Centaurea L.

Centaurea, as currently understood, is a large genus of ca. 500 species. It is most diverse in the Mediterranean region and southwestern Asia. Representatives in North America and tropical Africa probably belong to segregates of Centaurea s.str., e.g. Plectocephalus (D. Don) DC. (Susanna & al. 2011). The generic limits of Centaurea have long been controversial. As traditionally circumscribed it is a polyphyletic assemblage. Some segregate genera have now become more or less widely accepted, for instance Amberboa (Pers.) Less., Mantisalca Cass., Rhaponticum Vaill., Volutaria Cass., etc. (e.g. Tison & de Foucault 2014), also in this account. Molecular phylogenetic studies, however, will surely re-define the limits of Centaurea. Garcia-Jacas & al. (2000) already showed that Cnicus is in fact nested within Centaurea. Cyanus Mill. is also better segregated (see Greuter 2003) although this is not (yet) widely accepted (see for instance Mabberley 2008, Boršić & al. 2011, Hilpold & al. 2014). It comprises, among others, native Cyanus montanus (L.) Hill (syn.: Centaurea montana L.) and archaeophytic C. segetum Hill (syn.: Centaurea cyanus L.). Likewise, Psephellus Cass., with at least one species in our territory, has also been segregated (Wagenitz & Hellwig 2000) and its generic status has been confirmed by subsequent molecular studies (Aydin & al. 2013). For convenience, an overview of Belgian Centaurea s.l. (native as well as non-native) with – if applicable – their names in the new circumscription is presented here under.

Centaurea alba L.
Centaurea aspera L.
Centaurea calcitrapa L.
Centaurea calocephala Willd. 
Centaurea dealbata Willd. => Psephellus dealbatus (Willd.) K. Koch
Centaurea depressa Bieb. => Cyanus depressus (Bieb.) Soják
Centaurea diffusa Lam.
Centaurea diluta Ait. 
Centaurea glaberrima Tausch
Centaurea hyalolepis Boiss.
Centaurea jacea Group
Centaurea macrocephala Willd.
Centaurea melitensis L.
Centaurea moschata L. => Amberboa moschata (L.) DC.
Centaurea nigrescens Willd.
Centaurea orientalis L.
Centaurea paniculata L.
Centaurea pectinata L.
Centaurea phrygia L.
Centaurea repens L. => Rhaponticum repens (L.) Hidalgo
Centaurea salmantica => Mantisalca salmantica (L.) Briq. & Cavill.
Centaurea scabiosa L.
Centaurea solstitialis L.
Centaurea stoebe L.
Centaurea triumfettii All. => Cyanus triumfettii (All.) Á. Löve & D. Löve

Cnicus benedictus L. => Centaurea benedicta (L.) L.

Three species are presumably native (or at least archaeophytic) in Belgium: Centaurea calcitrapa L., C. jacea group and C. scabiosa L. (Lambinon & Verloove 2012). All also occur as aliens outside their native distribution range (as grain aliens, garden escapes, etc.). In recent times several of these are included in wild flower seed mixtures. However, these might represent non-native infraspecific or even related taxa. Centaurea jacea subsp. pannonica (Heuff.) Hayek (syn.: C. pannonica (Heuff.) Simonk. subsp. pannonica) and C. scabiosa subsp. fritschii (Hayek) Hayek (syn.: C. grinensis Reut. subsp. fritschii (Hayek) Dostál) are frequent components in such mixtures (Frank & John 2007) and should be looked for (see also Kruk & Szymańska 2012). Likewise, Centaurea scabiosa subsp. sadleriana (Janka) Aschers. et Graebn. and subsp. spinulosa (Spreng.) Arcang. were recently recorded in seed mixtures in northeastern Germany (Kiesewetter & Henker 2010).
Several species of Centaurea are cultivated as ornamentals in Europe (see Cullen 2000, Jäger & al. 2008). Many others are noxious agricultural or environmental weeds, some are even considered invasive.
Despite several previous attempts there is still no satisfactory treatment for the Centaurea jacea group. In Belgium five taxa are usually recognized (currently as subspecies of Centaurea jacea L.; see Lambinon & Verloove 2012). A sixth subspecies, subsp. nigrescens (Willd.) Čelak. (syn.: C. nigrescens Willd.), has been recorded as an alien. It was recently confirmed in at least two localities (Hoste & al. 2015) and is here considered a separate species (see also Sommer 1990, Tison & de Foucault 2014).
Hybridization may also account for difficulties in naming Centaurea (Vonica & Cantor 2011).

