2. Carduus nutans L. (musk thistle, nodding thistle)

Pl. 250 g–i; Map 1039

Stems 40–300 cm long, not brittle, glabrous or finely hairy between the ribs, usually felty-hairy toward the tip. Leaves and stem wings armed with relatively hard, straw-colored spines. Basal leaves 15–60 cm long, lanceolate to broadly elliptic, glabrous or less commonly sparsely to moderately hairy on both surfaces. Stem leaves 3–30 cm long, lanceolate to broadly elliptic, the lobes mostly triangular to ovate, glabrous or less commonly sparsely to moderately hairy on both surfaces, occasionally somewhat glaucous. Heads solitary, all or mostly nodding, 2–7(–8) cm in diameter, the stalk usually long and relatively naked (a few small, scattered, bracteal leaves often present), less commonly short (normal-sized leaves present nearly to the head), usually densely felty-hairy. Involucral bracts 15–50 mm long (including the spiny tip), 2–8 mm wide, lanceolate to narrowly ovate (often slightly constricted in the basal 1/2), the outer and median ones spreading to reflexed above the midpoint, gradually or more commonly abruptly tapered to a hard, straw-colored or occasionally purple, spiny tip, the surfaces glabrous or felty- to cobwebby-hairy. Pappus 13–18 mm long, white. Corollas 18–30 mm long. 2n=16. June–October.

Common nearly throughout the state, but very undercollected (native of Europe, Asia, widely introduced nearly throughout the U.S. and adjacent Canada). Upland prairies and potentially glades; more commonly pastures, crop fields, fallow fields, old fields, fencerows, roadsides, railroads, and open, disturbed areas.

This species was introduced in the mid-nineteenth century in the eastern United States, possibly as a contaminant in ballast dumped from ships (Stuckey and Forsyth, 1971). By the early twentieth century, it was recognized widely as a noxious weed in pastures and crop fields. Its spread into Missouri was slow. Although musk thistle was declared a noxious weed by the state legislature in 1909, the first infestation in Missouri was not recorded until 1941 (Marion County). Steyermark (1963) knew it from only four widely scattered counties, including the St. Louis and Kansas City metropolitan regions. Since that time, it has spread into probably every county using roadsides and railroads as dispersal corridors and as a contaminant in hay. Control of musk thistle involves spraying with a foliar herbicide (best during the rosette stage) or digging up rootstocks in combination with mowing prior to fruiting. The U.S. Department of Agriculture also has attempted biological control of the species. Beginning in 1979 in Missouri (earlier elsewhere), controlled releases were carried out involving two small insects found to be predators of the species in Europe: rosette weevils of the Trichosirocalus horridus (Panz.) complex and a flower head weevil, Rhinocyllus conicus Froel. (Puttler and Bailey, 2001). Unfortunately, although these insects can be effective in controlling the spread of Carduus species, it has since been shown that Rhinocyllus conicus also attacks many North American species of the thistle genus Cirsium (Louda, 2000). In Missouri, the effects of so-called nontarget infestations have not been studied in detail, but anecdotal observations by a number of botanists suggest that Cirsium muticum, which is of restricted distribution in the state, has been impacted the most adversely. The release of Rhinocyllus as a biological control agent in North America without sufficient evaluation of its host range has been among the most widely cited examples of the dangers involved in the deliberate introduction of foreign organisms into natural ecosystems in America.

Within its native range, a number of subspecies of C. nutans appear to be separable. However, the relationship of these to populations introduced in North America is not clear. The following key serves to separate typical examples of the variation:

1. Involucral bracts tapered relatively evenly to the spiny tip ... ssp. macrolepis

1. Involucral bracts tapered relatively abruptly to the spiny tip

2. Heads with relatively long, leafless stalks, the stems below these glabrous or sparsely hairy between the ribs; leaves glabrous or both surfaces sparsely hairy ... ssp. leiophyllus

2. Heads short-stalked, the stems more or less persistently hairy and often densely felty-hairy, at least toward the tip; leaves moderately hairy on both surfaces ... ssp. macrocephalus

McGregor (1986e), in dealing with populations in the Great Plains, where some of the largest infestations occur, noted that the degree of tapering of the bract tips varies greatly, even on different heads of a single individual, and also was more gradual in plants flowering the first growing season. He therefore chose not to recognize ssp. macrolepis (Peterm.) Kazmi within his region. He contrasted the more common ssp. leiophyllus (Petrovic) Stoy. & Stef. and the mostly western ssp. macrocephalus (Desf.) Nyman, noting that intermediates were relatively uncommon. Musk thistle is greatly underrepresented in the Missouri herbaria, making it difficult to evaluate the infraspecific variation present in the state. Most specimens of well-developed plants appear to represent ssp. leiophyllus, as keyed above. However, occasional specimens, especially from the St. Louis and Kansas City metropolitan areas, appear to key better to ssp. macrocephalus. Missouri materials mostly have glabrous or very sparsely hairy leaves, but the density and persistence of stem pubescence appear to be more variable than in plants of the Great Plains. Efforts should be made to sample more populations from throughout the state to address the question of whether musk thistle subspecies can be separated in Missouri.