Horsetails are an ancient group of plants, little changed over 300 million years. The
genus Equisetum means “horse bristles,” and refers to the plants’ unusually high levels
of silica. They are often classified with ferns, but they are a distinct group of
vascular plants. Like ferns, they reproduce by spores, not seeds.
Although Equisetum includes all the now-living relatives of these plants, extinct relatives
reached 100' (30 m) in height as far back as the Carboniferous era, and were dominant
species during the late Paleozoic Era. All horsetails prefer damp to wet freshwater soils.
Biological Glassworks
Horsetails are pretty improbable-looking: tall
thin stems, some as hollow as soda straws, almost hairlike branches.
How do they achieve enough stiffness? Might be because they are glassmakers.
They incorporate
silica, tiny little glass beads, into their structure. Horsetails
absorb silicic acid—Si(OH)₄—a mild acid present in small quanitites in
the wet soil. They convert it to silica (SiO₂), depositing it in stems
and branches. That’s what makes most horsetails feel rough to the
touch, and abrasive enough to sand or polish
wood and metal, or to scour pots. Today this
“biogenic silica,” extracted from horsetails, is used in abrasives, toothpaste,
protective cloth, optical fibers, and paint thickeners.
The silica is deposited as a thin layer of tiny, round, tightly packed glass beads, on
the outer parts of the stem and branches. The beads form an effective barrier against fungal
infections.
Tiny, dark brown sharp-tipped leaves surround the stem at nodes. Photo by Enrico Blasutto.
Each stem node is wrapped with tiny, black, sharp-tipped scale leaves. Branch nodes also have miniscule leaves that appear as black dots. The leaves do not perform photosynthesis; this is done by the stem and branches. Cropped from a photo by Luc Viatour.
Each stem is punctuated by nodes surrounded by sheaths. The sheaths are actually tiny leaves, fused to the stem.
Stem
Fertile stems, tipped by conelike structures, are pale brown, erect, and unbranched. They are 6-12" (15-30 cm) tall. They appear early in the season and are gone by May. Sterile stems are green, 6-24" (15-60 cm) tall, ¹/₁₆-³/₁₆" (3-5 mm) in diameter, with 4-14 ridges. They appear after the fertile stems have come and gone. They are highly variable, usually heavily branched in a series of upward-pointing whorls, usually erect, sometimes prostrate. Branches are thin, shaped like a plus sign in cross-section. Stem and branch surfaces feel rough.
In dense colonies, unbranched or multiply branched. Fertile and sterile stems look the same. 14-46" (35-116 cm) tall, with dark green stems ¹/₁₆-¼" (2-8 mm) in diameter. If branches are present, they are less than 3" (7.6 cm) long, with 8 or fewer nodes. Each segment boundary is ringed by a series of tiny, black-tipped scale leaves. The stems of water horsetails are entirely hollow, and smoother than marsh horsetails. The stems pull apart easily at the joints.
Typically a single, rough-textured unbranched, dark green stem, 7-86" (17-218 cm) tall. Rarely has a few branches that tend to be nearly attached to the vertical stem.
Fruit
Fertile stems have cones (strobili) 1-1½" (2.5-3.8 cm) long in the spring. Photo by Kristian Peters.
The spore-bearing, conelike strobili are yellow-green, ⅜-¾" (1-2 cm) long. Photo by joelpk@flickr.
Conelike structures at stem tips, strobili, are ½-1" (1.3-2.5 cm), with sharp tips. This one is immature.
Range/ Zones
USDA Zones: 4-9
Habitats
Fields, woods, marshes, roadsides, especially damp areas in partial shade
Sheaths appearing at nodes along the stem contain 8-10 tiny, non-photosynthetic leaves, shaped like sharp dark spears.
Tiny, non-photosynthetic leaves called scales are fused to the stem at branch nodes, or sheaths. Sheaths are ¹/₁₆-⅛" (2-6 mm) long, with 8-10 brown, white-edged teeth.
Non-photosynthetic leaves are in thin sheaths of three (rarely four), black or dark brown in color, wrapped around the narrow stems. The sheaths have a white edge.
Stem
Plants are evergreen, rough-textured, 4-20" (10-50 cm) tall. They have 4-12 (usually 8-10) ribs, and are ¹/₃₂-⅛" (1-3 mm) around. In cross section, stems show vallecular canals about the same size as a central hollow region. The central hollow is much less than half the diameter of the stem. Fertile and sterile stems look similar, except for a cone at the tip of the fertile stems, vs. a stem that tapers to a point in sterile stems. The first sheath on the branch (aka first internode) has 5 or 6 narrow teeth, and the first branch internode is shorter than the stem sheath. Fertile and sterile stems also emerge at about the same time.
Thin light green fragile-looking branches, in symmetric whorls, <24" (60 cm) in height. Branches tend to be straight, or to droop somewhat, while in other species the branches are ascending. Stems are ¹/₃₂-⅛" (1-4 mm) in diameter, with 8-18 ridges. In cross-section, up to half the diameter of the stem is hollow, more than marsh horsetail. Fertile stems are 8-10" (20-25 cm) tall, and pale pink or brownish at first, tipped by a cone-like structure. After fruiting, fertile stems become green and develop branches, looking like the sterile stems. Sterile stems are 8-24" (20-60 cm) tall, and very rough.
The smallest horsetails, often occurring in low, twisted, tangled-looking mats, intertwined with mosses and grasses. 1-8" (2.5-20 cm) tall. Stems are unbranched, but thin enough to look like branches of other horsetails. Fertile and sterile stems are evergreen, with six broad ridges. They occur together and look almost identical, except that the fertile stems are a bit more erect, and tipped by a small black conelike structure; while the sterile stems are more curved and unstructured. Stems have a central hollow less than a third of the diameter, smaller than most horsetails.
Fruit
A conelike structure about ¾-1" (2-3 cm) long, the strobilus, appears at the tip of each fertile stem.
Fertile stems are tipped by a single blunt-tipped cone-like strobilus, ½-1½" (1.5-4 cm) long, atop a stalk. Spores are released in April.
Fruits are a narrow conelike structure called a strobilus, appearing at the tip of each fertile stem. Each “cone” is ¹/₁₆-⅛" (2-5 mm) long.
Tiny, non-photosynthetic leaves form small sheaths that encircle nodes along the stem. There are 3-14 tiny, sharp-pointed black leaves, often called teeth, per sheath. Each sharp black tooth is surrounded by white. The teeth tend to persist all season, and they are sometimes tipped with hairlike bristles.
Stem
Usually stems are erect, though they may bend under their weight. Sterile stems are evergreen, rough, 3-19" (7.6-48 cm) tall, ¹/₃₂-⅛" (1.5-4 mm) in diameter, dark green and unbranched. They have 3-12 vertical ridges. In cross section, the stems have a central cavity that is ¼ to ⅓ of the diameter of the stem. Vallecular canals, which ring the central cavity, are a bit less than half the diameter of the central cavity. Fertile stems resemble the sterile stems, but there is a “cone” atop each stem.
Fruit
A “cone”—strobilus—appears at the tip of each fertile stem. It is green, with a black, sharp-pointed tip, and ⅛-¼" (5-8 mm) long. The spores are green, and spherical.
ID is uncertain 
