Smooth Horsetail
Equisetum laevigatum
Horsetail family (Equisetaceae)

Description: This perennial plant is –3' tall and usually unbranched. Each plant consists of a central stem that has several cylindrical joints along its length; it is deciduous, dying down to the ground before the end of winter. Individual joints are a few inches long, up to 10 mm. in diameter, medium green, and glabrous; adjacent joints along the stem are interconnected by ring-like sheaths. The joints have 14-30 fine longitudinal ridges that are smooth to slightly rough-textured. The interior cavity of each joint is very large, extending across at least 70% of the joint's diameter. Individual sheaths are 1" long or less; they have 14-30 grayish black teeth (3 mm. or less in length) along their upper rims; except for the teeth of the uppermost sheath, these teeth are usually early-deciduous The small teeth are actually leaves that have been reduced to scales. The sheaths are mostly green, except for narrow bands of broken-off teeth along their upper rims that are whitish black. However, some older sheaths toward the bottom of the stem may become gray. The central stem rarely branches. However, an uncommon form of Smooth Horsetail, f. prolifera, produces narrow cylindrical branches in whorls along the upper one-half of the central stem.

Each fertile shoot terminates in a spore-bearing cone at its apex. This spore-bearing cone is up to 1" long, ovoid in shape, and usually rounded at the top, rather than apiculate (abruptly tapering to a short narrow point); it is either sessile or short-stalked. Immature cones are mostly olive green, but they may become yellow or pale red as they approach maturity. Each cone has several rows of angular sporangiophores (spore-bearing structures) that become increasing dark-colored and more elevated with age. The infertile shoots have the same appearance as the fertile shoots, except that they never produce spore-bearing cones. These cones are produced by fertile shoots during late spring or early summer for about 3-4 weeks. After releasing their spores to the wind, they become dark brown or black and gradually wither away. The root system consists of spreading rhizomes with secondary fibrous roots. Smooth Horsetail often forms clonal colonies of plants.

Cultivation: The preference is full sun, moist to mesic conditions, and poor soil containing gravel, clay, or sand (especially the latter). Smooth Horsetail will also grow in fertile soil, but it dislikes competition from taller plants. Both acidic and alkaline conditions are tolerated, as well as significant variations in moisture levels.

Range & Habitat: The native Smooth Horsetail is occasional to locally common in central and northern Illinois, while in the southern section of the state it is uncommon or absent (see Distribution Map). Habitats include moist to mesic sand prairies, sloughs, riverbanks, disturbed grassy meadows, ditches, and gravelly areas along railroads (including the ballast). This plant can be found in both degraded and higher quality habitats; tolerance to occasional wildfires is high.

Faunal Associations: Relatively few insects feed on horsetails (Equisetum spp.). Among them, the aphid Anoecia equiseti has been found to feed on the roots of Smooth Horsetail (Equisetum laevigatum); see Blackman & Eastop (2013). The larvae of several sawflies (Dolerus spp.) feed on the stems of horsetails, including Dolerus apricus and Dolerus tibialis conjugatus (Smith, 2006; Eastman, 2003). The larvae of a weevil, Grypus equiseti (Horsetail Weevil), develop within the stems, and the leafhopper Macrosteles borealis also feeds on these plants (Harms & Grodowitz, 2009; Eastman, 2003). Most of these insects are found primarily in boreal regions where horsetails are more common. Because horsetails often form dense colonies of clonal plants, they provide protective cover for small mammals, birds, and other vertebrate wildlife, otherwise they are little-used by these animals. The foliage is mildly poisonous, especially to horses, because it can cause thiamine deficiency. The foliage also contains high levels of silicate compounds; this makes it rather coarse and unpalatable to most mammalian herbivores, although moose occasionally feed on horsetails (Eastman, 2003).

Photographic Location: A disturbed grassy meadow along an abandoned railroad near Urbana, Illinois.

Comments: This is my favorite Horsetail. It is less weedy and aggressive than Equisetum arvense (Field Horsetail) and Equisetum hyemale var. affine (Scouring Rush). Horsetails can be divided into two large groups: those that produce whorled secondary branches versus those that produce unbranched stems. Field Horsetail belongs to the former group, while Scouring Rush and Smooth Horsetail belong to the latter group (although Smooth Horsetail may produce secondary branches on rare occasions). The stems of Scouring Rush are a darker shade of green and more rough-textured, while the stems of Smooth Horsetail are usually a lighter shade of green and more smooth-textured. Scouring Rush is usually a larger plant with stems that are less hollow; its sheaths are often dark-colored or gray, while those of Smooth Horsetail are inconspicuously green. The cones of Scouring Rush are apiculate at their apices, while those of Smooth Horsetail are more likely to be rounded at their apices. Sometimes these two species hybridize, producing plants with intermediate characteristics. Such plants are referred to as Equisetum ferrissii (Intermediate Scouring Rush). Another common name for Equisetum laevigatum is Smooth Scouring Rush.