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Cascade knotweed

seabeach knotweed, seaside knotweed

Habit Herbs. Plants silvery, homophyllous.
Stems

spreading to erect, zigzagged, green, simple or branched from base, wiry, 5–12(–15) cm, glabrous.

prostrate to ascending, branched from base, not wiry, 20–70 cm.

Leaves

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

ocrea 2–5 mm, glabrous, proximal part funnelform, distal part lacerate;

petiole essentially absent;

blade 1-veined, not pleated, oblanceolate to obovate, 5–20 × 2–5 mm, margins revolute, never touching along midrib, sparsely papillose-denticulate, apex rounded or apiculate.

ocrea persistent, 7–15 mm, proximal part cylindric, pruinose, distal parts silvery, margins overlapping, entire or lacerate;

petiole 0.5–3 mm;

blade bluish green, lanceolate, 10–30 × 2–8 mm, coriaceous, margins revolute, apex obtuse or acute, rugulose when fresh, markedly rugulose when dried, glaucous;

middle stem leaves slightly larger than adjacent branch leaves, distal leaves overtopping flowers they subtend.

Inflorescences

axillary and terminal, spikelike, dense;

cymes congested at tips of stems and branches, 3–5-flowered.

axillary;

cymes uniformly distributed, 1–3-flowered.

Pedicels

enclosed in ocreae, erect to spreading, 2–3 mm.

enclosed in ocreae, 3–5 mm.

Flowers

open;

perianth 2–2.5 mm;

tube 12–25% of perianth length;

tepals overlapping, uniformly white, petaloid, oblong to obovate, cucullate, navicular in distal 1/4, apex rounded;

midveins unbranched;

stamens 8.

semi-open;

perianth (2–)3–4 mm;

tube 12–26% of perianth length;

tepals ± recurved, overlapping, white, margins white or pink, petaloid, not keeled, oblong-obovate to spatulate, not cucullate;

midvein usually unbranched;

stamens 8.

Achenes

enclosed in or exserted from perianth, black, ovate to ovate-oblong, 1.8–2.1 mm, faces subequal, shiny, smooth.

exserted from perianth, reddish brown to dark brown, ovate, 3-gonous, 2.5–3(–4) mm, faces subequal, apex not beaked, edges straight, shiny, smooth; late-season achenes common, 3–5 mm.

2n

= 40.

Polygonum cascadense

Polygonum glaucum

Phenology Flowering Jun–Sep. Flowering Jul–Nov.
Habitat Dry, usually rocky slopes, often on serpentine Coastal beaches, sand dunes, margins of salt ponds
Elevation 1600-1800 m (5200-5900 ft) 0-10 m (0-0 ft)
Distribution
from FNA
OR
[WildflowerSearch map]
[BONAP county map]
from FNA
CT; DC; DE; FL; GA; MA; MD; NC; NJ; NY; RI; SC; VA
[BONAP county map]
Discussion

Polygonum glaucum is restricted to maritime beaches along the Atlantic coast from Massachusetts south to Georgia. Over most of its range it is rare and declining; populations on coastal islands of Massachusetts and along the shore of Long Island appear to be secure. Seabeach knotweed appears to be related to P. oxyspermum.

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

Source FNA vol. 5, p. 571. FNA vol. 5, p. 553.
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. 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. sawatchense, 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. erectum, P. fowleri, 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
Name authority W. H. Baker: Madroño 10: 62, plate 1, fig. 1. (1949) Nuttall: Gen. N. Amer. Pl. 1: 254. (1818)
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