Some 25 species of annual and perennial horsetails in the Equisetaceae family of plants grow in wetlands and marshy, wet areas at the edge of swamps, ponds and lakes. Horsetails are vascular plants that reproduce by spores, not seeds, and by spreading underground stems called rhizomes. The common field horsetail (Equisetum arvense) is a perennial that grows up to 2 feet tall.
Horsetails, related to ferns, are considered living fossils. Fossils of horsetail the size of trees have been found in coal beds that date to the Paleozoic era, 251 to 542 million years ago. Horsetails grow two hollow stems. In the early spring horsetails grow single stems each of which bears a reproductive cone called a strobilus. A horsetail strobilus is ¾ to 1 ½ inches long. Later in the spring, horsetails grow vegetative stems that have whorls of slender, jointed-branches. What appear to be small, scale-like leaves grow in a whorl at the base of each stem forming a sheath. These are not true leaves; horsetails conduct photosynthesis through chlorophyll in their stems. An unbranched, fertile shoot of the common field horsetail is from 4 to 6 inches tall. The infertile shoots are from 6 inches to 2 feet tall. Scouring rushes (Equisetum hyemale L.) resemble horsetails but do not have branches. Mare's tail (Hippuris vulgaris) also resemble horsetails.
The cone-like reproductive strobilus on the top of the thick horsetail stem releases spores in the form of a light yellow powder. The spores burrow into the soil. Branching, underground horsetail rhizomes grow up to 3 feet deep and contain nodes that can grow new plants.
Because of the high concentration of silica in their tissue, horsetails have been traditionally used by Europeans, Native Americans and American pioneers to polish furniture and scrub pots and cooking utensils. This silica has been extracted for use in diuretic medicines. More than 20 percent horsetail in the forage can paralyze or kill horses.
Eliminating horsetails by digging them up is nearly impossible because their rhizomes are brittle and break easily; each piece can sprout and grow into a new plant. Horticulturalists at the University of Minnesota recommend triclopyr, a systemic herbicide. You can use triclopyr near wetlands but not in the water. You should contact your local agricultural extension office or department of natural resources before you use triclopyr in a wetland.