Polygonum douglasii
Douglas' knotweed
(c) Steve Matson, some rights reserved (CC BY), uploaded by Steve Matson · licence
Family: Polygonaceae · Type: Herb · Native
Polygonum douglasii (Douglas' knotweed) is a native California herb in the Polygonaceae family, recognized by large stature, large black fruits, and reflexed exserted pedicels.
Douglas' knotweed is a California native annual found especially in mountain regions on dry, rocky outcrops, sandy ground, and disturbed places at elevations below 3,500 meters. Flowering from June to October, this plant produces small green to tan flowers with white or pink margins, clustered in spike-like inflorescences. Growing with erect stems 5 to 80 centimeters tall, the plant has angular green stems that are glabrous or sparsely scabrous. Its lance-linear to oblanceolate leaves measure 15 to 55 millimeters long, with margins rolled under and occasionally fine-toothed, and basal leaves that quickly drop off. The fruit is a small, shiny to dull black elliptic seed 3 to 4 millimeters long.
Identification and appearance
Key characters: 30 sympatric species; large stature, large black fruits, and reflexed exserted pedicels
Habitat and range
Habitat: Dry, rocky outcrops, sandy ground, disturbed places
Elevation: < 3500 m
California counties: Mono, Siskiyou, Calaveras, Fresno, Tehama, Mariposa, San Bernardino, Plumas, Trinity, Tulare, Alpine, Shasta, Placer, Lassen, San Diego, Modoc, Sierra, Del Norte, El Dorado, Butte, Humboldt, Tuolumne, Nevada, Lake, Inyo, Los Angeles, Amador, Riverside, Yuba, Glenn, Kern, Madera, Mendocino, Sonoma, Merced, Ventura
Outside California: to Canada, eastern North America
Bloom period
Polygonum douglasii blooms Jun-Oct.
How to tell it apart
vs Euphorbia: Polygonum has swollen stem joints with papery sheaths; Euphorbia has milky sap when broken.; vs Persicaria: Hold node sheaths to light—Polygonum's are translucent and split; Persicaria's are opaque and intact.; vs Eriogonum: Polygonum has papery sheaths at nodes; Eriogonum has flowers in cup-like involucres.; vs Acmispon: Polygonum has swollen stem nodes; Acmispon has compound leaves with 3-5 leaflets.; vs Atriplex: Polygonum has papery stem sheaths; Atriplex has mealy or scurfy coating on leaves.
Frequently asked questions
Is Polygonum douglasii native to California?
Yes. Polygonum douglasii is native to California (recorded as native).
When does Polygonum douglasii bloom?
Polygonum douglasii blooms Jun-Oct.
Where in California does Polygonum douglasii grow?
Polygonum douglasii occurs in Mono, Siskiyou, Calaveras, Fresno, Tehama, Mariposa, San Bernardino, Plumas and other counties, at elevations of < 3500 m.
What plant family does Polygonum douglasii belong to?
Polygonum douglasii is in the Polygonaceae family, genus Polygonum.
What habitat does Polygonum douglasii grow in?
Polygonum douglasii grows in Dry, rocky outcrops, sandy ground, disturbed places.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · Index of California Plant Names · CalPhotos
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.