| 1. 3 outer perianth lobes ± keeled to widely winged, especially in fruit uncommon aliens (subg. Fallopia) |
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| 2. Leaf generally 310 cm, ovate-sagittate; vine (sect. Fallopia) |
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| 3. Woody perennial herb, perianth bright pink |
| P. baldschuanicum |
| 3' Non-woody annual, perianth greenish white |
| P. convolvulus |
| 2' Leaf generally 1030 cm, widely ovate to round; stout per from rhizome; (sect. Reynoutria) |
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| 4. Stem generally 12 m; leaf blade 1015 cm, ± round, tip abruptly pointed; perianth ± white |
| P. cuspidatum |
| 4' Stem generally 23 m; leaf blade 1530 cm, widely ovate, acuminate; perianth ± green |
| P. sachalinense |
| 1' 3 outer perianth lobes not keeled or winged, even in fruit |
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| 5. Leaf generally 535 cm, petioled (except sometimes P. davisiae), blade generally lanceolate or wider; if annual, stem generally 50200 cm; filaments thread-like (subg. Persicaria) |
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| 6. Stem unbranched, erect; generally subalpine; leaves ± basal, blade generally oblong, strap-shaped; perianth lobes nearly free (sect. Bistorta) |
| P. bistortoides |
| 6' Stem branched, generally ± sprawling; lower elevations; leaves cauline, blade rarely oblong; perianth lobes fused to nearly free |
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| 7. Leaf widely lanceolate to round |
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| 8. Inflorescence head-like, ± spheric, dense; perianth fused ± 1/3 its length (sect. Cephalophilon) |
| P. capitatum |
| 8' Inflorescence a panicle or appearing as a small, dense cluster; perianth fused << 1/3 its length (sect. Aconogonon) |
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| 9. Inflorescence an open, ± leafless panicle, flowers > 50; stem erect, > 1 m, stout; some leaves > 10 cm |
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| 10. Montane native; stem from thick caudex; leaf blade base round to tapered, basal lobes 0 |
| P. phytolaccifolium |
| 10' North coastal alien; stem from extensive rhizome; leaf blade base tapered, often with 2 small, round lobes |
| P. polystachyum |
| 9' Inflorescence a small axillary cluster, flowers 225; stem prostrate to decumbent, < 1 m; leaves < 8 cm |
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| 11. Petioles < 1 cm; leaf blade generally lanceolate, surfaces ± glabrous to scabrous, yellow-green, margin scabrous, tip acute; flowers 25 per axil |
| P. davisiae |
| 11' Some petioles > 1 cm; leaf blade generally ovate to round, surfaces ± glabrous to velvety, dark green becoming reddish, tip generally ± round, margin glabrous to soft-hairy; flowers 525 per axil |
| P. newberryi |
| 7' Leaf ± narrowly lanceolate (except in P. amphibium) inflorescence spike-like; perianth fused ± 1/3 its length (sect. Persicaria) |
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| 12. Perianth gland-dotted; inflorescence ± open, slender; fruit 23-angled |
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| 13. Perianth margins generally ± red; fruit dark brown, dull, bitter-tasting; annual |
| P. hydropiper |
| 13' Perianth margins generally white; fruit black, shiny, not bitter-tasting; perennial herb from rhizome |
| P. punctatum |
| 12' Perianth not gland-dotted; inflorescence open or dense; fruit generally 2-angled or ± flat |
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| 14. Perianth greenish to light pink; inflorescence spikes 26 mm wide |
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| 15. Perianth generally light pink, opening, veins at fruiting inconspicuous; fruit 3-angled; perennial herb from extensive rhizome |
| P. hydropiperoides |
| 15' Perianth generally greenish, remaining closed, veins at fruiting prominent, forked, recurved; fruit flat, generally indented on both sides; annual |
| P. lapathifolium |
| 14' Perianth deep pink; inflorescence spikes 720 mm wide |
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| 16. Annual; ± terrestrial; stem ± erect (lower nodes sometimes rooting); leaf narrowly lanceolate |
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| 17. Stipule (including margin) glabrous; peduncle glandular; fruit ± 3.5 mm, round, nearly flat to indented |
| P. pensylvanicum |
| 17' Stipule strigose, margin bristly; peduncle glabrous; fruit ± 2.5 mm, widely ovate, often 3-angled |
| P. persicaria |
| 16' Perennial from rhizome; aquatic or on shorelines; stem generally floating or decumbent; leaf widely lanceolate to elliptic |
| P. amphibium |
| 18. Stem terrestrial or emergent in flower; leaf blade lanceolate, tip ± acuminate; inflorescence > 4 cm |
| var. emersum |
| 18' Stem floating in flower; leaf blade oblong, tip round to acute; inflorescence < 3.5 cm |
| var. stipulaceum |
| 5' Leaf generally 15 cm, ± sessile, generally linear to lanceolate (to round in P. minimum); annual, subshrub, or shrub, stem generally 335 cm; filaments much wider at base (subg. Polygonum) |
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| 20. Stem ± erect (gnarled at base), flower branches erect, whip-like; foothills or low mtns |
| P. bolanderi |
| 20' Stem prostrate to ascending throughout; coast or high mtns |
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| 21. Subshrub; stipule ± 20 mm, persistent; coastal dunes |
| P. paronychia |
| 21' Shrub, gnarled; stipule ± 5 mm, ± falling with leaf; high mtns |
| P. shastense |
