Time of bloom: July to September.
Seed-time: Late August to November.
Seeds of this thistle have been noted by Alfalfa growers as a common impurity in Alfalfa seed, especially of states of the Ohio River Valley. They are so nearly the same weight of alfalfa seed as to make removal difficult. When in the soil the seeds have a vitality of about three years, and are a most undesirable acquisition.
Stem stout, rigid, erect, fifteen to thirty inches tall, widely branched, gray with loose woolly hair, and broadly winged by the decurrent bases of the leaves; these are also gray-woolly, the lower ones pinnate with terminal segment large (lyrate), and lateral lobes narrow with wavy or sparsely toothed edges; upper leaves small, entire, nearly linear, but all strongly decurrent. Heads terminal, solitary, more than an inch broad, bright yellow; involucre broadly ovoid or nearly globular, the inner row of its bracts ending in shining, scarious tips; the intermediate row armed with rigid, yellow, divergent spines nearly an inch long, with one or two shorter ones at the base; and the outermost row having short, palmately branched spines. Achenes light-colored, smooth, shining, with soft, white pappus much longer than the achene. (Fig. 358.)
Fig. 358 - St. Bar-naby's Thistle (Centaurea solstitialis). X 1/6.
Prevent seed production, either by cutting while in first bloom or by wholly uprooting. The labor of hand-pulling in alfalfa and grain fields is worth the cost, if expensive seed is saved from contamination and the ground is kept from being fouled for another season.