All species are truly aquatic, although some species with broad leaves may survive at least temporarily on wet shores after a lowering of water levels. Species with broad leaves (whether floating or submersed) have spikes held above the surface of the water, and hence the pollen is readily wind-dispersed. Linear-leaved species mostly have spikes at or below the surface of the water, which then becomes the important dispersal medium for the pollen.

Fruiting specimens of Potamogeton generally give no trouble in identification; however, vegetative material is so often collected that it has become customary to expect to be able to name such specimens. The present key should work fairly well with vegetative material, though the effort should always be made to collect fruit. The problem of identifying vegetative plants is made even more difficult in Potamogeton by the great variability associated with conditions of the water in which the plants are growing, whether deep or shallow, quiet or flowing.

Several linear-leaved species are characterized by having the stipule margins connate. The only sure way to determine whether these margins are connate is to moisten (if the specimen is dry) a young branch tip with a drop or two of a wetting agent (such as detergent solution), cut one or more sections about 1–1.5 mm long cleanly across it with a sharp razor blade, and examine these in water under 10× to 20× magnification. If the sections have been cut where the internodes are short and the stipules overlap, at least the inner ones in those species with connate stipules (P. foliosus, P. friesii, P. strictifolius, and P. pusillus) will form cylinders or tubes, which will be evident when the sections are carefully sorted out with dissecting needles. Older stipules tend to split, so this character must be determined with caution from the youngest stipules possible. The stipule margins may overlap strongly in species in which they are not connate. In some species winter-buds (turions) are produced, which offer the botanist a source of diagnostic characters. These structures are firm and much-shortened shoots with crowded stipules and reduced leaves.

Species of Potamogeton have long been known to hybridize, and recent molecular studies (Kaplan et al. 2009; Les et al. 2009) show that there is a great deal of sometimes cryptic hybridization present when populations are studied extensively.

In relatively quiet calcareous waters, there is often a considerable deposition of marl (chiefly insoluble calcium carbonate) on submersed parts of Potamogeton and other aquatic plants.

1. Leaf blades (all submersed) ± auricled at the base, (2.5–) 3–8 (–9) mm wide, with many (usually 20–40) nerves, strongly and rather stiffly 2-ranked, the margins ± thickened and usually minutely spinulose-serrulate (at least toward the apex—use strong lens); sheathing portion of leaf adnate to the prominent whitish, ± fibrous stipules; inflorescence (very rarely fruiting) usually branched; nodes of lower part of stem often with slender adventitious roots.

P. robbinsii

1. Leaf blades (submersed or in some species partly floating) not auricled (if clasping, then broad and without sheathing base), the width, veins, and stipules various but not combined as above; margin usually perfectly entire; inflorescence unbranched; nodes of stem very seldom with a few adventitious roots.

2. Margins of leaves undulating and finely but definitely and sharply toothed, especially toward the apex; fruit (very rare) with curved beak about 2 mm long; leaves all submersed, oblong, ± rounded at apex.

P. crispus

2. Margins of leaves undulating or flat but not toothed; beak of fruit shorter or absent; leaves various.

3. Submersed leaves (at least fully developed ones) with blades more than 5 mm wide.

4. Blades of principal (not the youngest) submersed leaves sessile and subcordate to clasping at base; floating leaves never developed.

5. Stipules conspicuous, persistent (sometimes fibrous), (2–) 2.5–6.5 (–11) cm long when fully developed; body of fruit 3.5–5 mm long, sharply keeled; tips of leaves usually boat-shaped (when pressed and flattened therefore split); rhizome white with reddish spots; leaves up to 24 cm long, often over 10 cm; peduncles (9–) 18–55 (–100) cm long.

P. praelongus

5. Stipules inconspicuous or soon disintegrating into fibers, less than 2 (–2.3) cm long (if present); body of fruit 2.5–3 (–3.5) mm long, with keel weak or absent; tips of leaves ± flat (not boat-shaped and hence not splitting when pressed); rhizome unspotted; leaves usually not over 10 (very rarely up to 19) cm long; peduncles occasionally as much as 17 cm long but usually less than 10 cm.

6. Stipules early disintegrating into ± coarse whitish fibers (the youngest entire); leaves ovate to lanceolate, mostly 3–10 cm long and with more than 12 nerves.

