Platanthera is sometimes split further with recognition of such genera as Limnorchis Rydb., Lysiella Rydb., and Blephariglottis Raf.

1. Lip prominently ciliate or fringed.

2. Lip simple, not deeply divided (except for fringe).

3. Flowers white; longest cilia (usually lateral) of fringe about half as long as the undivided portion of the lip, or even shorter.

P. blephariglottis

3. Flowers yellow to orange; longest cilia of fringe distinctly more than half the length of the undivided portion of the lip.

P. ciliaris

2. Lip deeply 3-parted in addition to fringe.

4. Flowers pink-purple; divisions of the lip broadly fan-shaped, copiously lacerate-fringed, but the fringe usually cut less than half the distance to the base of the division of the lip.

P. psycodes

4. Flowers yellowish, cream, or greenish; at least the lateral divisions of the lip more narrowly cuneate, mostly cut into a long fringe more than half their length.

5. Sepals (6–) 6.5–9 (–10) mm long; lateral petals broadly obovate or cuneate, erose or denticulate at apex.

P. leucophaea

5. Sepals 3.5–5 (–6) mm long; lateral petals linear-oblong or almost lanceolate, usually essentially entire.

P. lacera

1. Lip entire or toothed, but not fringed.

6. Leaves all basal, the stem at most with reduced bracts.

7. Leaves about twice as long as wide, or longer; spur less than 12 mm long.

8. Lip ± cuneate, with truncate 3-toothed or crenate apex; spur 7–11 mm long, much exceeding the lip.

P. clavellata (in part)

8. Lip tapered to a pointed or rounded apex untoothed; spur about equaling lip or at most ca. 2 mm longer.

9. Leaves 1 (–2), present through anthesis; ovary short-pediceled (evident on older flowers or fruit), the inflorescence usually a raceme shorter than 10 (–12) cm; lateral sepals 3.5–6 mm long.

P. obtusata

9. Leaves usually 2–3, withering during or before anthesis; ovary sessile, the inflorescence a spike (5–) 10 cm or more long; sepals 1.5–3 (–3.5) mm long.

P. unalascensis

7. Leaves less than twice as long as broad, orbicular or almost so; spur 16–40 mm long.

10. Scape naked (rarely with a bract); spurs (16–24 mm long) tapered ± evenly to rounded apex; lip yellowish green, tending to turn upward near the end.

P. hookeri

10. Scape with 1–6 bracts between leaves and inflorescence; spurs parallel-sided or even somewhat club-shaped toward apex; lip whitish green, tending to turn downward.

11. Spurs 16–27 (–29) mm long.

P. orbiculata

11. Spurs (27–) 29–43 mm long.

P. macrophylla

6. Leaves cauline.

12. Lip truncate and 2–3-toothed or -lobed at apex.

P. clavellata (in part)

12. Lip tapered, rounded (or almost truncate and obscurely crenulate) but not 2–3-toothed at apex.

13. Lip much shorter than the spur, broadly rounded (or almost truncate) at apex, with an erect tubercle near the base and a lateral tooth or projection on each side near the base.

P. flava

13. Lip 1–2 mm shorter than, about equaling, or slightly longer than the spur, tapered to narrow apex, with neither a tubercle nor lateral teeth (at most, broadly widened basally).

14. Flowers pure white, lip strongly expanded basally.

P. dilatata

14. Flowers green, greenish yellow or greenish white, lip cuneate to strap-shaped, not or only slightly widened basally.

15. Anther sacs essentially in contact above the rounded stigma (separated at apex by less than 0.3 mm); lips (pressed) 2.5–5 (–5.4) mm long.

P. aquilonis

15. Anther sacs separated at apex by ca. 0.4 mm or more, stigma pointed; lips (pressed) (3–) 4–8 mm long.

P. huronensis