Key to North American Potamogetonaceae

  1.  Stipular sheaths adnate to leaf base for 2/3’s of more of the stipule. ………………………..……………………...2

2.  Leaf margins serrate, plants stiff, two-ranked……..…………………………………...….Potamogeton robbinsii

2.  Leaf margins entire, plants limp, not two-ranked,  Stuckenia…………….………………………………………3

      3.  Sheaths open to base, leaf tips acute, apiculate, cuspidate, or acuminate, rarely obtuse.

                 4.  Lower sheaths not inflated to 1.5× wider than stem ………….……..……………………………………...5

        5.  Leaves 0.2-1.0 mm wide, tips acute; lower leaf sheaths 1.5× wider than stem; rhizomatous tubers usually

              developed; fruits flesh-colored, 3.3-4.7 (-5.1) mm, beak to 1.2 mm.  Common in alkaline and brackish

              water, throughout.………………………..………………………….……………..……..….S. pectinata

        5.  Leaves (0.4-)1.5-3(-8.5) mm wide, tips acute, apiculate, or cuspidate; lower leaf sheaths not inflated;  

              rhizomatous tubers not developing; fruits green, 3.0-3.9 mm,, beak less than 0.5 mm. Uncommon in     

              alkaline waters of western U.S. ……………………………………………….………...……S. STRIATA

                4.  Leaf tips rounded, obtuse, rarely acute, lower leaf sheaths 2× or more the stem with fruits not developing. 

                       Uncommon hybrid of  S. pectinata × P. vaginata…………………..…….………….…….…..Sbottnica

            3.  Sheaths tubular or partially tubular, leaf tips notched, obtuse, or rounded………...............................................6

                6.  Fruits commonly developed ………………………...………………………………………………………..7

         7.  Leaves 0.2-1 mm wide, tips notched, blunt, or short-apiculate; stipular sheaths slightly inflated, with

              distinct ligules 2-20 mm long; fruits 2-3 mm wide. Mostly alkaline waters of northeastern and western

              U.S., widespread in Canada.  Two subspecies have been identified in North America depending on

              peduncle length…………………………………………………………………..……….……S. filiformis

        7.  Leaves 1-2 mm wide, tips obtuse, rounded or slightly notched; stipular sheaths inflated 3-5× stem

              thickness, to 10 mm wide, 30-70 mm long, lacking ligules or to 0.2 mm; fruits 3-8 mm wide.  In still,

              alkaline waters of western U.S. and Canada…………………………………………..….….S. VAGINATA

    6.  Fruits not developed……………………………………………..………………………………………….8

          8. Mature leaves to 18 cm long, tips obtuse to acuminate; sheaths enclosing 2 branches, lower ones usually

               less than 2× the width of the stem, up to 6 cm long; ligules mostly greater than 15 mm long; stigmas

               sessile or borne on an obscure style. (S. filiformis× S. pectinata)…………………..………S. ×suecica

           8. Mature leaves more than 20 cm long, tips truncate, rounded, or subretuse;  some sheaths enclosing 3

               branches, the lower branches  2× or more the width of the stem, greater than 6 cm long;  ligules up to 15

               mm long; stigmas sessile (S. filiformis × S. vaginata)……………………..…………….…S. ×fennica    

 1.  Stipular sheaths free from leaf base, or if adnate, less than ½ the stipule length, or with basal lobe at  junction with

      adnate stipule; inflorescenses submersed or erect on surface………………………………..…………………………9

       9.  Leaves with adnate stipules, (all adnate species can produce floating leaves so only keyed out here)….….…….10

            10.  Submersed and floating leaves blunt-tipped; fruit keel blunt, not well developed; acid. to neutral waters of                

                    northern U.S. and southern Canada ……………………………………………………………….…P. spirillus

            10.  Submersed and floating leaves acute or blunt-tipped; fruit keel well-developed, beak present  or absent …..11

      11.  Floating leaves 3-7-veined; submersed leaves 0.1-0.6 mm wide, no rows of lacunae, fruits lacking beak;

              acidic waters of northeastern U.S. ……………………………………………………..…..P. bicupulatus

      11.  Floating leaves 5-23-veined; submersed leaves 0.9-2.0 mm, 1-many rows of lacunae; fruits with beak;

             southern U.S, …………………………………..…..………………………………………….…………..12

           12.  Fruits 1-1.5 mm long, beak 0.1 mm long; floating leaves 5-15-veined; all stipules adnate; acid to   

                    neutral waters throughout southeastern states and scattered though western states ……………………

