Potamogeton obtusifolius (Blunt-leaved Pondweed)

Plant Info
Also known as:
Genus:Potamogeton
Family:Potamogetonaceae (Pondweed)
Life cycle:perennial
Origin:native
Habitat:part shade, sun; shallow to deep water; moderate to soft water lakes, ponds, rivers
Bloom season:July - September
Plant height:1 to 3 feet
Wetland Indicator Status:GP: OBL MW: OBL NCNE: OBL
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: 4-petals Flower shape: indistinct Cluster type: spike

[photo of spike arrangement] Short, head-like cluster held above or below the surface of the water, up to ½ inch (to 13 mm) long, arising from the upper leaf axils. Spikes commonly have 3 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green to brownish, ladle-shaped, sepal-like appendage.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaf tip and veins] Leaves are all submersed and more or less spirally arranged along the stem; no floating leaves are produced. Blades are flexible, green to reddish, linear, 1 to 4 inches long, 1 to 4 mm wide, rounded at the tip often with a minute point (mucronate), toothless, not wavy along the edges. The midvein is prominent with 1 to 3 narrow rows of large, empty cells (known as the lacunar band) along each side and flanked by a single pair of lateral veins.

[photo of stipules and glands] Leaf bases are stalkless and not clasping, with a pair of greenish to yellow to orange glands .2 to 1 mm wide at the nodes. At the base of the leaf is a pale, membranous appendage (stipule), not connected to the leaf blade, blunt at the tip and not usually shredding, less than ¾ inch (to 18 mm) long but may be longer than the internodes. Stems are slightly compressed, branched, and may zig-zag somewhat between the leaf nodes. Rhizomes are mostly absent.

[photo of turions] Numerous vegetative buds (turions), also known as winter buds, develop later in the summer at branch tips. Turions are tightly wrapped in several leaves forming a spindle-shaped structure 1¼ to 3 inches long, less than ¼ inch (to 5.1 mm) diameter, and flanked by leaves on opposite sides of the branch (2-ranked), 3 or 4 leaves per side. Turions drop off, overwinter, and form new plants the next season. Dense mats may result.

Fruit: Fruit type: seed_without_plume

[photo of developing fruit] Fruit is a dry seed (achene), olive to brown when mature.

[photo of achenes] Achenes are irregularly oval, 2.5 to 4 mm long, usually with a low, somewhat knobby keel along the back edge, but is sometimes absent. The beak is erect and up to 1 mm long.

Notes:

Blunt-leaved Pondweed is occasional in central and northeastern Minnesota, usually found in the quiet or slow-flowing waters of lakes, ponds and rivers. It is recognized by its stalkless linear leaves 1 to 4 inches long, 1 to 4 mm wide, flexible but not wavy along the edges, 3 veins, a rounded tip sometimes with a minute point at the apex; a pair of glands at the leaf nodes; persistent pale stipules less than 1 inch long, not attached to the leaf blade and not shredding; achenes 2.5 to 4 mm long usually with (sometimes without) a low, slightly knobby keel along the back edge; spindle-shaped turions at branch tips flanked by 2-ranked leaves, 3 or 4 per side. Floating leaves are absent.

This is one of several Pondweeds with no floating leaves and linear, submersed leaves, the widest between 1 and 4 mm wide: Potamogeton foliosus lacks glands at the leaf nodes, leaves have pointed tips, and stipules are attached to the blades; P. friesii, has leaves with 5 to 7 veins and its turions have leaves that are on 2 planes (4-ranked); P. pusillus leaves have pointed tips, stipules are fused together on the lower half, and turions have 2-ranked leaves 1 to 3 per side; P. spirillus (often without floating leaves) lacks turions and stipules are attached to leaf blades for half the stipule length; P. strictifolius has stiff, straight leaves pointed at the tip and usually lacks glands at the leaf nodes; P. zosteriformis has a conspicuously flattened stem and lacks glands at the leaf nodes. P. hillii, present in Wisconsin, is also similar and has pointed or bristle tips on leaves and achenes with 3 low keels.

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More photos

Photos by K. Chayka and Peter M. Dziuk taken in Becker County.

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