Description: 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.
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 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.
Faunal Associations: 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.
Photographic Location: A disturbed
grassy meadow along an abandoned railroad near Urbana, Illinois. The
grassy-looking plants in the background of the above photograph are
Equisetum arvense (Field Horsetail).
Comments: 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.