                    …………………………………………………………………..…………...……..P. DIVERSIFOLIUS

           12.  Fruits 2.5-3 mm long, beak 0.5 mm long, floating leaves 9-23-veined; most stipules not adnate; acid

                    waters of the Cumberland Plateau, PA south to TN……………………..P. TENNESSEENSIS (in part)

      9.  Leaves lacking adnate stipules………………………………………………………………………………………13

           13.   Plants with floating leaves……………………………………………………………………………………..14

                   14.   Floating leaves 5-9-veined; submersed leaves, thread-like, 1(-3)-veined, acid to neutral waters of 

                           Northeastern U.S.; appears like a linear-leaved pusilloid pondweed ….……..………...P. vaseyi (in part) 

                   14.   Floating leaves to 51-veined;. submersed leaves not thread-like, 1-37-veined,.……….………………...15

                          15.  Submersed leaves linear, flattened or phylloidal………….………………..……………………..…16

                      16.  Submersed leaves flat, linear 1-13-veined with a broad lacunae band on either side of mid-vein

                              fruits with distinct pointed keels…….…………..……………………………………………..17

                            17.  Floating leaves acute at tip; submersed leaves 1-3(-7)-veined, few stipules may be adnate.

                                    acid waters of the Cumberland Plateau, PA s. to TN…………P. TENNESSEENSIS (in part)

                             17.  Floating leaves rounded at tip; submersed leaves 3-13-veined, , no stipules adnate; narrow-

                                     leaved plants from acidic waters, broader-leaved from alkaline waters, common in north..

                                     ……….………………………………………………………………P. epihydrus (in part)

                      16.   Submersed leaves phylloidal (similar to leaf petiole) to slightly linear, 1-5-veined, lacking                     

                               Lacunae band; fruit if produced lacking keels or no keel present…………….……………..18

                               18.  Floating leaves 17-35-veined with the petiole at leaf attachment a different color than rest

                                       of petiole, cordate at base; in northern waters; fruits 3.8-5 mm long  .………..…………

                                       ………………………………………………………………………..P. natans (in part)                          

                                18.  Floating leaves 7-23-veined with petiole at leaf attachment not of different color than rest

                                       of petiole; rounded or tapering at base……………………………………..…….19


                                       19.  Submersed leaves 3-veined; floating leaves (7-)9-19(-23)-veined, rounded or tapering

                                              at base; fruits 2.5-3.5(-3.7) mm long; northern acidic waters…P. oakesianus (in part)

                                    19.  Submersed leaves 1-veined, floating leaves 7-11-veined, acute at base; fruits not

                                               produced (possibly a hybrid);endemic to panhandle of Florida …………………….             

                                               ……………………………………………………………  P. floridanus (in part)

               15.  Submersed leaves broadly linear-oblong, lanceolate to elliptic…………………………….………20

                      20.  Submersed leaves petiolate……………………………………………….………………….…21

                             21.  Floating leaves 19-49-veined; submersed leaf margins crispate; stems rusty to black-

                                    spotted……………………………………………………………………….…………..…22

                                    22.  Stems black-spotted; submersed leaves, 9-21-veined, 10-25 (-35) mm wide, slightly                

                                           arcuate; fruits 3.1-4.1(-4.7) mm with distinct dorsal keel; acidic water of coastal and                

                                           southern U. S…………………………..………..……………….…P. pulcher (in part)

                                    22.  Stems often rusty-spotted; submersed leaves 27-49-veined, 25-75 mm wide, arcuate;

                                           fruits 3.9-5.2(-5.7) mm with indistinct keel; common, northeastern U.S……………….

