Potamogeton robbinsii (Robbin's Pondweed)
|Also known as:|
|Habitat:||part shade, sun; shallow to 20+ feet deep water; soft water lakes, ponds, slow flowing rivers|
|Bloom season:||June - August|
|Plant height:||20 to 40 inches|
|Wetland Indicator Status:||GP: OBL MW: OBL NCNE: OBL|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Small cylindrical spike, weakly held above or near the surface of the water, ¼ to ~¾ inch (to 20 mm) long at the tips of branching stems. Spikes have 3 to 5 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green, paddle-shaped, sepal-like appendage.
Leaves and stems:
Leaves are all submersed with no floating leaves produced, alternately attached on opposite sides of the stem (2-ranked), ascending to spreading and often arching (recurved), usually closely spaced but may be more widely spaced on flowering stems. Blades are firm and somewhat stiff, dark green to reddish, lance-linear, ¾ to 2¾(4¾) inches long, 1/8 to 1/3 inch (3 to 8 mm) wide, pointed at the tip and minutely toothed around the edges, at least near the tip. The midvein is conspicuous yellow, flanked by 20 to 60 lateral veins, about 5 of which are prominent and the rest faint.
Leaves are stalkless with a pair of rounded lobes (auricles) at the base that do not clasp the stem. A translucent, membranous appendage (stipule) is connected (adnate) for about ¼ its length at the leaf base, the tip free and shredding. Stems are round, without spots, few-branched on the lower plant, branching frequently in the upper plant when flowering. Colonies are often formed from creeping rhizomes. Vegetative buds (turions) are not produced. Glands at the leaf nodes are absent.
Fruit is a dry seed (achene) but is not often produced, the flowering spikes showing mostly aborted fruit. Fruit matures from green to brown.
Achenes are irregularly oval, 3 to 5 mm long, a conspicuous, sharp, cock's-comb like keel along the back edge flanked by a pair of more obscure, smooth, rounded, lateral keels. The short, abrupt beak is erect or recurved at the tip.
If you've ever put your face underwater in one of the thousands of lakes in central or northern Minnesota, you may have spotted Robbins' Pondweed. Its not uncommon at all to pull these up on your anchor; it's one of Minnesota's more common and easily recognized aquatic species. Found in waters 2 to 20+ feet deep, it can create large, dense colonies in mid to deeper depths with dark green foliage and crowded, strongly 2-ranked leaves, giving it a fern frond appearance.
Most references note that it rarely flowers or produces fruits, and the flowering period is sometimes noted as late summer into fall. Without knowing this, we found it flowering the last week of June and ran into fruits several times in one summer. When flowers or fruits are present they further distinguish this species by the few to many upper branches each tipped a single, small spike—it's the only pondweed with such branching.
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- Potamogeton robbinsii plant
- Potamogeton robbinsii plants
- a colony of Potamogeton robbinsii, with P. amplifolius
- flowering stems are branched
Photos by Peter M. Dziuk taken in Aitkin, Beltrami, Kanabec and Lake counties.
Have you seen this plant in Minnesota, or have any other comments about it?