1 Stem, petioles, and lower surface of major leaf veins with abundant recurved prickles; [section Echinocaulon].

..2 Ocreae foliaceous, green, orbicular, perfoliate; tepals becoming fleshy and blue in fruit

..2 Ocreae scarious, not as above; tepals not becoming fleshy or blue in fruit.

....3 Leaf blades triangular in outline, the larger 6-11 cm wide; perianth 4-parted

....3 Leaf blades lanceolate to narrowly elliptic, the larger 0.8-3 cm wide; perianth 5-parted.

......4 Inflorescence branches glandular-pubescent; stamens 5, in 1 whorl; leaves sessile (rarely shortly petiolate), usually cuneate or rounded at the base (rarely slightly cordate)

......4 Inflorescence branches glabrous; stamens 8, an outer whorl of 5 and an inner whorl of 3; leaves petiolate, sagittate at the base

1 Stem, petioles, and lower surface of major leaf veins unarmed.

........5 Styles exserted, persistent on achenes; inflorescences spikelike, interrupted; [section Tovara].

..........6 Leaf base cuneate to attenuate; leaves obovate and acute to short-acuminate; leaves with persistent dark purplish-brown chevron on adaxial surface; leaves with 7-13 pairs of lateral veins; flowers red (very rarely, if ever, white)

........5 Styles included, rarely exserted, deciduous; inflorescences capitate, paniclelike, or spikelike, uninterrupted or interrupted.

............ 7 Inflorescences capitate; leaf blades < 6 cm long (except P. chinensis); leaf blades usually > 2× as long as wide; [section Cephalophilon].

............ ..8 Achenes biconvex; lower leaf surface glandular-punctate

............ ..8 Achenes trigonous; lower leaf surface not glandular-punctate.

............ ....9 Leaf margins ciliate with reddish hairs; ocreae lanate; peduncles glabrous or stipitate-glandular above

............ ....9 Leaf margins glabrous or scabrous with whitish hairs; ocreae glabrous or pubescent; peduncles stipitate-glandular throughout

............ 7 Inflorescences panicle-like or spikelike; leaf blades usually > 6 cm long; leaf blades < 2× as long as wide (except P. orientalis).

............ ......10 Inflorescence panicle-like

............ ......10 Inflorescence spike-like; [section Persicaria].

............ ........11 Ocreae with a green, herbaceous flange; leaves 3-17 cm wide

............ ........11 Ocreae hyaline, tan, brown, or reddish throughout; leaves < 6 (-8) cm wide.

............ ..........12 Ocreae lacking cilia or with cilia 0-1 mm long.

............ ............ 13 Plants perennial, with rhizomes or stolons; leaves lacking a triangular reddish blotch in the middle of the upper surface.

............ ............ ..14 Achenes biconvex; styles 2; leaf base cuneate; ocreae 12-23 mm long

............ ............ ..14 Achenes triangular in ×-section; styles 3; leaf base rounded to cordate; ocreae 6-12 mm long

............ ............ 13 Plants annual, lacking rhizomes or stolons; leaves often with a triangular reddish blotch in the middle of the upper surface (except for in P. minor).

............ ............ ....15 Peduncles glabrous; leaves lacking a triangular reddish blotch in the middle of the upper surface

............ ............ ....15 Peduncles usually stipitate-glandular; leaves often with a triangular reddish blotch in the middle of the upper surface.

............ ............ ......16 Outer tepals with 3 strong veins, each forked in an anchor shape; tepals 4 (-5); inflorescences usually arching-drooping

............ ............ ......16 Outer tepals with inconspicuous and irregularly-forking veins; tepals 5; inflorescences erect.

............ ............ ........17 Flowers heterostylous; achenes usually with central hump on 1 side

............ ............ ........17 Flowers homostylous; achenes without central hump on 1 side

............ ..........12 Ocreae with cilia 1-12 mm long.

............ ............ ..........18 Perianth with glandular punctae.

............ ............ ............ 19 Achenes minutely textured, dull; axillary inflorescences sometimes included within ocreae

............ ............ ............ 19 Achenes smooth, shiny; axillary inflorescences never included within ocreae.

............ ............ ............ ..20 Glandular punctae not uniformly distributed on the tepals, mainly on the lower portions of the outer tepals and on the inner tepals

............ ............ ............ ..20 Glandular punctae uniformly distributed on the tepals, not noticeably absent on the upper portions of the outer tepals.