                                           ……………………………………………………………………P.amplifolius (in part)

                             21.  Floating leaves 11-29-veined; submersed leaf margins rarely crispate; stem lacking

                                     spots…………………………………………………………………………………….….23

                                     23.  Fruits (1.6-)2-2.5 mm, lacking well-developed keel; submersed leaf margin entire;

                                             eastern Nfld and Cape Sable Is.,N. S ……….…...……P. POLYGONIFOLIUS (in part)

                                     23.   Fruits 2.5-4.3 mm, with well-developed keel; submersed leaf margins  minutely

                                             denticulate to entire; widespread…………………………………………………….24

                                             24.  Submersed leaf petioles (7-)11-15 cm long,  leaves with 2-5 rows of lacunae on

                                                     either side of mid-vein; leaf tips acute to rounded; widespread in alkaline waters             

                                                     and acidic rivers in New England……………………..….…P. nodosus (in part)

                                             24.  Submersed leaf petioles 0.5-4 cm long, leaves with 1-2 rows of lacunae on either

                                                     side of  mid-vein; leaf tips acute; (rarely petiolate) widespread in alkaline waters

                                                     of the United States ……………………………….………P. illinoensis (in part)

                      20.  Submersed leaves sessile………………………………………………………………………..25

                             25.  Stems branched; submersed leaf margin denticulate, tip acute to obtuse, stipules acute;

                                    fruits laterally compressed, sessile 1.9-2.3 mm long ………………..P. gramineus  (in part)

                             25.  Stems unbranched; submersed leaf margin entire, tip blunt, stipules blunt; fruits plump,

                                    pedicillate, 1.9-2.3 mm long…………………….…………………….…P. alpinus (in part)

           13.  Plants lacking floating leaves…………………………………………………………………………………..26

                   26.  Leaves linear, threadlike, flattened or phylloidal……………………………………….…………………27

                           27.  Plants strongly rhizomatous……………….…………………………………………………………28

                                   28.  Leaves thread-like, 0.1-0.5 mm wide; peduncles (3-)5-25 cm long; in extremely acidic waters                                                                                  

                                           in northeastern U. S. and Canada…………………..………………………….…P. confervoides

                                   28.   Leaves flattened or phyllodial, 2-11 mm wide; peduncles 1.5-8 cm long………...…………..29

                                           29.  Submersed leaves flat, linear 1-13-veined with a broad lacunae band on either side of mid-

                                                   vein; fruits with distinct pointed keels…….……………………………….……………..30

                                      30.  Submersed leaves 1-3(-7)-veined, few stipules may be adnate; acid waters of the                  

                                              Cumberland Plateau, PA south to TN…………..…………TENNESSEENSIS (in part)                                 

                                      30.  Submersed leaves 3-13-veined, no stipules adnate; narrow- leaved plants from acidic

                                              waters, broader-leaved from alkaline waters, common in north …………….………..            

                                             ………………………………………………………………….. P. epihydrus (in part)

                              29.   Submersed leaves phylloidal (similar to leaf petiole) to slightly linear, 1-5-veined, lacking  

                                       lacunae band; fruit if produced lacking keels or no keel present…………………….…..31

                                          31.  Submersed leaves 1-veined; rare endemic of Florida panhandle ...........................

                                                      ……………………………………….……………………….…P. floridanus (in part)

                                          31.  Submersed leaves 3-5 veined……………………………………………………...32

                                                 32.  Submersed leaves 0.3-1 mm, veins 3-5; common thoughout north……………..

                                                         ……………………………………………………………...P. natans  (in part)                          

                                                 32.  Submersed leaves 0.7- 2.5 mm, veins 3; northern acidic waters ………………

                                                         ………………………………………………….…….P. oakesianus  (in part)  

                         27.  Plants lacking rhizomes or not apparent; leaves not threadlike, 0.1- mm wide; peduncles 0.3-7 cm…     

                          ……………………………………………………………………………………………....33

                         33.  Leaves 1(-3)-veined…………………………………………...……………………….…………34

                                34.  Leaf width 0.1-1 mm, ……………………………………………………………………….35

                                       35.  Stipules 4-12 mm, axillary; nodal glands usually absent; fruits absent in plants …….…..

                                               lacking floating leaves; uncommon…….…………………..……….…P. vaseyi (in part)

                                       35.  Stipules 5-20 mm, terminal; nodal glands present; fruits 2.2 mm long, rare…..………….

                                               ……………………………………………………….P. berchtoldii subsp. gemmiparus

                         33.  Leaves 3-15-veined…………………………………..……………………………………………36

                                 36.  Nodal glands rare or absent;…………………………………………………………………37

.

