Intermediate Horsetail
Equisetum ferrissii
Horsetail family (Equisetaceae)

Description: This perennial spore-bearing plant is 1–2' tall and usually unbranched. Each plant consists of a jointed central stem that is about 1/3" (8 mm.) across; it is more or less erect. This stem is medium green, cylindrical, and hollow; a larger interior cavity extends to about 80% of the stem's diameter. The stem also has 15-32 fine ridges extending along its length; the stem feels rough-textured and gritty when these ridges are rubbed across. Each joint of the stem extends for several inches or centimeters. At the junction of each pair of joints, there is a sheath up to " long with 15-32 fine teeth along its upper side. These teeth are early-deciduous and they soon turn black. Directly underneath the teeth, there is usually a narrow band of gray, while the rest of the sheath is green and inconspicuous. However, a broad band of gray or grayish brown often develops on older lower sheaths. Short ascending lateral branches can develop from the upper central stem of a plant, but this happens rarely. The stems of this horsetail are usually semi-evergreen; they die down slowly (from top to bottom) during the winter. 

Each fertile stem terminates in a spore-bearing cone at its apex. These sessile or short-stalked cones are up to 1" long, ovoid in shape, and either rounded or apiculate at their apices (i.e., they may taper abruptly to form short pointed tips). Immature cones are olive green, but they may become pale yellow or pale red at maturity. Each cone has several rows of angular sporangiophores (spore-bearing structures) that become increasingly dark-colored and elevated with age. Fertile stems usually produce spore-bearing cones during late spring or early summer for about 3-4 weeks. Because this hybrid horsetail is sterile, the released spores of its cones are abortive. As a result, this hybrid horsetail spreads asexually only by means of its long rhizomes. Secondary fibrous roots develop from these rhizomes. Clonal colonies of plants are often produced.

Cultivation: The preference is full sun, wet to mesic conditions, and poor soil containing gravel, clay, or sand (especially the latter). Intermediate Horsetail will also grow in fertile soil, but it dislikes competition from taller plants. This plant tolerates both acidic and alkaline soil, as well as significant variations in moisture levels. In open sunny areas, it can spread aggressively.

Range & Habitat: Intermediate Horsetail is occasional to locally common in central and northern Illinois, but it is rare or absent in the southern part of the state (see Distribution Map). Habitats include moist sand prairies, sloughs and prairie swales, riverbanks, marshes, shores of ponds, disturbed grassy meadows, gravelly areas along railroads, and roadside ditches. This plant can be found in both degraded and higher quality habitats.

Faunal Associations: Relatively few insects feeed 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 and wetland birds, otherwise they are little used. 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. The grassy-looking plants in the background of some photographs are Equisetum arvense (Field Horsetail).

Comments: The bright green stems of this horsetail are quite attractive. Intermediate Horsetail (Equisetum ferrissii) is a sterile hybrid of Equistem hyemale var. affine (Scouring Rush) and Equisetum laevigatum (Smooth Horsetail). Its physical appearance is intermediate between these two species and it can be difficult to distinguish this hybrid from its parents. Generally, Scouring Rush is a slightly larger horsetail with stems that are 2–3' long and about –" across; its stems are evergreen throughout and the joints of its stems are conspicuously grey or black. Smooth Horsetail usually has stems that are more narrow (about " across) and entirely deciduous; when its stems are rubbed against, they feel smooth because its ridges are rounded, rather than angular. For a sterile hybrid, Intermediate Horsetail is surprisingly common within the state.