pondweed, common name for the family Potamogetonaceae, and for weedy aquatic herbs of the genus Potamogeton, of which about 50 known species inhabit North American ponds and slow streams. They have slender stems and small spikes of inconspicuous flowers appearing above the water. A few species are cultivated—some as waterfowl food and for fish protection in conservation projects. The saltwater eelgrass (genus Zostera ) of gently sloping shores is closely related. Pondweeds are classified in the division Magnoliophyta, class Liliopsida, order Najadales.