Douglas' Knotweed

Polygonum douglasii Greene

Polygonum douglasii
Stephen M. Young

Dicotyledoneae (Dicots)
Polygonaceae (Buckwheat Family)
State Protection
Listed as Threatened by New York State: likely to become Endangered in the foreseeable future. For animals, taking, importation, transportation, or possession is prohibited, except under license or permit. For plants, removal or damage without the consent of the landowner is prohibited.
Federal Protection
Not Listed
State Conservation Status Rank
Critically Imperiled or Imperiled in New York - Especially or very vulnerable to disappearing from New York due to rarity or other factors; typically 20 or fewer populations or locations in New York, very few individuals, very restricted range, few remaining acres (or miles of stream), and/or steep declines. More information is needed to assign either S1 or S2.
Global Conservation Status Rank
Secure globally - Common in the world; widespread and abundant (but may be rare in some parts of its range).


Did you know?

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).

State Ranking Justification

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.

Short-term Trends

The few verified occurrences of this species appear to have stable populations.

Long-term Trends

The long-term trends for this species are unclear; there are many historical records which need to be checked.

Conservation and Management


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).

Associated Ecological Communities

  • Appalachian oak-hickory forest (guide)
    A hardwood forest that occurs on well-drained sites, usually on ridgetops, upper slopes, or south- and west-facing slopes. The soils are usually loams or sandy loams. This is a broadly defined forest community with several regional and edaphic variants. The dominant trees include red oak, white oak, and/or black oak. Mixed with the oaks, usually at lower densities, are pignut, shagbark, and/or sweet pignut hickory.
  • Calcareous shoreline outcrop (guide)
    A community that occurs along the shores of lakes and streams on outcrops of calcareous rocks such as limestone and dolomite. The vegetation is sparse; most plants are rooted in rock crevices.
  • Calcareous talus slope woodland* (guide)
    An open or closed canopy community that occurs on talus slopes composed of calcareous bedrock such as limestone or dolomite. The soils are usually moist and loamy; there may be numerous rock outcrops.
  • Northern white cedar rocky summit* (guide)
    A community that occurs on cool, dry, rocky ridgetops and summits where the bedrock is calcareous (such as limestone or dolomite), and the soils are more or less calcareous. The vegetation may be sparse or patchy, with numerous rock outcrops. The species have predominantly boreal distributions.
  • Red cedar rocky summit* (guide)
    A community that occurs on warm, dry, rocky ridgetops and summits where the bedrock is calcareous (such as limestone or dolomite, but also marble, amphibolite, and calcsilicate rock), and the soils are more or less calcareous. The vegetation may be sparse or patchy, with numerous lichen covered rock outcrops.

* probable association but not confirmed.

Associated Species

  • Agrostis scabra (northern tickle grass)
  • Ambrosia artemisiifolia (common ragweed)
  • Boechera missouriensis
  • Cardamine douglassii (purple spring cress, pink spring cress)
  • Cardamine parviflora (small-flowered bitter cress)
  • Carex brevior (short-beaked sedge)
  • Carex merritt-fernaldii (Fernald's sedge)
  • Carex pensylvanica (Pennsylvania sedge)
  • Carex siccata (dry-spiked sedge)
  • cladina rangiferina
  • Comandra umbellata
  • Corydalis sempervirens
  • Danthonia spicata (poverty grass)
  • Deschampsia flexuosa
  • dicanthelium linearifolium
  • Dryopteris marginalis (marginal wood fern)
  • Elymus trachycaulus
  • Fallopia scandens (climbing false buckwheat)
  • Geranium carolinianum (Carolina crane's-bill)
  • Hedwigia ciliata
  • Helianthus decapetalus (thin-leaved sunflower)
  • Helianthus divaricatus (woodland sunflower)
  • Hypericum perforatum
  • Juncus secundus (secund rush)
  • Juniperus communis
  • Juniperus virginiana
  • Lactuca canadensis (tall lettuce)
  • Lechea intermedia (large-podded pinweed)
  • Lespedeza intermedia
  • Maianthemum racemosum
  • Minuartia michauxii
  • Myosotis verna (spring forget-me-not)
  • Poa compressa (flat-stemmed blue grass, Canada blue grass)
  • Polytrichum juniperinum
  • Potentilla argentea (silvery cinquefoil)
  • Potentilla arguta
  • Quercus rubra (northern red oak)
  • Rhus aromatica
  • Ribes cynosbati (prickly gooseberry, dogberry)
  • Rosa canina (dog rose)
  • Saxifraga virginiensis
  • Schizachyrium scoparium
  • Selaginella rupestris (rock spikemoss)
  • Silene noctiflora (night-flowering campion)
  • solidago bicolor
  • Solidago caesia
  • Solidago juncea (early goldenrod)
  • Sorghastrum nutans (Indian grass)
  • Triodanis perfoliata (common Venus's looking-glass)
  • Verbascum thapsus (common mullein)
  • Veronica arvensis (corn speedwell)
  • Woodsia ilvensis (rusty woodsia, rusty cliff fern)
  • xanthoparmelia sp
  • Zanthoxylum americanum (prickly-ash)


New York State Distribution

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.

Global Distribution

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.

Identification Comments

General Description

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).

Best Life Stage for Proper Identification

This species is best identified when flowering or fruiting.

Similar Species

Polygonum tenue has shorter (1 to 3 cm) leaves which are folded from the midvein, and only 1 flower per ochrea.

Best Time to See

The flowers appear in June or July, and the fruits may persist through October.

  • Flowering
  • Fruiting

The time of year you would expect to find Douglas' Knotweed flowering and fruiting in New York.

Douglas' Knotweed Images


Douglas' Knotweed
Polygonum douglasii Greene

  • Kingdom Plantae
    • Phylum Anthophyta
      • Class Dicotyledoneae (Dicots)
        • Order Polygonales
          • Family Polygonaceae (Buckwheat Family)

Comments on the Classification

We follow the treatment within Flora of North America where all former subspecies of Polygonum douglasii have been raised to full species.

Additional Resources


Charters, Michael L. 2008. California Plant Names: Latin and Greek Meanings and Derivations. Published on the internet

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. 2024. 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 University of South Florida]. New York Flora Association, Albany, New York

Zaremba, Robert E. 1991. Corrections to phenology list of April 9, 1991.


About This Guide

Information for this guide was last updated on: May 22, 2008

Please cite this page as:
New York Natural Heritage Program. 2024. Online Conservation Guide for Polygonum douglasii. Available from: Accessed June 23, 2024.