10. Salsola L. (Russian thistle, tumbleweed)

(Mosyakin, 1996; Rilke, 1999)

Plants annual (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.

The vernacular 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.