                                        37.  Leaves 3(-5)-veined, acute -tipped; peduncles 0.3-1.1(-3.7) cm, recurved, axillary;

                                                fruit 1.4-2.7 mm long, beak 0.2-0.6 mm, keeled;  neutral to alkaline waters....P. foliosus

                                        37.  Leaves 3-veined, bristled, apiculate, rarely blunt-tipped; peduncles 0.6-1.4 cm. straight

                                               or slightly curved, axillary and terminal; fruits 2.3-4 mm, keeled; uncommon, alkaline

                                               waters…………………………………………………………………………..…..P. hillii

                                 36.  Nodal glands present……………..…………………….……………………………………38

                                         38.  Stipules fibrous, often whitish or brown………………. ………….…………………..39

                                                39.  Winter buds with leaves and stipules 90° to each other; northern alkaline waters….

                                                        ………………………...……….…………………………………….……..P. friesii

                                                39.  Winter buds with leaves and stipules in same plain……………………………….40 

                                                        40.  Leaves with 3 bold veins and ± 30 additional faint veins; stem broad

                                                               0.6-3.2 mm wide, extremely flattened; fruit 4-5 mm long, northern U. S. and

                                                               Canada…………………………………………………………P. zosteriformis

                                                        40.  Leaves with 3-13 veins; stems; narrow, less than 1 mm wide, not flattened;

                                                               fruits 1.4-2 mm long…………………………………………  …………..…41

                                                               41.  Veins 3(-5), marginal vein faint; leaf tips acute; fruits 1.4-2.5mm long

                                                                       with distinct keel to 0.2 mm, uncommon, often in warm spring waters of

                                                                       western U.S …..……………….……………..………P ×TURIONIFERA                             

                                                                              previously identified as P. FOLIOSUS SUBSP. FIBRILLOSIS

                                                                 41.  Veins 3-9(-13), marginal vein bold; leaf tips acute, cuspidate or bristled;

                                                                         fruits 1.9-3 mm long, lacking distinct keel…………………………….42

                                                                          42.  Fruits 2.5-3 mm long, infrequently formed; peduncles terminal and

                                                                                 axillary; leaves 3-9(13)-veined; rare in northeast U.S. and southern

                                                                                 Canada………………………………………………..…..P. ogdenii

                                                                           42.  Fruits 1.9-2.1 mm long, frequently formed; peduncles terminal;

                                                                                   leaves 3-5(-7)-veined; northern alkaline waters……P. strictifolius

                                         38.  Stipules not fibrous, green, brown, or white……………………………………………43

                                                43.  Leaf veins 7-17…………………………………………………………………….44

                                                       44.  Fruits 3-4 mm long, peduncles 1.7-3 cm long, axillary; leaves 1.5-2 mm wide

                                                               with 1-2 row of lacunae on either side of mid-vein, marginal vein bold;

                                                               northern Canada and Alaska……………………………..…..P. SIBiRICUS

                                                             44.  Fruits 2.6-2.7 mm long, peduncles to 2 cm. long, terminal; leaves 0.7-1.7 mm

                                                               wide with 0-3 rows of lacunae on either side of mid-vein, marginal vein faint;

                                                               Greenland…………………………………..……..……P. GROENDLANDICUS

                                                     43.  Leaf veins 3-5 ….………………………………..………………………..……….45

                                                        45.  Fruits 2.5-3.6 mm long; winter buds 3.5-7.8 cm × 2.3-5.1 mm; leaves of ten

                                                                reddish; neutral to alkaline waters, n. U.S. and Canada………..P. obtusifolius                                                                

                                                        45.   Fruits 1.9-2.8 mm long; winter buds  0.9-3.2 cm  × 0.3 -1.8 mm; leaves not

                                                                 reddish……………………………………………………………………….46

                                                                 46.  Stipules mostly connate (surrounding stem); peduncles usually terminal;

                                                                         common in alkaline waters…………………..………………..P. pusillus

                                                                 46.  Stipules convolute (wrapped around stem); peduncles axillary and

                                                                         terminal………………..……………………………………………….47

                                                                             47.  Fruits 2.5-2.8 mm with basal tubercles and bumpy surface; leaves         

                                                                                    3-5- veined; endemic to Davis Mts., w. Texas ……..……………                                                                                   

                                                                                    ……………..…P. BERCHTOLDII SUBSP. CLYSTOCARPUS



                                                                       47.  Fruits 2-2.5 mm, lacking with basal tubercles and bumpy  

                                                                                     surface; leaves 3-veined; common in acidic to neutral waters …..

