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Johnston's knotweed, knotweed, sawatch knotweed

devil's shoestring, erect knotweed, renouée dressée, upright knotweed, wireweed

Habit Herbs. Plants light green or yellowish, heterophyllous.
Stems

erect, green to brownish, simple or branched from base, not wiry, 4–50 cm, glabrous or papillose-scabrid-ulous.

erect to ascending, sparingly branched in distal 1/2, not wiry, 15–75 cm.

Leaves

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

ocrea 4–10 mm, glabrous or papillose-scabridulous, proximal part cylindric, distal part lacerate or disintegrating into a few persistent fibers;

petiole 0.1–2 mm;

blade 1-veined, not pleated, linear or narrowly oblong to oblanceolate, 8–45 × 2–8(–12) mm, margins revolute or flat, smooth or papillose-denticulate, apex acute, mucronate, rarely obtuse.

ocrea 7–12 mm, proximal part cylindric, distal part usually persistent, with strong veins, margins entire or lacerate, silvery, later disintegrating into ± persistent brown fibers;

petiole 1–5 mm;

blade light green or yellowish, elliptic to obovate, 30–60(–80) × (8–)10–25 mm, margins flat, apex obtuse;

stem leaves 1.5–3.5(–4) times longer than branch leaves;

distal leaves overtopping flowers in distal part of inflorescence.

Inflorescences

axillary or axillary and terminal, spikelike, elongate;

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

axillary;

cymes in axils of most leaves and toward tips of stems and branchs, 1–5-flowered.

Pedicels

enclosed in or exserted from ocreae, erect, 1–4 mm.

mostly exserted from ocreae, 3–7 mm.

Flowers

closed or at least some open;

perianth (2.5–)4 mm;

tube 20–40% of perianth length;

tepals overlapping, greenish or reddish, sometimes flushed with purple, with white or pink margins, sepaloid or petaloid, oblong or oblong-elliptic, cucullate, navicular, apex rounded;

midveins unbranched or with few branches at base;

stamens 3–8.

closed;

perianth 2.8–3.8(–4.2) mm;

tube 20–37% of perianth length;

tepals overlapping, green with yellowish, rarely whitish green, margins, sepaloid, not keeled, oblong to obovate, cucullate;

midveins branched, moderately to heavily thickened;

stamens 7–8.

Achenes

enclosed in perianth, black, elliptic or ovate, 2.5–3.3 mm, faces subequal, shiny, smooth.

enclosed in perianth, brown to tan, ovate, 3-gonous, 2.3–3.5 mm, faces subequal, ± concave, apex not beaked, edges concave, dull, striate-tubercled; late-season achenes uncommon, 4–5 mm.

Polygonum sawatchense

Polygonum erectum

Phenology Flowering May–Oct.
Habitat Dry, waste ground
Elevation 10-300 m (0-1000 ft)
Distribution
from FNA
AZ; CA; CO; ID; MT; ND; NE; NM; NV; OR; SD; UT; WA; WY; AB; BC; SK
[WildflowerSearch map]
[BONAP county map]
from FNA
AL; AR; CO; CT; DC; DE; GA; IA; IL; IN; KS; KY; LA; MA; MD; ME; MI; MN; MO; MS; MT; NC; NE; NH; NJ; NM; NV; NY; OH; OK; OR; PA; RI; SC; TN; VA; VT; WA; WI; WV; WY; AB; ON; QC; SK
[WildflowerSearch map]
[BONAP county map]
Discussion

Subspecies 2 (2 in the flora).

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

Polygonum erectum was cultivated in the midwest by Native Americans for its starchy seeds (C. M. Scarry 1993). It was formerly confused with P. achoreum (T. R. Mertens and P. H. Raven 1965).

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

Key
1. Stems and ocreae glabrous; leaf blades linear-lanceolate to narrowly oblong or oblanceolate, margins smooth; flowers closed
subsp. sawatchense
1. Stems and ocreae papillose-scabridulous; leaf blades elliptic or oblong-elliptic, margins papillose-denticulate; at least some flowers open
subsp. oblivium
Source FNA vol. 5, p. 568. FNA vol. 5, p. 550.
Parent taxa Polygonaceae > subfam. Polygonoideae > Polygonum > sect. Duravia Polygonaceae > subfam. Polygonoideae > Polygonum > sect. Polygonum
Sibling taxa
P. achoreum, P. argyrocoleon, P. austiniae, P. aviculare, P. bidwelliae, P. bolanderi, P. californicum, P. cascadense, P. douglasii, 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. shastense, P. spergulariiforme, P. striatulum, P. tenue, P. utahense
P. achoreum, P. argyrocoleon, P. austiniae, P. aviculare, P. bidwelliae, P. bolanderi, P. californicum, P. cascadense, P. douglasii, P. engelmannii, 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
Subordinate taxa
P. sawatchense subsp. oblivium, P. sawatchense subsp. sawatchense
Name authority Small: Bull. Torrey Bot. Club 20: 213, plate 156. (1893) Linnaeus: Sp. Pl. 1: 363. (1753)
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