Flower:

Two forms of flowering spikes are produced. Dense cylindrical spikes on stalks up to 1 inch long, with up to 8 whorls of flowers, are held above the surface of the water at the tip of the stem and arising from the axils of floating leaves. Shorter, nearly round spikes on very short stalks (to 3mm), with only 1 or 2 whorls of flowers, arise from the axils of submersed leaves. Flowers have a 4-parted style surrounded by 4 stamens, each stamen with a green to orange-brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, though floating leaves are few or may be absent. Floating leaves are firm, green, oval-elliptic, less than 1½ inches (to 3.5 cm) long, up to ½ inch (to 13 mm) wide, toothless, rounded to bluntly pointed at the tip, rounded to tapering at the base, on a stalk shorter than the blade. Blades have 5 to 15 veins.

Submersed leaves are more or less spirally arranged, green to reddish, linear and ribbon-like, often arching or coiled, up to 3 inches long, .5 to 2 mm wide, bluntly to sharply pointed at the tip, stalkless, and have 1 to 3 veins. The midvein is flanked by a few to several narrow rows of large, empty cells (known as the lacunar band), the band sometimes extending nearly to the leaf edge.

At the base of the leaf is a membranous appendage (stipule), connected to the leaf blade for more than half the stipule length, green to reddish, up to ½ inch long, the tip somewhat rounded and not shredding. Stems are flattened and branched. Colonies are often formed from spreading rhizomes. Vegetative buds (turions) are not produced. Glands at the leaf nodes are absent.

Fruit:

Fruit is a dry seed (achene), the flowering spikes forming densely packed seed heads, greenish-brown when mature.

Achenes are nearly round, 1.3 to 2.4 mm long, with a bumpy keel along the back edge and a strong spiral shape on each side, conforming to the embryo shape. A beak is lacking.

Notes:

Spiral Pondweed, known as Potamogeton dimorphus in older references, is common in the northeastern quadrant of Minnesota, usually in the shallow, quiet waters of soft water lake and pond margins, most often on sandy or boggy shores, less often in flowing water or deeper than 3 feet. It is distinguished by flattened stems; stipules connected to the leaf blade for more than half the stipule length; small oval-elliptic floating leaves less than 1½ inches (7 to 35 mm) long on stalks shorter than the blades; submersed leaves stalkless, flexible and ribbon-like (often arching or coiled) to 3 inches long, .5 to 2 mm wide, commonly with a broad lacunar band along each side of the midrib; round achenes with a knobby keel along the back, a strong spiral shape on the sides, and no discernible beak. The floating leaves are usually few and often absent but the numerous submersed, small flowering/fruiting spikes should be evident.

Potamogeton spirillus is one of 3 Pondweeds that have 2 forms of flowering spikes (emersed and submersed), the others are P. bicupulatus and P. diversifolius, both of which are quite rare in Minnesota and have submersed leaves that are much more thread-like, less than .5 mm wide.