Water-willow is a rooted emergent plant that spreads by rhizomes to form large colonies.  Colonies are mostly found along edges of rivers in shallow water, and less often along the marshy shores of lakes or ponds.

The leaves are opposite, liner to lanceolate and tapered at the base.  Flowers appear in dense spikes that are longer than the surrounding leaf.  The blooms are pollinated by various insects.  Flowering occurs from late June to September.  The fruit is a capsule that bursts open when ripe propelling the seeds some distance from the plant.