                                                                                     ………………………….………………………..P. berchtoldii

            26.   Leaves broadly linear-oblong, lanceolate, elliptic, or subcircular ……………………………..48

                             48.  Submersed leaves petiolate ……………………….……………………………………..49

                           49.  Submersed leaf margins crispate; stems rusty to black-spotted…….………………………50

                                   50.  Stems black-spotted; submersed leaves, 9-21-veined, 10-25 (-35) mm wide, slightly                

                                          arcuate; fruits 3.1-4.1(-4.7) mm with distinct dorsal keel; acidic water of coastal and               

                                          southern U. S…………………………..………..………………...P. pulcher (in part)

                                   50.  Stems often rusty-spotted; submersed leaves 27-49-veined, 25-75 mm wide, arcuate;

                                          fruits 3.9-5.2(-5.7) mm with indistinct keel; common, northeastern U.S…….….…….

                                           ………………………………………………………………..…P.amplifolius (in part)

                           49.  Submersed leaf margins rarely crispate; stem lacking spots………………………………..51

                                   51.  Fruits (1.6-)2-2.5 mm, lacking well-developed abaxial keel; submersed leaf margin

                                          entire; eastern Nfld and Cape Sable Is.,N. S …………P. POLYGONIFOLIUS (inpart)


                                   51.   Fruits 2.5-4.3 mm, with well-developed abaxial keel; submersed leaf margins                 

                                            minutely denticulate to entire; widespread……………………………………..…….52

                                            52.  Submersed leaf petioles (7-)11-15 cm long,  leaves with 2-5 rows of lacunae on

                                                    either side of mid-vein; leaf tips acute to rounded; widespread in alkaline waters            

                                                    and acidic rivers in New England………………………..…..P. nodosus  (in part)

                                            52.  Submersed leaf petioles 0.5-4 cm long, leaves with 1-2 rows of lacunae on either

                                                    side of  mid-vein; leaf tips acute; (rarely petiolate) widespread in alkaline waters

                                                    of the United States …………………………..………….…P. illinoensis (in part)

                   48.  Submersed leaves sessile or clasping stem………………………………………………..……..53

                           53.  Leaves sessile……………………………………………………….………………………54

                                   54.  Stems branched; submersed leaf margins mostly denticulate, tip acute to obtuse,

                                           stipules acute …………………………………………………………………………55

                                           55.   Leaves 3-9-veined with 1-2 rows of lacunae on either side of mid-vein, fruits

                                                   1.9-2.3 mm with beak 0.3-0.5 mm long; common………..P. gramineus (in part)

                                           55.   Leaves 7-19-veined with 2-5 rows of lacunae on either side of mid-vein. fruits

                                                   2.5-3.6 mm with beak 0.5-0.8 mm long, common in alkaline waters……………..

                                                   ………………………………………………………………P. illinoensis (in part)

                                   54.  Stems unbranched; submersed leaf margin entire, tip blunt, stipules blunt; ……………

                                           …………………………………………………………………….   P. alpinus (in part)

                                        53.   Leaves clasping stem………………………………………………………………………..56

                                                56.  Leaf tip with hoodlike tip, splitting when pressed; margins entire; stem "zig-zag" at

                                                       each node; fruits 4-5.7 mm long with beak 0.6-1 mm; deep water, deep neutral to

                                                       alkaline waters………………………….………………….………………P. praelongus



                                               56.  leaf tip not hoodlike and splitting when pressed, margins mostly denticulate; stem not

                                                      "zig-zag" at each node; fruits1.6-4.2 mm with beak 0.4-0.7mm…………………….57    

                                                      57.  Stipules delicate and deteriorating with age; leaves broadly lanceolate, orbiculate,

                                                              or ovate; fruits1.6-3 mm long; neutral waters, northeastern N.A. and south coast

                                                              ………………………………………………………..…….……….P. perfoliatus

                                                      58.  Stipules disintegrating to persistent fibers; leaves ovate-lanceolate to narrowly

                                                              lanceolate; fruits 2.2-4.2 mm long; alkaline waters throughout North America…..

                                                              …………………………………………………….………….……P. richardsonii

* All North American species are covered.  Those not found in the northeast and Great Lakes region are in CAPITALS.  Hybrids are extremely common in the Potamogetonaceae.  Hybrids involving P. gramineus, P. illinoensis, P. richardsonii, P. perfoliatus are common.  Hybrids in Stuckenia are common outside the northeast and are difficult to identify, thus hybrids within Stuckenia are keyed out here. These hybrids typically occur in fast-flowing rivers.  (Prepared by C. Barre Hellquist)

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith