Polygonum cespitosum or tufted knotweed is a summer annual weed plant of the family Polygonaceae, native to eastern Asia, from China to Japan and Southeast Asia, and introduced into North America. The plant grows to 3.5 feet (105 cm) in height with elliptic to lanceolate leaves, usually 20–75 mm (0.4-30.0 inches) 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 an 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.
- The Plant List, Polygonum posumbu Buch.-Ham. ex D. Don
- Yonekura K (2006) Polygonaceae. In: Iwatsuki K, Boufford DE, Ohba H, editors. Flora of Japan, Vol IIa. Tokyo, Japan: Kodansha Publisher. pp. 122-174.
- Mehrhoff LJ, Silander JAJ, Leicht SA, Mosher ES, Tabak NM (2003) IPANE. Invasive plant atlas of New England. Department of Ecology & Evolutionary Biology, University of Connecticut, Storrs, CT, USA.
- "Tufted Knotweed", Weed ID Guide, Weed Science Program, University of Missouri
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