Coefficient of Conservatism:
Coefficient of Wetness:
TUMBLE KNAPWEED, WHITE-FLOWERED KNAPWEED
Roadsides and dry fields, and other disturbed, usually sandy places.
Centaurea diffusa seems to hybridize with C. stoebe subsp. micranthos virtually whenever they come into contact to produce the fertile hybrid C. ×psammogena G. Gáyer. Plants with black-tipped phyllaries having a spine at the tip are apparently hybrids, as are plants otherwise like C. diffusa, but with a short pappus. Other combinations of characters occur as well. Centaurea stoebe has a rare white form (collected in Charlevoix, Iosco, and Kalamazoo Cos.), and C. diffusa, a rarer pink one, so flower color alone will not discriminate although it may be a clue in a given population where hybridization is occurring.
The hybrid and C. diffusa grade insensibly into one another, and for convenience, the map includes both ± pure C. diffusa (specimens in MICH from Benzie, Charlevoix, and Cheboygan Cos.) and C. diffusa-like (white-flowered) hybrids (which were mostly referred to C. diffusa in Michigan Flora). The first indication that C. diffusa occurred in Michigan was the collection of C. ×psammogena in Kalamazoo Co. in 1943, but it quickly spread northward (if not already there) to become locally common along with C. stoebe. Within a decade it was well established north to the Straits of Mackinac, often abundant over great areas.