Callitriche stagnalis is an aquatic perennial with elongate stems that can grow from 3.94-11.81 in. (1-3 dm) in length.
Foliage
Its leaves are opposite, entire and somewhat diverse in form, depending on whether they are submerged or floating. The floating leaves are spatulate-obovate in shape, have 5 to 7 veins, are 0.12-0.31 in. (3-8 mm) wide and up to 0.79 in. (2 cm) long. The submerged leaves are typically linear, have a single vein and grow 0.16-0.39 in. (4-10 mm) in length; however, they can also be broader and more closely resemble the floating leaves.
Flowers
Callitriche stagnalis is monoecious. The staminate and pistillate flowers are tiny, simple in structure and typically located right next to one another in the leaf axils. This close proximity facilitates aerial self-pollination. The flowers have two small bracts at their bases, which may help the flowers to float on the water, also facilitating pollination.
Fruit
The fruits are nearly round (suborbicular) and 0.06-0.08 in. (1.5-2 mm) thick. Each mericarp of the fruit has a thin (0.004 in. / 0.1 mm) winged margin running from its base to its apex.
Ecological Threat
Callitriche stagnalis is capable of creating locally dense mats of vegetation that may crowd out native aquatic vegetation.