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Douglas' knotweed, renouée de Douglas

Habit Herbs.
Stems

erect, green, simple or branched, not wiry, 5–80 cm, glabrous or sparsely papillose-scabridulous.

Leaves

uniformly distributed, articulated to ocreae, basal leaves caducous, distal leaves abruptly reduced to bracts;

ocrea 6–12 mm, glabrous or minutely papillose-scabridulous, proximal part cylindric, distal part hyaline, lacerate;

petiole 0.1–2 mm;

blade 1-veined, not pleated, linear, narrow-oblong, or oblanceolate, 15–55 × 2–8(–12) mm, margins revolute, smooth or papillose-denticulate;

apex acute to mucronate.

Inflorescences

axillary and terminal, spikelike, elongate;

cymes widely spaced along branches, 2–4-flowered.

Pedicels

mostly exserted from ocreae, reflexed, 2–6 mm.

Flowers

closed;

perianth 3–4.5 mm;

tube 20–28% of perianth length;

tepals overlapping, green to tannish with white or pink margins, petaloid, oblong, cucullate, navicular, apex rounded;

midveins usually branched, rarely unbranched;

stamens 8.

Achenes

enclosed in perianth, black, elliptic or oblong to ovate, 3–4(–4.5) mm, faces subequal, shiny or dull, smooth or minutely striate-tubercled.

Polygonum douglasii

Phenology Flowering Jun–Oct.
Habitat Dry, often disturbed places, rock outcrops, sandy ground
Elevation 300-3000 m (1000-9800 ft)
Distribution
from FNA
AZ; CA; CO; IA; ID; MI; MN; MT; NE; NH; NM; NV; NY; OR; SD; UT; VA; VT; WA; WY; AB; BC; MB; ON; QC; SK; YT
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[BONAP county map]
Discussion

Five taxa that have been included in Polygonum douglasii (E. Murray 1982; J. C. Hickman 1984; J. T. Kartesz and K. N. Gandhi 1990) are treated here as distinct species: P. austiniae, P. majus, P. nuttallii, P. sawatchense, and P. spergulariiforme. Hickman noted extensive intergradation and numerous intermediate specimens among those sympatric elements, but qualitative or quantitative characters allow reliable discrimination in most cases (M. Costea and F. J. Tardif 2005), and species are here circumscribed similar to C. L. Hitchcock (1964).

Greene described var. latifolium as having leaf blades and achenes broader than those of var. douglasii. C. L. Hitchcock (1964) recognized the former, but the characters used to distinguish it appear to vary continuously, and reliable separation is not possible.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 5, p. 567.
Parent taxa Polygonaceae > subfam. Polygonoideae > Polygonum > sect. Duravia
Sibling taxa
P. achoreum, P. argyrocoleon, P. austiniae, P. aviculare, P. bidwelliae, P. bolanderi, P. californicum, P. cascadense, P. engelmannii, P. erectum, P. fowleri, P. glaucum, P. heterosepalum, P. hickmanii, P. humifusum, P. majus, P. marinense, P. minimum, P. nuttallii, P. oxyspermum, P. paronychia, P. parryi, P. patulum, P. plebeium, P. polygaloides, P. ramosissimum, P. sawatchense, P. shastense, P. spergulariiforme, P. striatulum, P. tenue, P. utahense
Synonyms P. douglasii var. latifolium, P. emaciatum, P. montanum, P. tenue var. commune, P. tenue var. latifolium
Name authority Greene: Bull. Calif. Acad. Sci. 1: 125. (1885)
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