1 Heads (sub-) sessile, surrounded by involucre-like upper leaves. Stem spreading or prostrate. Corolla cream === Centaurea benedicta
Heads pedicellate, not surrounded by the upper leaves. Stem usually erect. Corolla pink, blue, purple, yellow or whitish === 2

2 Corolla pure yellow === 3
Corolla pink, blue, purple or whitish === 6

3 Appendages of bracts lacerate, without spines. Plant perennial === C. macrocephala
Appendages of bracts distinctly spiny. Plant annual or biennial === 4

4 Leaves not decurrent === C. hyalolepis
At least the upper leaves decurrent === 5

5 Florets glandular (with numerous minute sessile glands on the tube). Pappus at most as long as the achene, usually much shorter. Central spine of the appendages of the involucral bracts 5-10 mm long === C. melitensis
Florets eglandular. Pappus twice as long as the achene. Central spine of the appendages of the involucral bracts 10-25 mm long === C. solstitialis

6 Appendages of the involucral bracts spine-tipped (at apex with at least one small spine) === 7
Appendages of the involucral bracts entire or fimbriate, lacerate or pectinate, without a central spine === 11

7 Central spine of the appendage 10-25 mm long (native) === C. calcitrapa
Central spine of the appendage 1-5 mm long === 8

8 Appendages with 3-5 palmately arranged spines. Plant scabrid hairy === C. aspera
Appendages fimbriate-lacerate with a short, usually solitary spine at apex. Plant glabrous or variously hairy but usually not scabrid === 9

9 Upper leaves slightly decurrent. Heads always radiant, corolla purplish. Inner achenes with pappus as long as the achene === C. diluta
Leaves not decurrent. Heads radiant or not, corolla whitish to purple (often variable in a population). Achenes without or with an inconspicuous pappus (< 0,6 mm long) === 10

10 Heads disciform, never radiant, corolla white. Pappus absent or rudimentary === C. diffusa
Heads radiant or (less frequently) disciform, corolla purple or (less frequently) white (often variable in a population). Achenes with inconspicuous pappus (< 0,6 mm long) === C. xpsammogena p.p.

11 Appendages not decurrent on the involucral bracts === 12
Appendages decurrent on the bracts === 14

12 Appendages of the involucral bracts tapering to a subulate, recurved tip. Margins of the appendages densely pectinate with long fimbriae, these up to 3 mm long === C. pectinata
Appendages of the involucral bracts not tapering to a subulate apex, not recurved at tip. Margins of the appendages at most shortly fimbriate === 13

13 Heads relatively narrow, the involucres longer than wide. Phyllary appendages small, usually more or less triangular, always pectinately-fimbriate, the green parts of phyllaries not fully covered by black appendages; as a result, involucres black and green === C. nigrescens
Heads often broader, the involucres usually as wide as or wider than long. Phyllary appendages larger, usually roundish, undivided to lacerate-fimbriate, the green parts of phyllaries usually fully covered by ± brown, variably pectinately fimbriate appendages, involucres light to dark brown (native) === C. jacea group

14 Involucre 15-25 mm across. Involucral bracts with indistinct parallel veins === 15
Involucre 3.5-11 mm across. Involucral bracts with prominent parallel veins === 16

15 Appendages small (1-2 mm), not covering the bracts, long-decurrent. Fimbriae only slightly lighter in color than the appendages (native) === C. scabiosa
Appendages large (ca. 10 mm), completely covering the bracts, shortly decurrent. Fimbriae whitish (strongly contrasting in color with the dark brown appendages) === C. calocephala

16 Corolla pink (uniform in a population). Involucre 6,5-11 mm across. Appendages fringed at apex, not tapering to a filiform tip. Pappus always present, usually 1-2,5 mm long === C. stoebe
Corolla whitish to purple (often variable in a population). Involucre 3,5-8 mm across. Appendages often with a distinct filiform tip. Pappus absent or inconspicuous (< 0,6 mm long) === C. xpsammogena p.p.

Additional aliens: C. alba L. (S-Eur., vector unknown), C. glaberrima Tausch (ex-Jugosl., vector unknown), C. orientalis L. (SE-Eur., grain alien?), C. paniculata L. (SW-Eur., vector unknown) and C. phrygia L. (N-, C- and E-Eur., vector unknown).


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Taxonomic name: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith