Polygonum caespitosum or Tufted Knotweed is a summer annual weed

Weed

A weed in a general sense is a plant that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More specifically, the term is often used to...

 plant of the family Polygonaceae

Polygonaceae

Polygonaceae is a family of flowering plants known informally as the "knotweed family" or "smartweed family"— "buckwheat family" in the United States. The name is based on the genus Polygonum and was first used by Antoine Laurent de Jussieu in 1789 in his book, Genera Plantarum. The name refers...

, native to eastern and central North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

. The plant grows to 3.5 feet in height with elliptic to lanceolate leaves, usually 20-75 mm long. It has small pink or red flowers arranged in tight terminal spikes.

Tufted Knotweed is similar to other polygonum species, particularly Polygonum persicaria (Lady's thumb). Around the stem of both these species there is a papery sheath known as a ocrea with stiff spine-like hairs at the top, but in P. caespitosum these hairs are much longer, as long as the visible portion of the ocrea, whereas in P. persicaria they are much shorter.