Lemna trisulca is a species of aquatic plants in the genus Lemna with a subcosmopolitan distribution, occurring in quiet, freshwater habitats in cool, temperate regions. L. trisulca normally does not occur in warm, temperate regions.
Narrow-leaf duckweed, when it isn't flowering, usually floats in open water a little below the surface. On rare occasions, when this plant produces flowers, it floats on the surface of water. A non-flowering plant consists of a flat frond about 6-12 mm. long and 2-5 mm. across (excluding the basal stalk). This frond is ovate-oblong or lanceolate-oblong in shape and mostly toothless along the outer margin, except toward its tip, where poorly defined small teeth occur. The base of the frond tapers abruptly into a narrow and flat basal stalk that is 8-20 mm. long when it is fully developed. The frond of a flowering plant is similar, except it is more oval in shape and it has a shorter basal stalk.