Partly or fully mature fruit is desirable, sometimes essential, for certain identification of species.

1. Leaf blades mostly hastate or sagittate with acute basal lobes, ± pleasantly acid to the taste; plants dioecious.

2. Outer tepals not at all reflexed; inner tepals of pistillate flowers closely covering the achene, or smaller, but no larger.

R. acetosella

2. Outer tepals strongly reflexed; inner tepals of pistillate flowers much expanded in fruit, at least twice as broad as the achene.

3. Inflorescence with branches at most branched once again and ± loosely flowered; tepals green to pinkish.

R. acetosa

3. Inflorescence repeatedly branched and densely flowered; tepals bright pink.

R. thyrsiflorus

1. Leaf blades tapered, rounded, or subcordate at the base, the basal lobes, if any, not acute and the taste not sour; plants monoecious or with bisexual flowers.

4. Margins of inner tepals with elongate teeth or spines at maturity (teeth much longer than broad).

5. Swellings (“grains”) well developed on midribs of all 3 inner tepals; plant an annual, of moist shores and marshy ground, with leaf blades tapered at the base.

R. fueginus

5. Swellings (“grains”) well developed on midrib of only 1 inner tepal; plant a stout perennial of disturbed ground generally, with subcordate or cordate leaf blades.

R. obtusifolius

4. Margins of inner tepals entire to shallowly crenulate or toothed.

6. Well-developed swellings (“grains”) absent or present on at most 1 of the 3 inner tepals at maturity.

7. Inner tepals without well-developed swellings (“grains”), distinctly cordate, except in the rare R. occidentalis.

8. Joint in pedicel conspicuously swollen, appearing like a thickened, narrow ring; inner tepals distinctly cordate.

R. longifolius

8. Joint in pedicel inconspicuous, barely thicker than the pedicel; inner tepals truncate.

R. occidentalis

7. Inner tepals with a bulbous grain on 1 midrib (sometimes an obscure grain on others), scarcely cordate (except in the rare R. patientia) and usually as long as broad or longer.

9. Leaves entire, flat, those of the main stem with short axillary shoots (branches or tufts of leaves); swollen joint ca. 1 mm or less from base of all fruiting pedicels.

R. altissimus

9. Leaves crenulate-toothed, ± crinkly or undulate on the margins, without axillary shoots; swollen joint more than 1 mm from base of at least some fruiting pedicels.

10. Inner tepals 3.5–5 mm long at maturity; petioles with a narrow groove on each side of midvein above.

R. crispus (in part)

10. Inner tepals 6.5–8 mm long; petioles ± flat above.

R. patientia

6. Well-developed swellings (“grains”) present on all 3 inner tepals at maturity.

11. Leaves entire, flat, those of the main stem with short axillary shoots (branches or tufts of leaves); swollen joints on fruiting pedicels 1 mm or less (usually 0.5 mm) from the base.

12. Pedicels equaling or shorter than the perianth, ± curved, the inflorescence rather dense.

R. triangulivalvis

12. Pedicels at least 2.5–3 times as long as the perianth, sharply reflexed and straight most of their length, the inflorescence in distinct whorls.

R. verticillatus

11. Leaves crenulate-toothed and usually ± crinkly undulate on the margin, without axillary shoots; swollen joints on at least some fruiting pedicels more than 1 mm from the base or not evident at all.

13. Pedicels with a distinctly swollen joint on lower third; fruit ripe in June and July (rarely as late as August as far north as Isle Royale); base of swollen grain even with base of tepal midrib or bulging below it.

R. crispus (in part)

13. Pedicels with joint not swollen, barely or not apparent; fruit ripe in September (rarely as early as late August in Upper Peninsula); base of swollen grain distinctly above base of tepal midrib.

R. orbiculatus