The bright green stems of this horsetail are quite attractive. Intermediate Horestail 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 2–3½' long and about ½" across; these stems are evergreen throughout and their joints are conspicuously grey. Smooth Horsetail has stems that are more narrow (about ¼" across) and entirely deciduous; the stems of this horsetail are often bunched together in loose clusters, while the stems of Intermediate Horsetail and Scouring Rush are more evenly spaced (a consequence of their long rhizomes). For a sterile hybrid, Intermediate Horsetail is surprisingly common within the state.

This native perennial plant is 1–2½' tall and usually unbranched. Each plant consists of a central stem that has several cylindrical joints along its length; the upper half of each stem is deciduous and becomes greyish brown during the winter, while the lower half is evergreen. Each joint is several inches long and about 1/3" in diameter; its apex consists of a circular sheath, from which the base of the next joint develops. Each joint has 15-30 fine ridges along its length; it is medium green and its outer surface is slightly rough. The interior cavity of each joint is very large, extending across about 80% of the joint's diameter. The circular sheath at the apex of each joint is about ½" in length; it has 15-30 greyish black teeth along its upper rim. These tiny teeth are actually leaves that have been reduced to scales; they are usually early deciduous. The base of each sheath is usually green, although some lower sheaths become grayish brown with age. The central stem rarely branches. Each fertile shoot terminates in a spore-bearing cone at its apex. This spore-bearing cone is smooth or pointed at the top (depending on its maturity), and it is sessile or short-stalked. The infertile shoots have the same appearance as the fertile shoots, except that they never produce spore-bearing cones. These cones are produced by the fertile shoots from late spring to early summer. Because this horsetail is a sterile hybrid, its spores are abortive. The root system consists of long spreading rhizomes with secondary roots. Intermediate Horsetail often forms vegetative colonies.

Equisetum × ferrissii Clute:
Canada (North America)
United States (North America)

Canada

Origin: Unknown/Undetermined

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

United States

Origin: Unknown/Undetermined

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

Intermediate Horsetail is occasional to locally common in central and northern Illinois, but it is uncommon or absent in the southern part of the state. Habitats include moist sand prairies, sloughs and prairie swales, riverbanks, marshes and 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.

Type collection for Equisetum hyemale var. intermedium A.A. Eaton
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): C. Dodge
Year Collected: 1896
Locality: Port Huron., Michigan, United States, North America

Type collection for Equisetum hyemale var. intermedium A.A. Eaton
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): W. N. Suksdorf
Year Collected: 1902
Locality: Bingen., Washington, United States, North America

Type collection for Equisetum hyemale var. intermedium A.A. Eaton
Collection: Smithsonian Institution, National Museum of Natural History, Department of Botany
Preparation: Pressed specimen
Collector(s): C. Dodge
Year Collected: 1896
Locality: Port Huron., Michigan, United States, North America

Intermediate Horsetail is occasional to locally common in central and northern Illinois, but it is uncommon or absent in the southern part of the state. Habitats include moist sand prairies, sloughs and prairie swales, riverbanks, marshes and 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.

Horsetails (Equisetum spp.) provide cover for small mammals and wetland birds, otherwise they are little used. The foliage is mildly poisonous to mammalian herbivores 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 reportedly eat horsetails.

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 3
Species: 3
Species With Barcodes: 1

Canada

Rounded National Status Rank: NNA - Not Applicable

United States

Rounded National Status Rank: NNA - Not Applicable

Rounded Global Status Rank: GNA - Not Applicable

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.