10. Salsola L. (Russian thistle, tumbleweed)
(perennial herbs or shrubs elsewhere), the roots not tuberous-thickened. Stems
erect to loosely ascending, not succulent, not appearing jointed,
much-branched, sparsely pubescent with stiff, short, unbranched, pustular-based
hairs, occasionally appearing glabrous at maturity. Leaves alternate or
occasionally opposite toward the stem base, well developed, succulent, sessile.
Leaf blades linear, unlobed, circular to elliptic in cross-section, the base
clasping the stem, the tip with a slender, somewhat spinelike extension of the
midvein, the margins entire but pubescent with minute, spreading, unbranched,
pustular-based hairs (thus sometimes appearing finely toothed), the surfaces
glabrous or sparsely hairy, sometimes slightly glaucous. Inflorescences spikes,
terminal on the branches, the flowers solitary (rarely 2 or 3) per node, not
sunken into the axis. Bracts longer than and more or less enclosing the flower,
the middle bract only slightly longer than the lateral pair, narrowly
triangular, spreading or ascending, tapered to a stiff, hard, spinelike tip.
Flowers perfect. Calyx deeply 5-lobed or of 5 free sepals spaced around and
tightly enclosing the ovary, the tips sometimes erect or somewhat spreading,
becoming papery or somewhat hardened at fruiting, with a continuous horizontal
ridge or wing above the midpoint (the fruit including calyx thus appearing
concave at the tip). Stamens 3–5. Ovary superior. Style 1, short, the
stigmas 2(3), linear to narrowly club-shaped. Fruits broadly obovoid to
obconic, usually indehiscent, the wall somewhat fleshy, becoming membranous at
maturity. Seed adhering loosely to the fruit wall, positioned horizontally,
1.5–2.0 mm in diameter, somewhat flattened, the surface smooth, black,
shiny, the strongly coiled embryo usually apparent. About 130 species; native
to Europe, Asia, Africa; introduced nearly worldwide.
name tumbleweed refers to the habit of the spherical plants dispersing their
seeds by breaking off at the base after fruits have matured and rolling across
the countryside in response to breezes. Russian thistles are among the worst
range weeds in North America. The primary problem species is S. tragus,
which is present in nearly every state and throughout southern Canada, but S.
collina occurs sporadically in the central half of the United States as
well. Salsola tragus was present along the eastern seaboard as early as
1788, but its development into a noxious weed occurred only after a second
introduction into South Dakota around 1873, possibly as a contaminant in flax
seed from Russia (Rilke, 1999). From there it spread rapidly in all directions,
expanding its range through the Great Plains and subsequently crossing the
Rockies into the western states. Salsola collina apparently was a later
introduction and was first recorded from Minnesota in 1938 (Rilke, 1999).
Steyermark (1963) noted that the plants can be harvested for hay during times
of drought and also have helped to stabilize eroded soils in some regions, but
Russian thistle is blamed for the degradation of millions of hectares of
rangeland in this country. Thus the tumbleweed of Western novels and films is
actually Salsola, an invasive exotic species. In addition to lowering
the productivity of rangelands, tumbleweeds also impact fields when the dry,
spiny plants roll through them (Karpiscak and Grosz, 1979), depositing seeds
that sprout along the apparent furrows into linear arrays of plants the
following year. In the southwestern United States, tumbleweed trails are
sometimes so noticeable that they can be mapped using aerial photography.