Fl. Londin. 2(6,61): plate 55. 1789.
Annuals, 20–200 cm. Stems simple or openly branched, loosely tomentose with fine single-celled hairs and villous with curled, septate hairs; teeth of wings to 25 mm, wing spines to 15 mm. Leaves: basal tapered to winged petioles, blades 10–25 cm, margins pinnately 6–10-lobed, abaxial faces tomentose, adaxial faces loosely tomentose and villous or ± glabrate; cauline sessile, shorter, less divided. Heads clustered in ± tight arrays of 5–20+ at ends of stems, usually sessile, 15–22 mm × 7–12 mm. Involucres cylindric to ellipsoid (appearing campanulate when pressed), 15–20 × 7–12 mm. Phyllaries linear-lanceolate, bases appressed, 2–2.5 mm wide, ± glabrate, and ascending, appendages 0.5–1.5 mm wide, narrowly scarious-margined, distally glabrous or minutely ciliolate, spine tips 1–2 mm, inner phyllaries with erect, straight, unarmed tips. Corollas pinkish, 10–14 mm; lobes 1.5–2.5 times longer than throat. Cypselae brown, 4–5 mm, finely 10–13-nerved; pappus bristles 10–15 mm. 2n = 54.
Phenology: Flowering spring–early summer (Apr–Jul).
Habitat: Aggressive weed of waste ground, pastures, roadsides, fields
Elevation: 0–1000 m
Introduced; Calif., Oreg., Pa., s Europe (Mediterranean region).
Carduus tenuiflorus has been reported from New Jersey, Texas, and Washington; I have not seen specimens from those states.
Carduus pycnocephalus and C. tenuiflorus are similar annuals with small, usually tightly clustered heads. The number of heads per capitulescence is usually ultimately greater in C. tenuiflorus, but early season plants of this species often have only a few heads. At the end of the growing season the fruiting heads of C. tenuiflorus are aggregated in dense, subspheric clusters. Stem wings tend to be more pronounced in C. tenuiflorus. Fresh corollas of C. pycnocephalus are rose-purple whereas those of C. tenuiflorus have a more pinkish tinge, but this difference is subtle and not reliable on herbarium material. The phyllaries of C. tenuiflorus are membranous-margined, more or less glabrate, and lack the short, stiff, upwardly appressed trichomes of C. pycnocephalus. All published chromosome counts for Carduus tenuiflorus from both Old and New World material are the same.
The two species sometimes grow in mixed populations and at times appear to intergrade. Hybridization has been reported in Europe (S. W. T. Batra et al. 1981) and is suspected to occur in California. Hybrids between C. pycnocephalus and C. tenuiflorus have been designated Carduus ×theriotii Rouy.