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. It 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.