P. richardsonii

6. Stipules very delicate, soon disappearing; leaves orbicular to ovate, rarely as long as 5 cm, with not over 17 nerves.

P. perfoliatus

4. Blades of principal submersed leaves tapering or even petioled at base; floating leaves often developed (absent on young material or that from deep water).

7. Submersed leaves long and ribbon-like with essentially parallel sides for most of their length; midrib of some if not all submersed leaves with a conspicuous cellular-reticulate band (i.e., appearing lighter and different in texture) on each side, totaling at least 1 mm wide and usually about a fourth or more the total width of the leaf.

P. epihydrus (in part)

7. Submersed leaves with sides ± curved most of their length; midrib with cellular-reticulate band less than 1 mm wide or, if wider, still much less than a fourth the total width of leaf.

8. Submersed leaves (not transitional or floating ones) with 3–9 (–11) nerves and with stipules not over 3.5 cm long.

9. Blades of floating leaves merging gradually into petioles; fully developed submersed leaves 6–13 cm long (often with conspicuous band along midrib); mature flowers usually on pedicels 0.5–1 mm long; plants usually strongly tinged with red (more evident on drying), especially in upper portions and in rachis of inflorescence.

P. alpinus

9. Blades of floating (not transitional) leaves sharply distinguished from petioles; submersed leaves often less than 6 cm long (if longer, distinguished from P. alpinus by green or brownish leaves, usually with cellular-reticulate band along midrib faint tor virtually absent); flowers sessile or on pedicels up to 0.5 mm long; plants green or brown, with red suffusion nearly always lacking, at least on peduncle and rachis.

go to couplet 13

8. Submersed leaves with (7–) 9–40 (–52) nerves and with stipules (2–) 2.5–9 (–11) cm long.

10. Fully developed submersed leaves ± strongly arched (and often folded), with 24–40 (–52) nerves, mostly (2.5–) 3.5–7.2 cm wide; fruits 4–5.5 mm long (including short beak); floating leaves with nerves about 30 or more (28–50); stipules of submersed leaves (except for the lowermost, nearest rhizome) 4–9 (–11) cm long.

P. amplifolius

10. Fully developed submersed leaves flattish or at most slightly arched (undulating in P. pulcher), with not more than 19 nerves, with blade up to 3 (rarely 4, or very rarely 5.5) cm wide; fruits 2–4 (–4.2) mm long; floating leaves with up to 27 nerves; stipules various.

11. Stem and petioles rather conspicuously black-spotted or mottled; blades of floating leaves slightly cordate at base.

P. pulcher

11. Stem and petioles not black-spotted; blades of floating leaves ± rounded to broadly tapered at base, not cordate.

12. Larger submersed leaves with blades 1–2.5 (–3) cm wide, on petioles (2.5–) 5–11 cm long, acute at apex but without sharp awl-like tip; fruits 3.5–4 mm long, with ± knobby or warty keel and lateral ribs; stipules of submersed leaves 4–7.5 (–10.5) cm long; petioles of floating leaves up to 27 cm long, often equaling or exceeding the blades, which have 13–25 (usually 21) nerves.

P. nodosus

12. Larger submersed leaves with blades various, but sessile or on petioles not over 2 cm long, acute at apex or (especially on petioled leaves) with mucronate or awl-like tip; fruits with keels rounded or sharp, but not warty; stipules and floating leaves various.

13. Submersed leaves 8–32 (–50) mm wide, with (7–) 9–19 nerves, the apex mucronate or with awl-like tip up to 4 (–5) mm long; fruits 3–4 mm long, ± sharp-keeled; largest stipules of submersed leaves (2–) 2.5–6 (–7) cm long; floating leaves with petioles shorter than the blades (up to 3.5 (–8.5) cm long), the blades 1.7–3.5 (–5.7) cm wide.

P. illinoensis

13. Submersed leaves up to 9 (sometimes to 14) mm wide, with (3–) 5–9 (–11) nerves, the apex acute or with awl-like tip up to 1.5 mm long; fruits 2–2.5 (–2.8) mm long, the keel nearly always rounded or virtually absent; stipules of submersed leaves 0.7–3 (–4) cm long; floating leaves with petioles (up to 9 cm long) slightly shorter than to much exceeding the blades, the latter 0.8–3 cm wide.

