Leaves and stems:

The sterile stem is green and usually has whorled branches that are spreading to ascending. The “leaves” are reduced to a sheath that surrounds the stem, with 12 to 24 narrow, black-brown teeth around the top that occasionally have a narrow band of white around the edge.

The stem is green, stout and hollow with a large central cavity. The first branch internode is shorter than the stem sheath.

Fruit:

Fertile stems are like the sterile stems, but with a ½ to 1-inch, blunt-tipped cone at the tip of the stem. The cone matures in summer and falls off after spores are released.

Notes:

Water Horsetail can create very large, dense colonies, often in the quiet waters of lakes and ditches. There are 2 other Equisetum species that have spreading to ascending branches: Field Horsetail (Equisetum arvense) and Marsh Horsetail (E. palustre). Of these, only E. fluviatile has a stem with a large central cavity so it compresses easily when squeezed. When branches are absent, E. fluviatile resembles Tall Scouring Rush (E. hyemale) or Smooth Scouring Rush (E. laevigatum), both of which have sheath teeth that usually drop off and do not persist. E. fluviatile hybridizes with E. arvense and their offspring, Equisetum × litorale, may be expected where the parents are both present. The hybrid closely resembles E. palustre but the first branch internode is longer than the associated stem sheath, where on E. palustre it is shorter. When trying to determine if the first branch internode is longer or shorter than the stem sheath, it is important to look at the lowest branch on the stem as those higher up the stem may have different lengths.