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| 22. Stem ± round, obviously and ± regularly 816 ribbed (if stem ± angled, ribs obvious between angles); generally alien (sect. Polygonum) |
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| 23. Flower clusters scattered, some or all in axils of full-sized leaves; stem prostrate to erect |
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| 24. Fruit 3.54.5 mm, olive-brown, shiny uncommon, San Francisco Bay Area salt marshes, possibly native |
| 2 P. marinense |
| 24' Fruit 23 mm (late-season fruit sometimes larger), dark brown, generally ± dull |
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| 25. Perianth ± 2.5 mm, exserted from stipule; abundant |
| P. arenastrum |
| 25' Perianth ± 2 mm, ± enclosed in stipule; uncommon |
| P. prolificum |
| 23' Flower clusters ± restricted to branch tips, in axils of bracts; stem erect |
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| 26. All but lowest bracts < flowers or falling early, so inflorescence appears raceme-like and restricted to stem tips; perianth margin pink; fruit < 2 mm, brown, very shiny |
| P. argyrocoleon |
| 26' Most bracts > flowers, so inflorescence clusters appear axillary and less restricted to stem tips; perianth green or yellowish (margin rarely pink); fruit > 2 mm, not very shiny |
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| 27. Perianth ± green, ± included in stipule; fruit 22.5 mm, brown, ± shiny |
| P. patulum |
| 27' Perianth generally dark yellowish (especially margin), exserted from stipule; fruit ± 3 mm, dark brown, ± dull |
| P. ramosissimum |
| 22' Stem ± sharply angled (especially just below nodes), ribs between angles obscure or 0; native |
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| 28. Leaves firm, ± spiny, uniformly linear, joint with stipule obscure; styles free, bases hard, persistent in fruit (sect. Duravia, in part) |
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| 29. Plant dense, often cushion-like, 17 cm; stipule deeply fringed, segments generally curled |
| P. parryi |
| 29' Plant open, 240 cm; stipule bristly or shallowly fringed, segments not curled |
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| 30. Stipule prominent, silvery, widely lanceolate, ± obscuring leaves, margin fringed |
| P. bidwelliae |
| 30' Stipule not at all obscuring leaves, silvery or golden, lanceolate, margin bristly |
| P. californicum |
| 28' Leaves soft, not spiny, linear to round (generally lanceolate), joint with stipule obvious; styles fused, falling in fruit as a unit (sect. Monticola) |
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| 31. Fruit 1.52.5 mm, light brown to nearly black, dull to somewhat shiny (the darkest dull) [subspp. intergrade] |
| P. polygaloides |
| 32. Bract margin white; fruit brown to nearly black, generally dull (lanceolate if shiny) |
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| 33. Inflorescence ± 79 mm wide; bracts ascending to spreading, shorter, rounder upward, white margin wide, generally reflexed; fruit < 2 mm, ovate in outline, nearly black, dull; widespread |
| subsp. confertiflorum |
| 33' Inflorescence ± 46 mm wide; bracts ± appressed, uniformly lance-elliptic, white margin narrow, generally not reflexed; fruit 22.5 mm, lanceolate in outline, brown, generally ± shiny; RARE, Modoc Plateau |
| subsp. esotericum |
| 32' Bract margin green; fruit brown, generally ± shiny |
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| 34. Bracts lance-elliptic, stiffly appressed; fruit 22.5 mm, lanceolate in outline, brown, shiny to ± dull; RARE, Modoc Plateau |
| subsp. esotericum |
| 34' Bracts linear-lanceolate, soft, spreading; fruit ± 1.5 mm, ± widely ovate in outline, light brown, ± shiny; widespread |
| subsp. kelloggii |
| 31' Fruit 25 mm, shiny, black or olive brown |
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| 35. Fruit olive-brown; salt marshes, San Francisco Bay Area; uncommon |
| 2 P. marinense |
| 35' Fruit black; widely distributed; ± common |
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| 36. Leaves elliptic to round, persistent, regularly and widely spaced below, barely smaller upward, crowded at stem tips; stem reddish |
| P. minimum |
| 36' Lowest leaves linear to oblanceolate or narrowly elliptic, generally falling early, reduced, more widely spaced upward; stem ± green [subspp. intergrade extensively] |
| P. douglasii |
| 37. Perianth 35 mm, opening partly or widely; inflorescence open or dense; flowers and fruit generally spreading |
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| 38. Flower clusters scattered; perianth white, opening widely |
| subsp. majus |
| 38' Flower clusters crowded at stem tips; perianth ± pink, opening partly |
| subsp. spergulariiforme |
| 37' Perianth generally 23 mm, generally remaining closed; inflorescence ± open; flowers and fruit erect or becoming reflexed |
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| 39. Fruit erect, 23 mm; enfolding perianth not stalked at base; leaves uniformly lance-elliptic, gradually smaller upward |
| subsp. johnstonii |
| 39' Fruit reflexed, 24 mm; enfolding perianth generally with minute, curved stalk at base; leaves linear to elliptic, abruptly smaller upward |
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| 40. Fruit ± 2 mm; plant funnel-shaped from ascending basal branches; lower leaves ± elliptic, persistent, appearing crowded |
| subsp. austiniae |
| 40' Fruit 34 mm; plant ± open, not funnel-shaped; lower leaves linear to oblanceolate, falling early, ± widely spaced |
| subsp. douglasii
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