P. gramineus (in part)

3. Submersed leaves all less than 5 mm wide (often decayed and absent at fruiting time in P. natans and P. oakesianus).

14. Floating leaves with well-developed blades 1.2–4.7 cm wide (if rarely smaller, then narrowly elliptic and acute at both ends); submersed leaves appearing as if petioles only, rather coarse, not distinctly flattened, up to 2 (–2.5) mm in diameter, often decayed and absent at flowering time; fruit 3–4.5 mm long.

15. Blades of floating leaves definitely subcordate to cordate at base, 2–4.7 cm wide and (3.2–) 3.7–9 (–10) cm long, at least the larger ones (18–) 21–35-nerved; mature fruit (3.5–) 3.7–4.5 mm long (including beak), obscurely keeled, the surface usually brownish (sometimes green) and ± irregular or puckered.

P. natans

15. Blades of floating leaves rounded to acute at base, (0.5–) 1.2–2.2 (–2.8) cm wide and (2.5–) 3–4.7 cm long, 11–19-nerved; mature fruit 3–3.4 mm long, rather prominently keeled, the surface usually greenish and not puckered.

P. oakesianus

14. Floating leaves in many species never developed, if present, their blades less than 1.2 cm broad and/or the submersed leaves with definitely flat (though narrow) blades; fruit less than 3 mm long, or larger in a few species with definitely flattened submersed leaves.

16. Leaves (all submersed) over 2 mm wide, with many (15–30 or more) fine nerves; stem strongly flattened (appearing winged) and ca. half or more the width of the leaves; fruit 3.5–5.5 mm long.

P. zosteriformis

16. Leaves with fewer nerves, of various widths; stem obscurely or not at all flattened; fruit various.

17. Stipules of some or all submersed leaves adnate to the base of the leaf for up to 4 mm, the tip of the stipule projecting as a ligule; fruit suborbicular, flattened, nearly or quite beakless, the strongly coiled embryo showing through the thin wall (which appears distended by the embryo).

18. Submersed leaves filiform, sharp-tipped, not over 0.5 mm wide (usually 0.3 mm or less), their stipules free or adnate for less than the length of the free ligule; fruit ca. 1–1.5 mm in diameter.

P. bicupulatus

18. Submersed leaves broader, with blunt or rounded tip, the largest usually 0.6–1.6 mm wide, their stipules adnate for ca. 1.5–4 mm, the projecting free ligule shorter than adnate portion; fruit ca. 1.5–2 mm in diameter (including keel).

P. spirillus

17. Stipules of submersed leaves all free; the embryo not clearly seen through thin walls of a flattened fruit, not coiled more than 1 revolution; leaves various.

19. Floating leaves present; plants either very delicate with submersed leaves less than 1 mm wide and 1–3-nerved or with submersed leaves at least (1–) 2 mm broad, many if not all of them ± strongly 3–several-nerved, these broader leaved plants either with a broad cellular-reticulate band along the midrib of submersed leaves or with (usually) many bushy branches in which the internodes are mostly only 2–6 mm long.

20. Submersed leaves less than 1 mm wide and 1–3-nerved, lacking a cellular-reticulate band along the midrib; plants very lax and delicate.

P. vaseyi

20. Floating leaves always at least (1–) 2 mm broad, many if not all of them ± strongly 3–several-nerved, either with a broad cellular-reticulate band along the midrib of submersed leaves or with (usually) many bushy branches in which the internodes are mostly only 2–6 mm long [See also further descriptions in couplet 21. Narrow-leaved and often misidentified plants of species typically keying under the first half of couplet 3].

21. Submersed leaves ribbon-like, about 20 or more times as long as wide, distinctly parallel-sided, acute to blunt at tip, with a conspicuous cellular-reticulate band bordering the midrib and totaling a third or more the width of the leaf; plants not bushy-branched, the internodes mostly 10 mm or more in length.

P. epihydrus (in part)

21. Submersed leaves linear-lanceolate to lance-elliptic, mostly less than 15 times as long as wide, acute or often with short awl-like tip, without cellular-reticulate band; plants usually with bushy branches in which the internodes are frequently less than 6 mm long.

P. gramineus (in part)

19. Floating leaves absent; submersed leaves variable in width, but often usually 0.5–3 mm wide (except in P. confervoides); plants with cellular-reticulate band along midrib generally narrowed or absent, without bushy branches (though winter-buds often present) unless the leaves are under 2 mm wide and/or obtuse at apex.

