Flower:

Dense cylindrical spike held above the surface of the water, 1 to 3 inches (to 8 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 6 to 10+ whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, all spirally arranged. Floating leaves are firm, green, lance-elliptic to egg-shaped, 1 to 3 inches (2.5 to 6 cm) long, up to 1 inch wide, toothless, rounded to wedge-shaped at the base, softly to sharply pointed at the tip, with 9 to 19 veins. The leaf stalk is rather longer than the blade, usually the same color for its entire length.

Submersed leaves are thread-like, 2 to 6+ inches (5 to 16 cm) long, .25 to 1 mm wide, lax, blunt to pointed at the tip, somewhat narrowed at the base, stalkless, with 3 veins, usually bright green. Submersed leaves are sometimes reduced to bladeless stalks.

At the base of the leaf is a membranous appendage (stipule), not connected to the leaf blade, 1 to 1½ inches (to 4 cm) long, whitish, the tip rounded and not shredding. Stems are very slender, ~1 mm wide, and may be red-spotted. Loose colonies may be formed from creeping 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 irregularly oval, 2.5 to 3.5 mm long, with a prominent smooth keel along the back and a pair of more obscure lateral keels. The beak is short and erect.

Notes:

Oakes' Pondweed is rare in Minnesota. According to the DNR, after 20 years surveying over 2000 lakes, P. oakesianus was only found in 22 of them, making this one of the rarer aquatics in the state. Its habitat preference is in the shallow, quiet waters of soft water lakes and ponds and is most at risk from degraded water quality from various human activities, including herbicide use and nutrient enrichment from run-off. It was listed as Endangered in 2013. It is a fairly delicate thing with long, lax, thread-like submersed leaves (not more than 1 mm wide), lance-elliptic to egg-shaped floating leaves rounded to wedge-shaped at the base, and achenes 2.5 to 3.5 mm long with 3 smooth keels; the floating leaves are often mixed in among other aquatic species and may go unnoticed.

Potamogeton oakesianus is most similar to P. natans, which is more robust in all respects, has stiff submersed leaves up to 2.5 mm wide, larger floating leaves heart-shaped at the base with a stalk that's pale at the tip and strongly bent where it meets the blade, and its achenes lack any distinct keels. The floating leaves resemble those of some other Pondweeds as well, but their leaf stalks may be shorter than the blade, submersed leaves may be broader, and/or achenes may have knobby keels or none altogether.