florida Early Detection Network


Bristled knotweed, Bunchy knotweed, Oriental ladysthumb


Polygonum caespitosum Blume


Knotweed family


Polygonum caespitosum

Close-up of flower


Ocrea close-up



Synonyms: None


Botanical Glossary

Polygonum caespitosum is an herbaceous annual that can reach 1 m (3.2 ft.) in height. The stems can be erect or prostrate, are freely branched and can have a reddish color. The thin, dark green, alternately arranged leaves are lanceolate to elliptic in shape, and measure 2-7.5 cm (0.75-3 in.) long and 1.2-3 cm (0.5-1.25 in.) wide. The leaves are mostly glabrous, but older leaves can sometimes have hairs on the margins and veins on the lower leaf surface. The ocreae (membranous sheaths surrounding stem) have stiff hairs coming from the top which are 5-10 mm (0.2-0.4 in.) long. The small flowers of this plant are dark pink, and are arranged in few to many thin spikes. These spikes measure 2-4 cm (0.75-1.5 in.) in length and 5 mm (0.2 in.) across, which is relatively thin for most terrestrial Polygonum species. The flowers appear from July to October. The trigonous achenes are black, smooth and shiny. They are 2-2.5 mm (0.07-0.1 in.) in size. Page References Gleason & Cronquist 137, Magee & Ahles 442. See reference section below for full citations.


Polygonum pennsylvanicum L. (Pennsylvania smartweed), Polygonum persicaria L. (Lady's thumb)


The seeds of Polygonum caespitosum are dispersed mechanically. There is also some indication (Muhlenbach 1979) that railroads have played a role in its dispersal.


Polygonum caespitosum is native to eastern Asia in China, Japan, Korea, India and Malaysia. In the United States it has been reported from Maine to Florida and west to Texas, New Mexico, Kansas, Nebraska and Minnesota. Polygonum caespitosum has been reported from all the states of New England.


Though the exact date of introduction is unknown, Polygonum caespitosum was likely introduced into the North America in the 1920's. It spread rapidly across the northeastern, southeastern and Midwestern parts of the country. By 1959 it had been reported in Missouri as spreading. In 1976, this plant was reported as "new to Vermont and the Champlain Valley" (Zika et al. 1983). Thus, this plant's introduction and spread appear to be fairly recent.


Abandoned Field, Agricultural Field, Early Successional Forest, Edge, Floodplain Forest, Open Disturbed Area, Pasture, Railroad Right-of-Way, Roadside, Vacant Lot, Wet Meadow, Yard or Garden. Polygonum caespitosum prefers moist soil and can grow in low light situations. It is often found in poor soils and can tolerate a wide range of pH (4.8 to 8).


Polygonum caespitosum is usually found in highly disturbed situations. However, it has been seen in large, monotypic stands and can tolerate extreme shade and pH. Thus, this plant has the potential to invade shaded natural areas and to outcompete other native species that thrive in moist, shaded habitats. It can be found on ridge tops and open woods, usually near trails.


Documentation required: A photograph or mounted snippet of the inflorescence with stem and ocreae. Best time for documentation: Summer, fall


Integrated Taxonomic Information System Taxonomic information

PLANTS Database General information and map

Connecticut Botanical Society Images and description

GRIN database Native distribution information


Fernald, M.L. 1950. Gray's Manual of Botany 8th edition. American Book Company, New York.

Gleason, H.A. 1952. The New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada. Macmillan Publishing Co., Inc. New York

Gleason, H.A. and A.C. Cronquist. 1991. Manual of Vascular Plants of the Northeastern United States and Adjacent Canada. 2nd ed. New York Botanical Garden, Bronx, New York.

Holm, L.G., J.V. Pancho, J.P. Herberger and D.L. Plucknett. 1979. A Geographical Atlas of World Weeds. John Wiley and Sons, New York, USA.

Magee, D.W and H.E. Ahles. 1999. Flora of the Northeast. University of Massachusetts Press, Amherst.

Muhlenbach, V. 1979. Contributions to the Synanthropic (Adventive) Flora of the Railroads in St. Louis, Missouri, U.S.A. Annals of the Missouri Botanical Garden 66(1): 1-108.

Sultan, S.E., A.M. Wilczek, S.D. Hahn and B.J. Brosi. 1998. Contrasting Ecological Breadth of Co-Occuring Polygonum species. Journal of Ecology 86:363-383.

USDA, NRCS. 2001. The PLANTS Database, Version 3.1. (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.

Wheeler, C.T. and S.A. Mengel. 1984. Phytophagous insect fauna of Polygonum perfoliatum, an asiatic weed recently introduced to Pennsylvania (USA). Annals of the Entomological Society of America 77:197-202.

Zika, P.F., R.J. Stern and H.E. Ahles. 1983. Contributions to the Flora of Lake Champlain Valley, New York and Vermont (in Torreya). Bulletin of the Torrey Botanical Club 110(3): 366-369.