............ ............ ............ ....21 Inflorescences interrupted; ocreolae mostly not overlapping (especially the lower), the margins mostly ciliate with hairs < 2 mm long; leaves 0.6-2.4 cm wide

............ ............ ............ ....21 Inflorescences not interrupted; ocreolae usually overlapping, the margins usually eciliate or with cilia < 1 mm long; leaves 2-4.5 cm wide

............ ............ ..........18 Perianth lacking glandular punctae.

............ ............ ............ ......22 Plants perennial, with rhizomes or stolons; leaves lacking a triangular reddish blotch in the middle of the upper surface.

............ ............ ............ ........23 Achenes biconvex; styles 2

............ ............ ............ ..........24 Widest leaf blades 17-29 (-35) mm wide; principal emersed leaves with petioles 1-9 (-12) mm long, commonly with small, red-brown spots, usually otherwise green in drying; primary inflorescences 13-32 (-36) mm long; floating leaf blades cuneate to rounded (infrequently truncate) at the base; rhizome 1.5-3.5 mm thick between leafless nodes, bearing few-branched to moderately branched roots; stipules of emersed shoots usually developing sheathing stipules with a foliaceous, outward-flange at the summit, these pubescent with wide-ascending to spreading hairs

............ ............ ............ ..........24 Widest leaf blades (23–) 32­–63 mm wide; principal emersed leaves with petioles 5–28 (–38) mm long, lacking the mentioned fungal infection, usually tinged with red or brown in drying; primary inflorescences (25–) 29–90 mm long; floating leaf blades cordate to rounded at the base; rhizome (2.8–) 3–6.5 mm thick between leafless nodes, bearing highly branched roots; sheathing stipules of emersed shoots without a green, outward flange, glabrous or pubescent with appressed hairs

............ ............ ............ ........23 Achenes triangular in ×-section; styles 3.

............ ............ ............ ............ 25 Ocreae glabrous, or strigose toward the base (the hairs stiff and appressed)

............ ............ ............ ............ 25 Ocreae strigose and hirsute, at least some of the hairs loosely ascending to spreading.

............ ............ ............ ............ ..26 Leaf blades rounded to cordate at the base; stem internodes brownish-hirsute

............ ............ ............ ............ ..26 Leaf blades cuneate to truncate at the base; stem internodes glabrous or loosely spreading-hirsute near the nodes only

............ ............ ............ ......22 Plants annual, lacking rhizomes or stolons.

............ ............ ............ ............ ....27 Peduncles stipitate-glandular

............ ............ ............ ............ ....27 Peduncles not stipitate-glandular.

............ ............ ............ ............ ......28 Bristles of ocreae (0.5-) 1-4 (-6) mm long; achenes triangular in ×-section; styles 3.

............ ............ ............ ............ ........29 Leaves rhombic, bases cuneate, apices acute, adaxial and abaxial surfaces glabrous, purple blotch absent or faint, marginal bristles 0.1–0.3 mm long; inflorescences dense, fascicles congested (interrupted basally); pedicels included; proximal bracteole bristles surpassing the adjacent flowers; tepals dark pink, closed at anthesis; achenes < 2 mm long

............ ............ ............ ............ ........29 Leaves elliptic, bases rounded and abruptly cuneate to the petiole, apices caudate, adaxial and abaxial surfaces strigose, purple lunate blotch conspicuous and present at maturity, marginal bristles 0.5 mm long; inflorescences lax, fascicles remote; pedicels exserted; proximal bracetole bristles scarcely reaching the base of the tapals; tepals pale pink to white, open at anthesis; achenes > 2 mm long

............ ............ ............ ............ ......28 Bristles of ocreae 0.2-1.3 (-2) mm long; achenes biconvex or triangular in ×-section; styles 2-3.

............ ............ ............ ............ ..........30 Stems usually pilose; leaf margin setae 0.5-1.1 mm long, blade usually without a triangular reddish blotch; peduncles usually stipitate glandular; racemes usually 3-8 cm long, usually nodding; achenes lenticular (rarely trigonous), basally tumescent

............ ............ ............ ............ ..........30 Stems glabrous; leaf margin setae 0.2-0.5 mm long, blade often with a triangular reddish blotch; peduncles rarely stipitate-glandular; racemes 1-4 cm long, erect; achenes lenticular or trigonous, basally widest but not tumescent