Polygonum arenastrum Boreau

Nomenclature and Taproot

Polygonum arenastrum Boreau, commonly called knotweed, develops a deep taproot that can reach 46 cm (18 inches) deep.

Soil and Drought Tolerance

It grows well in compacted soil and survives drought conditions.

General Stem Growth Form

Numerous wiry stems grow outward from the plant's center, giving the plant a mat-like appearance.

Stem Morphology

Stems are very long, branching, and grow in a zigzag pattern.

Leaf Characteristics

Leaves are alternate, oblong, hairless, bluish-green, and measure 12 to 37 mm (1⁄2 to 2+1⁄2 inches) long.

Node Stipules

Each node has a short leaf stalk closely surrounded by papery stipules.

Common Name Origin

Stem nodes are slightly swollen and look somewhat like "knots", which gives the plant its common name of knotweed.

Flower Characteristics

The plant produces inconspicuous, petal-less flowers that grow along the entire length of stems; flower color ranges from white to green, and the sepals are pinkish with white edges.

Seed Characteristics

Seeds are egg-shaped, dark reddish brown, and have a smooth, shiny surface.

Similar Species Lookalike

In the United States, this knotweed can be mistaken for the native plant spotted spurge.

Species Distinction Method

An easy way to distinguish the two is to break off a stem: spotted spurges contain milky sap, while this knotweed does not.

Typical Habitat

Because it grows well in compacted soils, common knotweed is most often found on brown-field sites, along roadsides and pavements, in driveways, in heavily foot-trafficked turf grass, and in gardens.