Equisetum laevigatum

Native

Wet meadows, prairies.

12″ to 36″

This is an erect, 12″ to 36″ tall, unbranched perennial that rises from rhizomes.

The stems are erect, ⅛″ to 5 ⁄16″ in diameter, light green, and hollow. They are normally unbranched, but may develop one or a few branches after injury, or in the second year. They have 10 to 32, usually at least 16, fine, vertical ridges that are smooth to the touch. They are annual, lasting less than one year. The central cavity is ⅔ to ¾ the diameter of the stem. 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 a lighter shade of green than the stem, with a black band at the tip only, sometimes also the lowest ones with a black band at the base or black throughout. They are longer than wide, ¼″ to ⅝″ long, ⅛″ to ⅜″ wide. At the tip of the sheath are 10 to 32, usually at least 16, free lobes appearing as tiny, black teeth. The teeth are jointed and fall off at the joint promptly at maturity, leaving just a dark rim on the sheath.

A solitary, spore-bearing cone is borne at the end of each fertile stem. The cone is ⅜″ to 1″ long, circular in cross-section, and elliptic in long section, and rounded at the tip. It often ends with a tiny, inconspicuous, abrupt, flexible point at the tip (apiculate). Infertile stems are similar to fertile stems but lack the terminal cone. The cone falls off after releasing spores.

Scouring rush horsetail (Equisetum hymale var. affine) is a taller plant, usually reaching 24″ to 48″ tall. The stems are darker green, rough to the touch, and evergreen. The sheaths appear squarish, are tan, grayish, or white, and have a black band at both the base and the tip. The teeth on the sheath often persist after maturity. The cones are conspicuously apiculate.

Variegated scouring rush (Equisetum variegatum var. variegatum), is a shorter plant, only 4″ to 18″ tall. The stems are thinner, 1 ⁄32″ to 3 ⁄32″ in diameter. The sheaths are slightly flared outwards at the tip. There are 5 to 12 vertical ridges on the stem and the same number of teeth on the sheath. The teeth have conspicuous white margins. The teeth are not jointed and usually persist. The cone is shorter, ⅜″ long or less. It is found only in the upper third of the state.

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. smooth scouring rush is one of the eight species in the subgenus Hippochaete. Four of those eight species are found in North America. All four of those also occur in Minnesota.

In this subgenus, the stems are unbranched or have few irregular branches. They are coated with an abrasive silica, and were used for scrubbing cooking pots. This, along with their rush-like appearance, gave rise to the common name “scouring rush”.