Najas flexilis, commonly known as slender naiad, is an aquatic annual herb native to clear, oligotrophic lakes, slow-flowing rivers, and ponds in northern Europe and North America. Slender naiad has a delicate appearance, with thin, branching stems and narrow, translucent leaves that can give a bushy aspect under good growing conditions. It flowers from late summer to early autumn, producing inconspicuous greenish flowers that are submerged and not showy.

Slender naiad is valued for its role in aquatic ecosystems, providing habitat for fish and invertebrates, and oxygenating the water. It is often used in aquariums and water gardens for its fine-textured foliage and ease of maintenance. In cultivation, it requires submerged conditions in full sun to part shade, with clean, nutrient-poor water to mimic its natural habitat. While generally not prone to diseases, it can be sensitive to water quality changes and may compete with algae under certain conditions. Slender naiad is considered rare and is protected in parts of Europe, with significant populations such as the one in Lake Sīveri, Latvia, being of conservation interest.CC BY-SA 4.0