22. Stipules ± coarsely fibrous, whitish, the oldest tending to disintegrate into shreds; bases of winter-buds usually ± hardened and strongly ribbed.

23. Peduncles less than 1.3 (–1.5) cm long, even in fruit; inflorescence few-flowered, nearly spherical; fruit (including short beak) ca. 3–4 mm long; stipules 6–12 (–14) mm long, their margins not connate; leaves all 3-nerved, tapering to a bristle-like tip and the midrib bordered by a narrow cellular-reticulate band.

P. hillii (in part)

23. Peduncles 1.5–4 cm or more in length (except when very young); inflorescence definitely cylindric, the whorls of flowers becoming well separated at maturity; fruit (2–) 2.2–2.8 (–3) mm long; stipules (8–) 10–21 mm long, their margins connate at least basally when young; leaves mostly 3–7-nerved, tapering to bristle-tip or the midrib bordered by cellular band (or with neither condition).

24. Apex of leaves obtuse or rounded (usually slightly mucronate); leaves mostly 5 (–7)-nerved (a few, especially the smaller, often 3-nerved), the broadest usually 1.5–2.7 (–3) mm wide, frequently with a very narrow cellular-reticulate band along the midrib; glands at base of leaves (on stem) usually conspicuous; peduncles ± flattened upwardly.

P. friesii

24. Apex of leaves gradually tapered to a sharp, often bristle-like tip; leaves mostly 3-nerved (a few with 5 nerves often present), the broadest usually 0.8–1.3 mm wide, without any cellular-reticulate band; glands at base of leaves usually inconspicuous or apparently absent; peduncles ± terete (or flattened for only about 2 mm immediately below the inflorescence).

P. strictifolius

22. Stipules ± delicate and membranous, whitish to green or brownish, without persistent fibers on disintegration (tending to break across the delicate fibers); winter-buds, if present, not hardened or strongly ribbed at base.

25. Leaves weakly 1-nerved, less than 0.3 (–0.4) mm wide, very slenderly tapering to tip; peduncle solitary, 3–14 cm long (occasionally with one branch).

P. confervoides

25. Leaves wider, 1–5-nerved; peduncles up to 6 (–8) cm long, often shorter, usually in axils of several leaves.

26. Principal leaves 2–3.5 mm wide, usually somewhat flushed with maroon (especially when dried), very obtuse to rounded (and sometimes minutely mucronate) at apex, with prominent (about 0.3–0.5 (–1) mm wide) lighter-colored cellular-reticulate band bordering midrib; leaves 3-nerved, the lateral pair of nerves rather weak; glands at base of leaves mostly 0.5 mm or more across; fruit 3–4 mm long (including beak), with low but sharp keel and often rather knobby toward base.

P. obtusifolius

26. Principal leaves mostly narrower, seldom flushed with maroon, obtuse or acute, 1–3 (–5)-nerved, with band bordering midrib generally smaller or absent; glands at base of leaves smaller or absent; fruit keeled or not, 1.8–3 mm long (except in P. hillii with very sharply acute leaves).

27. Peduncles (0.8–) 1.1–6 (–8) cm long; fruits less than 3 mm long and with keel rounded or obscure, in nearly spherical to cylindric spikes; glands often present at base of some or all leaves; leaves obtuse to acute, with or without narrow cellular-reticulate border.

28. Margins of stipules connate below the middle, at least when young; largest leaves 0.8–1.2 (–2) mm wide, acute, midrib lacking cellular-reticulate border.

P. pusillus

28. Margins of stipules separate, often overlapping but not connate; largest leaves 0.5–2 (–2.7) mm wide, obtuse or rounded to acute; midrib often with a narrow cellular-reticulate border.

P. berchtoldii

27. Peduncles less than 1.3 (–1.5) cm long, even in fruit; fruit 3–4 mm long or with thin sharp keel; spike few-flowered, nearly spherical; glands often absent at base of leaves; leaves acute at tip, the midrib often bordered by narrow reticulate band.

29. Fruit less than 3 mm long, with a sharp often thin keel, and often knobby toward base; glands absent at base of leaves; stipules delicate, the margins connate.

P. foliosus

29. Fruit 3–4 mm long (including short beak), without thin sharp keel; glands present or absent at base of leaves; stipules somewhat fibrous, not connate.

P. hillii (in part)