Equisetum fluviatile

Native

Moist. Marshes, shallows, springs, water less than 40″ deep.

June to August

14″ to 40″

This is an erect, semi-aquatic, emergent aquatic perennial that rises from widely creeping rhizomes. It often forms dense colonies.

The stems are erect, 14″ to 40″ tall, 1 ⁄16″ to 5 ⁄16″ in diameter, hollow, stiff, and green to dark green. The central cavity is 8 ⁄10 to 9 ⁄10 the diameter of the stem. The stems have 15 to 25 fine, vertical ridges. The ridges are smooth to the touch but do not have silica deposits. They are annual, lasting just one year. The portion of the stem between the nodes is up to several inches near the bottom, becoming progressively shorter as they ascend the stem.

The leaves are reduced in size, fused together for most of their length, and appressed against the stem, forming a collar-like sheath around the nodes. The sheaths are green to brownish-green, with a black band at the tip only. They are ⅛″ to ⅜″ long. At the tip of the sheath are 12 to 24 free lobes appearing as tiny, dark brown or black teeth. The teeth are narrow, 1 ⁄16″ to 3 ⁄32″ long, and occasionally have a white border. The teeth are not jointed and usually persist. As they age the sheath and teeth become ash gray, and the sheath often develops a narrow black band at the base.

A whorl of slender branchlets is sometimes produced at the middle nodes. The branchlets are 4- or 6-angled, solid, ascending or sometimes horizontal, never drooping, and usually do not themselves branch. Like the stems, the branchlets have segments with sheaths. The first segment of each branchlet is shorter than the sheath below the node from which it extends. The branchlet sheath has 4 to 6 narrow teeth along the top rim, the number of teeth equaling the number of branchlet ridges.

Spore cones appear in June to August at the end of the main stem or occasionally on upper branches. They are attached to the stem on a long stalk. They are yellowish-green, blunt, and ⅜″ to ¾″ long. They wither away after shedding their pollen.

Marsh horsetail (Equisetum palustre) stems are rough to the touch and have 4 to 8 vertical ridges. The hollow in the stem center is less than ⅓ the diameter of the stem. The cone is longer, ¾″ to 1½″ long.

Taxonomy
There are 15 species of Equisetum, which is the only living genus in the family Equisetaceae, which is the only family in the order Equisetales, which is the only order in the class Equisetopsida. The history of Equisetum has been traced 300 million years to the Cretaceous period, and possibly to the Triassic period. That could make Equisetum the oldest living genus of vascular plants.

The genus Equisetum is divided into two subgenera, Equisetum and Hippochaete. water horsetail is one of the eight species in the subgenus Equisetum. Six of those eight species are found in North America. Five are found in Minnesota.

In this subgenus, the stems tend to be regularly branched, the branches appearing in a whorl at each stem node. This gives the plant the appearance of a horse’s tail, giving rise to the common name “horsetail”. The aerial stems of most of the species in this subgenus are annual.