Potamogeton robbinsii Oakes
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Family: Potamogetonaceae
Genus: Potamogeton
Species Synonyms: none
Common Names: Robbins’s pondweed
flat-leaf pondweed
fern pondweed
Canada: southwestern British Columbia, central Alberta, north – east-central Saskatchewan, southeastern Manitoba – western Ontario – Quebec – New Brunswick – Nova Scotia
Saskatchewan: north – east-central Saskatchewan; Cluff Lake - Lac la Ronge – Prince Albert National Park – Porcupine Hills
Ecoregion: Mid-Boreal Upland, Mid-Boreal Lowland, Churchill River Upland
Saskatchewan: shallow water of quiet lakes and ponds
Associated Species: flat-stem pondweed, small pondweed, water milfoil, white water crowfoot, yellow pond lily
Provincial Status According
to Harms (2003):
Nature Conservancy Status:
G5 S2
Saskatchewan Species at
Risk Status:
Robbin’s pondweed is threatened because it is rare or uncommon in Saskatchewan. It is usually locally numerous within limited areas. No threats are known or anticipated for this species.
Height: to 100 cm
Roots: roots fibrous; rhizomes present
Stems: branched, round, stout, without spots, glands absent; overwintering shoots absent
Leaves: submersed, crowded, 2-ranked, sessile, 2 – 7 cm long, 3 – 4 mm wide, linear to lance-shaped, base rounded and lobed, stiff, air spaces absent, dark green to reddish-green, veins 20 – 60, margin minutely spiny to saw-toothed; stipules fused to the base of the blade and to each other, greenish brown to white, fibrous, shredding at tip
Inflorescence: spikes uniform, like a string of beads, 7 – 20 mm long, often in branched clusters 3 – 5 cm long, axillary, erect, cylindrical
Flowers: small, greenish; tepals 4; stamens 4; carpels 4
Fruits: one-seeded, and fleshy, stalked, brown, ridged, beak erect
1 Stipular sheaths of submersed leaves fused to base of leaf blade; leaf margins cartilaginous
P. robbinsii
1 Stipular sheaths of submersed leaves free from base of leaf blade; leaf margins not cartilaginous
2 Submersed leaves oblong to lance-shaped to nearly round
2 Submersed leaves ribbon-like or thread-like
3 Leaf margins conspicuously saw-toothed and wavy; overwintering stems present
P. crispus
3 Leaf margins entire or wavy; overwintering stems absent
4 Submersed leaves clasping; floating leaves absent
4 Submersed leaves not clasping, may be stalked; floating leaves present or absent
5 Stipules persistent; rhizome with rusty spots
P. praelongus
5 Stipules shredding into fibres; rhizome not obviously spotted
P. richardsonii
6 Floating leaf blade rounded to heart-shaped; stem rusty or black-spotted; submersed leaves arced
P. amplifolius
6 Floating leaf blade wedge-shaped or rounded; stems without spots; submersed leaves not arced
7 Submersed leaves stalked
P. nodosus
7 Submersed leaves sessile
8 Fruits plump, stalked; submersed leaf blade 7 – 9-veined
P. alpinus
8 Fruits compressed, not stalked; submersed leaf blade 3 – 19-veined
P. gramineus
9 Floating leaves present on at least come plants in a population
9 Floating leaves absent
10 Air spaces in leaf tissue present and prominent; submersed leaves with distinction between blade and stalk; leaves of two types, the floating ones long-stalked and wider than the submersed ones
P. epihydrus
10 Air spaces in leaf tissue absent or not prominent; blade and petiole not well-differentiated on submersed leaves; leaves uniform, no distinction between the floating and submersed leaves
P. natans
11 Nodes without glands
11 Nodes glandular
12 Leaves 15 – 35-veined, greater than 2 mm wide; stem wing-flattened
P. zosteriformis
12 Leaves 3 – 5-veined, less than 2 mm wide; stem not wing-flattened
P. foliosus ssp. foliosus
13 Stipules fibrous, whitish
13 Stipules not fibrous, green, brown or whitish
14 Leaf tip tapered to pointed, leaves thin, 5 -7-nerved; inflorescence stalks at most 5 cm long
P. friesii
14 Leaf tip bristle-tipped; leaves firm, 3-nerved; inflorescence stalks to 9 cm long
P. strictifolius
15 Fruits 2.5 – 3.5 mm long
P. obtusifolius
15 Fruits 1.5 – 2 mm long
16 Mature fruits widest at the top; inflorescence stalks 1 – 3 per plant; leaf blade with less than 2 rows of air spaces along midrib
P. pusillus ssp. pusillus
16 Mature fruits widest at the middle; inflorescence stalks more than 3 per plant; leaf blade with 1 – 5 rows of air spaces along midrib
P. pusillus ssp. tenuissimus