Equisetum fluviatile, the water horsetail or swamp horsetail, is a vascular plant that commonly grows in dense colonies along freshwater shorelines or in shallow water in ponds, swamps, ditches, and other sluggish or still waters with mud bottoms.
Equisetum fluviatile is a perennial herbaceous species, growing 30–100 cm (rarely 140 cm) tall with erect dark green stems 2–8 mm in diameter, smooth, with about 10–30 fine ridges. At each joint, the stem has a whorl of tiny, black-tipped scale leaves 5–10 mm long. Many, but not all, stems also have whorls of short ascending and spreading branches 1–5 cm long, with the longest branches on the lower middle of the stem. The side branches are slender, dark green, and have 1–8 nodes with a whorl of five scale leaves at each node. The water horsetail has the largest central hollow of the horsetails, with 80% of the stem diameter typically being hollow. This horsetail is sometimes seen as an invasive species because it is very hardy and tends to overwhelm other garden plants unless it is contained.