3. Centaurea diffusa Lam. (diffuse knapweed, tumble knapweed)
Pl. 251 g; Map
1042
Plants annual or
less commonly perennial, usually with a taproot, pubescent with cobwebby hairs
when young, not appearing pale or whitened, sometimes nearly glabrous at
maturity. Stems 20–90 cm long, erect or ascending with loosely ascending or
spreading branches at or above the midpoint, somewhat angled but not winged.
Leaves 2–20 cm long, often dotted with minute, yellow to brown resin glands;
basal and lower stem leaves with the blades mostly 12–80 mm wide, oblanceolate
to obovate, rounded or angled to a bluntly pointed tip, tapered gradually to a
sessile or short-petiolate, usually somewhat expanded base, 2 times pinnately
lobed, the margins otherwise entire or finely toothed; median and upper stem
leaves gradually reduced, mostly sessile, the base sometimes slightly clasping
the stem, not or only slightly decurrent, the blades linear to
oblong-oblanceolate, entire or rarely toothed or lobed, occasionally 2 times
pinnately lobed. Heads solitary at the branch tips. Involucre 10–13 mm long,
longer than wide, narrowly ovoid to narrowly ovoid-cylindrical. Lower and
median involucral bracts with the body ovate to narrowly ovate, the margins
fringed with small, slender, straw-colored spines, the outer surface glabrous
or slightly cobwebby-hairy, not concealed by the appendages; the apical
appendage well differentiated, ascending, narrower than the main body, not or
only slightly overlapping, straw-colored or pale greenish-tinged, the involucre
often entirely straw-colored, the margins with a fringe of slender, spreading
teeth and a short central spine, this 1–3 mm long. Upper involucral bracts
lanceolate, the appendages not well differentiated from the relatively broad
(at least toward the tip), thin, white, papery margins, the tips usually
sharply pointed, irregularly toothed or narrowly lobed. Florets all discoid,
the marginal florets appearing discoid but functionally sterile. Pappus absent
or of minute, bristly scales, these 0.1–0.5 mm long, white, usually persistent
at fruiting. Corollas 9–13 mm long, cream-colored to nearly white, rarely
pinkish- or purplish-tinged. Fruits 2.5–3.5 mm long, somewhat flattened, the
attachment scar appearing lateral, the surface usually dark brown, usually
glabrous and somewhat shiny. 2n=18. June–September.
Introduced,
known thus far only from the city of St. Louis (native of Europe; introduced
widely in the western and midwestern U.S. and adjacent Canada, also sporadically
in the northeastern U.S.). Railroads.
Steyermark
(1963) reported this species based on collections made by Viktor Mühlenbach in
the St. Louis railyards. However, some of Mühlenbach’s collections and also a
later one from Howard County are actually the fertile hybrid between C.
diffusa and C. stoebe, which has been called C. Hpsammogena
Gáyer. This hybrid is variable, but generally differs from C. diffusa in
its less divided leaves with narrower lobes (more like those of C. stoebe),
slightly broader heads with well-developed marginal raylike florets, and
involucral bracts with a shorter central spine at the tip (Ochsmann, 1997,
2001b). True C. diffusa is represented by only two of Mühlenbach’s
several voucher specimens. The species is a widespread weed in the northern and
western United States and Canada that may have been spread into Missouri by the
transport of hay or livestock.