They grow attached to the leaf axils and float on the water surface. Male flowers have three white to red narrow petals about 2 mm long and three white, red or brown sepals about 3 mm long by 2 mm wide. They are also formed in the leaf axils, but break free of the plant when mature and float to the water surface. On the surface the male flowers expel pollen which is then caught by the female flowers. In North America, all dioecious plants are female.
Hydrilla spreads via underground rhizomes and above ground stolons.
It also forms vegetative propagules called tubers and turions. These propagules are a characteristic unique to hydrilla amongst the Hydrocharitaceae. Turions are compact buds produced in the leaf axils or, in the case of monoecious plants, on stem tips. They break off the parent plant and drift or settle to the bottom to start a new plant. They are 5 - 8 mm long, dark green and appear spiny.