Leaves petiolate or subsessile, mostly paler beneath, not articulate, nectaries absent; ocrea tubular, brown, membranous, entire, mostly long-ciliate on the upper margin, occasionally with a green limb.
Flowers usually bisexual, clustered in terminal or axillary spike-like inflorescences (usually long, occasionally short and capitate), usually forming panicle-like compound inflorescences Perianth segments 4 or 5, usually distinctly petaloid, pink, white or occasionally green, slightly enlarged in fruit. Stamens 4–8; anthers versatile. Style bifid or trifid, mostly short. Stigmas capitate, smooth.
| | Key to the species | |
| 1 | Inflorescence small, 1–several few-flowered clusters or very short loose spike-like racemes on slender branches; stems creeping, prostrate to decumbent | 2 |
| Inflorescence of numerous flowers in mostly dense, elongated or more rarely subglobose spike-like racemes, usually arranged in large panicles; stems erect to decumbent or prostrate, occasionally with slender antrorse or spreading hairs but never strongly retrorse-strigose | 5 |
| 2 | Stems strongly retrorse-strigose; perianth not enlarged or changing colour when fruiting | 3 |
| Stems not retrorse-strigose; peranth enlarged, succulent, blue-black and surrounding the fruit
Back to 1 | Persicaria chinensis |
| 3 | Leaf bases hastate, sagittate or occasionally truncate | 4 |
| Leaf bases acute to cuneate, occasionally obtuse to truncate
Back to 2 | Persicaria dichotoma |
| 4 | Inflorescence of 2–4 branches with small terminal subglobose flower clusters 5–10 mm long with bracts crowded at ends of branches; inflorescence branches with stalked glandular hairs and usually some shorter simple hairs; leaves 15–32 mm wide | Persicaria strigosa |
| Inflorescence of 2 or 3 branches but bracts 5–15 mm apart so that flowers appear solitary; inflorescence branches with stalked glandular hairs only or with a few simple hairs as well; leaves 6–17 mm wide
Back to 3 | Persicaria praetermissa |
| 5 | Spikes globose or ovoid or short-cylindrical with length no more than twice as great as diam. (occasionally slightly more in Persicaria maculosa and then spikes interrupted below) | 6 |
| Spikes elongate-cylindrical, at least four times as long as wide
Back to 1 | 8 |
| 6 | Ocreas not developed into a green limb; perianth pink to whitish, not thickened and corky | 7 |
| Ocreas developed into spreading green toothed limb; perianth green, becoming truncate, thickened and corky in fruit
Back to 5 | Persicaria prostrata |
| 7 | Perianth c. twice as long as the nut; inflorescence bracts subtriangular, stiff; plants prostrate; spikes ovoid or globose, dense, not interrupted | Persicaria capitata |
| Perianth only slightly exceeding nut; inflorescence bracts funnel-shaped, soft-textured; plants ascending or erect; spikes short-cylindrical, dense but often interrupted below
Back to 6 | Persicaria maculosa |
| 8 | Ocreas developed on upper margin into a spreading green limb | 9 |
| Ocreas not developed into green limb
Back to 5 | 10 |
| 9 | Large erect herb; perianth pale pink, not becoming thick and corky in fruit; ocrea limb collar-like, to 7 mm wide, entire; leaf base obtuse, often slightly oblique and petiole winged for 1–2 cm | Persicaria orientalis |
| Small decumbent to prostrate herb; perianth green, becoming truncate, thickened and corky in fruit; ocrea limb lobed and toothed; leaf base attenuate
Back to 8 | Persicaria prostrata |
| 10 | Stalked glandular hairs present on most parts, occasionally sessile glands present too; simple hairs present on leaf margins | Persicaria elatior |
| Hairs simple on stems or absent, but sessile glands may be present on stem, leaf, ocrea or perianth
Back to 8 | 11 |
| 11 | Stigmas tough and persistent on fruit, hook-like | Persicaria filiformis |
| Stigmas remaining delicate when fruiting, usually deciduous
Back to 10 | 12 |
| 12 | Plants glabrous or with hairs only on leaf margins, veins and ocreas | 13 |
| Plants hairy, usually densely so, not just on leaf veins and margins
Back to 11 | 16 |
| 13 | Perianth densely glandular, pink to whitish or greenish white | 14 |
| Perianth not densely glandular, pink to white
Back to 12 | 15 |
| 14 | Perianth greenish white; upper margins of ocreas mostly with cilia 3–5 mm long; petioles 1–5 mm long; spikes slender, lax, with each flower cluster distinct | Persicaria hydropiper |
| Perianth pink to whitish; ocreas without marginal hairs; petioles 5–22 mm long; spikes dense, with flowers crowded and overlapping
Back to 13 | Persicaria lapathifolia |
| 15 | Leaves with glands (usually yellow) dense on lower surface and scattered on upper; spikes dense, 4–7 mm diam | Persicaria lapathifolia |
| Leaves not conspicuously glandular but usually minutely warty, often with purplish blotch near the middle of the upper surface; spikes slender, rather lax, 3–4 mm diam
Back to 13 | Persicaria decipiens |
| 16 | Leaves subsessile, petiole no more than 2 mm long | Persicaria subsessilis |
| Leaves obviously petiolate, petiole 5–40 mm long
Back to 12 | 17 |
| 17 | Plants erect; perianth sparsely sessile-glandular; leaves densely to sparsely tomentose, sometimes mixed with more or less glabrous leaves on same plant | Persicaria lapathifolia |
| Plants decumbent; perianth not glandular; leaves not tomentose but densely and shortly pubescent beneath, hairs on upper surface more or less densely and coarsely antrorse
Back to 16 | Persicaria attenuata |
