Prostrate knotweed

Prostrate knotweed (Polygonum aviculare) is a low, wiry, mat-forming summer annual that radiates outward from a central taproot in a flat, tangled, often circular patch hugging the ground. Stems are slender, tough, and much-branched, with small bluish-green, oblong to lance-shaped leaves arranged alternately along them. Like its smartweed relatives it has a thin papery ocrea (membranous sheath) at each swollen node, which is a key diagnostic feature. Tiny, inconspicuous greenish-white to pinkish flowers are tucked in the leaf axils. Knotweed is frequently mistaken for early spreading spurge or for prostrate grasses, but it lacks the milky sap of spurge and has the wiry jointed stems and ocrea of the buckwheat family; it is one of the earliest summer annuals to germinate.

In turf, knotweed forms flat, dense, wiry mats in worn and compacted areas, smothering or excluding thin grass and leaving an unsightly, weedy patch that signals an underlying compaction problem. Its tough prostrate stems tolerate traffic and close mowing, so it persists in exactly the high-wear zones where turf is weakest, accentuating bare, hardened areas and reducing surface quality and uniformity.

Prostrate knotweed is a summer annual reproducing only by seed. It is among the first summer annuals to germinate, often emerging in very early spring while soils are still cool, sometimes before crabgrass. Plants spread into low mats through the growing season, flower and produce abundant seed in late summer, and die at frost; the seed overwinters and is long-lived in the soil.

Knotweed is a strong indicator of soil compaction and is classic on heavily trafficked, compacted, and poorly aerated sites, often appearing first along walkways, gateways, sidelines, paths, and high-wear areas where turf has thinned. It also tolerates drought and low fertility. Compacted, worn, open soil that grass cannot reoccupy provides the niche where its very early-germinating seedlings establish and dominate.