Equisetum arvense

Native

Moist to moderately dry.

This is an erect, bushy perennial that rises on separate fertile and unfertile stems from long, branched, creeping rhizomes with fleshy tubers. The rhizomes are similar to the aerial stems but are not hollow. They extend to a depth of 40″ or more. The tubers are ¼″ to ½″ in diameter and are arranged singly or in pairs along the rhizomes.

Fertile stems are produced in early spring, late April to early May. They are erect, 4″ to 8″ tall, up to ⅓″ in diameter, unbranched, brittle, and succulent. They resemble an asparagus sprout but are brownish because they lack chlorophyll. The leaves are reduced in size, fused together for part of their length, and appressed against the stem, forming a collar-like sheath around the nodes. The sheaths are ½″ to ¾″ long. They have fewer than 14 large, 3 ⁄16″ to ⅜″ long, dark brown to black teeth along the top rim. The teeth may be separate or they may be stuck together, but not fused, into more than 4 small groups. A solitary, spore-bearing cone is borne on a stalk at the end of the stem. The cone is 3 ⁄16″ to 1⅜″ long, circular in cross-section, elliptic in long section, and rounded at the top. It is the same brownish color as the stem the stalk to which it is attached. It is covered with spore-bearing tubercles. The tubercles are darker brown to black with white markings. After the spores are shed the fertile stems die back.

Infertile stems are produced in late spring, after the fertile stems have wilted, and die back in the fall. They are erect, 6″ to 24″ tall, 1 ⁄16″ to 3 ⁄16″ in diameter, hollow, and green. The central cavity is about ¼ the diameter of the stem. The stems have 4 to 14 vertical ridges, though they usually have no fewer than 10 ridges. The ridges are rough to the touch but do not have silica deposits. The stems are segmented by up to 20 nodes about 1″ apart. At the top of all but the uppermost segment is an appressed, ring-like sheath. The sheaths are 3 ⁄16″ to ⅜″ long, green, with a narrow black band at the base and wider black band at the tip. At the top rim of the sheath are 4 to 14 brown teeth, the number of teeth equaling the number of stem ridges. The teeth are about 1 ⁄16″ to ⅛″ long. They may be separate or they may be partially fused together. They are persistent, remaining after maturity.

A whorl of slender branchlets is produced at each node. The branchlets are 3- or 4-angled, solid, ascending or sometimes horizontal, never drooping, and usually do not themselves branch. Both 3-angled and 4-angled branchlets may occur on the same stem. Like the stems, the branchlets have segments with sheaths. The first segment of each branchlet is longer than the sheath below the node from which it extends. The branchlet sheath has only 3 or 4 teeth along the top rim, the number of teeth equaling the number of branchlet ridges. By mid-summer the branchlets may become as long as the stem.

Meadow horsetail (Equisetum pratense) has fertile stems that are initially yellowish-brown and unbranching, but then turn green and sprout branches after the spore-producing cones have disappeared. The stems have short, pointed, epidermal projections made of silica. The branches are 3-sided and horizontal to drooping.

Woodland horsetail (Equisetum sylvaticum) has branches that are themselves branched. The teeth of the leaf sheaths are reddish brown.

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. field 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.