The genus name "Polygonum" is derived from the Greek for "poly", meaning many, and "gonu", meaning knee -- a reference to the many swollen joints, or nodes, along the stems of the plants (FNA 2005). Douglas' Knotweed is named for David Douglas, a Scottish plant collector of the early 19th century, who explored the northeast, Canada, California, and Hawaii for the Horticultural Society of London (Charters 2008).
There are only 5 verified occurrences in the state, 3 of which are ranked "good" with more than 100 individuals. There are 17 historical records, most of which have never been checked.
The few verified occurrences of this species appear to have stable populations.
The long-term trends for this species are unclear; there are many historical records which need to be checked.
No immediate threats are apparent at the known sites for this species. The thin soil and in some cases steep slopes at the sites could make the plants vulnerable to erosion.
In New York this plant is known from dry, often calcareous sites with shallow soils over bedrock, including soil-bearing cracks in exposed ledges or bedrock outcrops (New York Natural Heritage Program 2008). Sandy soil of open places, dry rock outcrops (Voss 1985). Rocky or gravelly slopes and open areas (Haines 1998).
There are historical records widely scattered in northern and eastern New York from Jefferson, and St. Lawrence Counties in the north, and a few in Ulster and Westchester Counties. The only verified occurrences are from Clinton, Essex, Warren, and Washington counties in the northeast.
Polygonum douglasii is found in Canada and in most of the western and northern plains states, and east to Michigan, New York, and northern New England, and slightly disjunct to Maryland.
Polygonum douglasii is a slender annual herb growing from 20 to 60 cm tall, with numerous ascending branches. A characteristic of all Polygonum species is that the leaves have stipules thattheir stipules are united to form a sheath around the stem (called an "ocrea") at each node. In this species the ocrea is 6 to 12 mm long, and has a jagged top edge. The leaves are linear to lance-shaped, 2 to 5 cm long and only 2 to 8 mm wide, and have an awl-shaped tip. The flowers have white petals 3 to 4 mm long and cleft nearly to their bases, and are in groups of 1 to 3 per ochrea. The flowers and subsequently the fruit (shiny black achenes 1 to 3mm long) become strongly reflexed or bent back towards the ground (Gleason and Cronquist 1991).
This species is best identified when flowering or fruiting.
Polygonum tenue has shorter (1 to 3 cm) leaves which are folded from the midvein, and only 1 flower per ochrea.
The flowers appear in June or July, and the fruits may persist through October.
The time of year you would expect to find Douglas' Knotweed flowering and fruiting in New York.
Polygonum douglasii Greene
We follow the treatment within Flora of North America where all former subspecies of Polygonum douglasii have been raised to full species.
Charters, Michael L. 2008. California Plant Names: Latin and Greek Meanings and Derivations. Published on the internet http://www.calflora.net/botanicalnames/index.html.
Flora of North America Editorial Committee. 2005. Flora of North America North of Mexico. Vol. 5. Magnoliophyta: Caryophyllidae: Caryophyllales, Polygonales, and Plumbaginales. Oxford Univ. Press, New York. vii + 656 pp.
Gleason, Henry A. and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York. 910 pp.
Holmgren, Noel. 1998. The Illustrated Companion to Gleason and Cronquist's Manual. Illustrations of the Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York.
House, Homer D. 1924. Annotated list of the ferns and flowering plants of New York State. New York State Museum Bulletin 254:1-758.
Mitchell, Richard S. and J. Kenneth Dean. 1978. Polygonaceae (buckwheat family) of New York State. Contributions to a flora of New York State. Richard S. Mitchell, ed. New York State Museum Bulletin No. 431. 79 pp.
New York Natural Heritage Program. 2020. New York Natural Heritage Program Databases. Albany, NY.
Smith, Gerald A. No date. Bird breeding season survey at El Dorado Beach Preserve 1981-.
Weldy, T. and D. Werier. 2010. New York flora atlas. [S.M. Landry, K.N. Campbell, and L.D. Mabe (original application development), Florida Center for Community Design and Research http://www.fccdr.usf.edu/. University of South Florida http://www.usf.edu/]. New York Flora Association http://newyork.plantatlas.usf.edu/, Albany, New York
Zaremba, Robert E. 1991. Corrections to phenology list of April 9, 1991.
Information for this guide was last updated on: May 22, 2008
Please cite this page as:
New York Natural Heritage Program. 2020. Online Conservation Guide for Polygonum douglasii. Available from: https://guides.nynhp.org/douglas-knotweed/. Accessed December 3, 2020.