Flora of North America species comparison

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Centaurea diffusa

centaurée diffuse, diffuse knapweed, diffuse or tumble or white knapweed, tumble knapweed, white knapweed
Habit Annuals or perennials, 20–80 cm.
Stems

1–several, much-branched throughout, puberulent and ± gray tomentose.
Leaves

hispidulous and ± short-tomentose;

basal and proximal cauline petiolate, often absent at anthesis, blades 10–20 cm, margins bipinnately dissected into narrow lobes;

mid cauline sessile, bipinnately dissected;

distal much smaller, entire or pinnately lobed.
Involucres

narrowly ovoid or cylindric, 10–13 × 3–5 mm.
Florets

25–35;

corollas cream white (rarely pink or pale purple), those of sterile florets 12–13 mm, slender, inconspicuous, those of fertile florets 12–13 mm.
Inner phyllaries

lanceolate, ± acute, appendage lacerate or spine-tipped.
Heads

disciform, in open paniculiform arrays.
Cypselae

dark brown, ca. 2–3 mm;

pappi 0 or less than 0.5 mm, only rudimentary.
Principal

phyllaries: bodies pale green, ovate to lanceolate, glabrous or finely tomentose, with a few prominent parallel veins, margins and erect appendages fringed with slender stramineous spines, each phyllary tipped by spine 1–3 mm.
2n

= 18, 36.

Centaurea diffusa
Phenology Flowering summer (Jun–Aug).
Habitat Disturbed sites in grasslands, woodlands, open coniferous forests
Elevation 100–2200 m (300–7200 ft)
Distribution
from FNA
AZ; CA; CO; CT; IA; ID; IL; IN; KY; MA; MI; MO; MT; NE; NH; NJ; NM; NV; OR; RI; TN; UT; WA; WY; AB; BC; ON; QC; SK; YT; Europe [Introduced in North America]
[WildflowerSearch map]
[BONAP county map]
Discussion

Centaurea diffusa is native to southeastern Europe and casually adventive in central and western Europe.

Centaurea diffusa readily hybridizes with C. stoebe subsp. micranthos and is often confused with their fertile hybrid (C. ×psammogena G. Gáyer); the latter can be recognized by its cypselae bearing pappi and having conspicuously radiant heads. Morphologically the hybrids are extremely variable; they may be intermediate or may closely resemble one or the other of the parents. Conspicuously radiant heads and pappi are always present; appendages of the phyllaries are brown to black, or rarely stramineous; spines are absent or short and 2n = 18. Centaurea ×psammogena is known from waste places, roadsides, railway tracks; 50–2500 m; B.C., Ont., Que.; Colo., Mass., Mich., Mo., N.C., Oreg., Tenn., Wash. It may occur spontaneously where the ranges of the parent species overlap; they may also be distributed separately. In mixed stands it replaces C. diffusa by introgression. Hybrids are often misidentified as C. diffusa.

(Discussion copyrighted by Flora of North America; reprinted with permission.)
Source FNA vol. 19, p. 190.
Parent taxa Asteraceae > tribe Cardueae > Centaurea
Sibling taxa
C. benedicta, C. calcitrapa, C. cyanus, C. depressa, C. diluta, C. iberica, C. jacea, C. macrocephala, C. melitensis, C. montana, C. nigra, C. nigrescens, C. phrygia, C. scabiosa, C. solstitialis, C. stoebe, C. sulphurea, C. virgata, C. ×moncktonii
Synonyms Acosta diffusa
Name authority Lamarck: in J. Lamarck et al., Encycl. 1: 675. (1785)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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