweed). However, swamp smartweed may be distinguished by its flower clusters, which are taller and more slender than those of water smartweed. In addition, water smartweed bears leaf-like fringes at the top of the nodes of the stem. Swamp smartweed lacks this trait.


Swamp smartweed blooms from July to September. The spreading root system allows for competition with other plants while making it difficult to kill the plants. For this reason, cultivation, hoeing, and harrowing may be used to control the plants. In addition, frequent mowing of fields and pastures can be used in conjunction with improving soil drainage.



Facts and Folklore:

  • Swamp smartweed has several close relatives one of which is Polygonum hydropiper, or water-pepper smartweed. In the past, extracts of water-pepper smartweed were applied to ulcers and hemorrhoids. The uses gave rise to two of its more earthy folk names, arse-smart and smartass.