Centaurea

Centaurea appendicigera, photographed by Degoeje.

Belongs within: Cardueae.

Centaurea, cockspurs and knapweeds, is a genus of thistle-like plants that is most diverse in the Mediterranean region. A number of species have become more widespread as synanthropic weeds such as the yellow cockspur C. solstitialis and star thistle C. calcitrapa.

Stars and blessings
Published 20 December 2015
Yellow starthistle Centaurea solstitialis, copyright Franco Folini.

The first thing that struck me when I was looking up material on Centaurea was how evocative some of the vernacular names associated with this genus are: starthistle, blessed thistle, dusty miller, sweet sultan. Centaurea, the starthistles and knapweeds, is a genus of composite-flowered plants native to Eurasia and northern Africa, with the highest diversity of species in the Mediterranean region. A handful of species have been spread to other parts of the world in association with humans; a handful of these are significant pasture pests such as spotted knapweed C. maculosa and yellow starthistle C. solstitialis, whereas others such as dusty miller C. cineraria are grown as garden plants. Centaurea is a large genus: depending on how you count them, it may contain anywhere between 300 and 700 species. The greater number of species are perennial herbs, but the genus varies from small spiny shrubs to low spreading annuals (Wagenitz 1986). Some arise from a single central tap-root; others grow from spreading rhizomes. Some species have spiny leaves and conform to our general idea of a ‘thistle’; others do not. The leaves are often deeply divided at the base of the plant, becoming entire towards the top. Flowerheads may be borne singly or in a corymbiform arrangement (a flat-topped cluster); the phyllaries (the bracts surrounding the flowerhead) often extend outwards around the head, and may be themselves tipped with spines.

Squarrose knapweed Centaurea triumfettii, copyright Kristian Peters.

With a genus of this size, it should be hardly surprising that taxonomic complications are involved. Long recognised as morphologically diverse, it has been confirmed as polyphyletic by more recent molecular analyses (Garcia-Jacas et al. 2001). The majority of Centaurea species fall within a single derived clade within the composite subtribe Centaureinae, united both by molecular data and by a number of morphological synapomorphies including adaptations for myrmecochory, dispersal of the seeds by ants (the seeds carry an attached oily body called an elaiosome; ants carry the seeds back to their nest where they may eat the elaiosome but leave the seed to sprout). A handful of species, though, lack these synapomorphies and lie in scattered segregate clades among the remainder of the Centaureinae. Some of these segregate clades, such as the former section Psephellus, have been straightforwardly promoted to the status of separate genera. One small segregate clade, however, is a little more problematic because it happens to include the north African Centaurea centaurium, the original type species of the genus Centaurea. Under normal circumstances, then (other than lumping the entirety of centaureines in a single genus), the name Centaurea would apply only to this small clade (including only about a dozen species) while the hundreds of species in the main ‘Centaurea‘ clade would have to be renamed. In this case, the name with priority for this large clade would be Cnicus, generally used to date for only a single species, the blessed thistle Cnicus benedictus (no, I haven’t been able to establish why it is called the ‘blessed thistle’; I have found references to a tradition of medicinal use for this species, including its supposedly encouraging milk production in nursing mothers, but I haven’t been able to confirm if this is the reason for the name). In order to stave off this nomenclatural turmoil, it has been proposed that the official type species of Centaurea be changed to a member of the main clade (Greuter et al. 2001), so this clade keeps the name Centaurea (and the blessed thistle becomes referred to as Centaurea benedicta) whereas the small clade including the prior type species becomes known as the genus Rhaponticoides. I haven’t found whether a final decision has been made on this proposal (the process for such nomenclatural decisions is a bit more involved for plants than animals, requiring an open vote at an international botanical conference rather than just being decided on directly by a select committee) but it seems to have general support. Less certain is the status of the cornflowers of the section Cyanus, which some have proposed recognising as a separate genus but which is closely related to the main clade, making the case for its separation a bit less compelling.

Centaurea acaulis, stemless star-thistle
Published 19 February 2022

In the section above, I commented on the diversity of species of the star-thistle genus Centaurea. Among the many, many species that have been assigned to this genus is the stemless star-thistle Centaurea acaulis* of northern Africa.

*Though dissolution of the polyphyletic Centaurea may lead to this species changing places. Banfi et al. (2005) listed it under the name of Colymbada acaulis.

Patch of stemless star-thistles Centaurea acaulis, from L’herbiel de Gabriel.

Centaurea acaulis is an inhabitant of dry, rocky habitats that is native to Tunisia and northeastern Algeria. As indicated by both the vernacular and botanical names, its growth habit lacks a central stem. Instead, the long, lobed leaves (which can be up to about a foot in length going by photos provided by Agut Escrig et al., 2021) lie prostrate on the ground. These leaves end in a large, ovate apical section with lobes running down the side of the central rib, becoming smaller towards the base. Flower heads are solitary and carry a mass of bright yellow florets. The involucral bracts (the ‘scales’ around the outside of the base of the flower head) are flat and green with darker longitudinal veins. The distal section of the bracts is triangular with a membranous, ciliate margin and typically (though not always) ends in a long spine. A closely related species found in northwestern Algeria and Morocco, C. oranensis, has historically been treated as a subspecies of C. acaulis (under the name C. acaulis ssp. boissieri, because botanical nomenclature is weird). However, C. oranensis was raised to species level by Greuter & Aghababian (in Greuter & von Raab-Straube, 2005) on the basis of its distinct involucral bracts, which are distally blackish, ovate and concave, with a margin of dense, long, stiff setae.

Close-up of flower head of Centaurea acaulis, copyright Stephen Mifsud.

Recent years have seen this species extending its range northwards with populations now found in Spain, Italy and Malta. In Malta, it was initially found grown in a disturbed area with particularly alkaline soil (Buttigieg & Lanfranco 2001). The mechanism of its arrival is uncertain. It could have dispersed naturally across the Mediterranean, or it may have arrived mixed into bird seed. However it got there, one might expect that as the south of Europe becomes increasingly hotter and drier, the stemless star-thistle will continue to spread.

Systematics of Centaurea

Characters (from Black & Robertson 1965): Involucre of numerous unequal bracts ending in a pungent spine or scarious fringed or jagged appendage; receptacle beset with dense soft hairy bristles; flowers all tubular, outer ones usually sterile and much exceeding incolucre, sometimes radiating but never ligulate; achene oblong, compressed, hilum at base of inner margin (sometimes basal); pappus of short free scales in several unequal rows or none.

Centaurea
|--C. sect. CentaureaWED98
| |--C. iconiensis Hub.-Mor. 1981WED98
| `--C. mykalea Hub.-Mor. 1979WED98
|--C. sect. AcrolophusWED98
| |--C. cariensiformis Hub.-Mor. 1982WED98
| `--C. yozgatensis Wagenitz 1997WED98
`--C. sect. PsephelloideaeWED98
|--C. appendicigeraWED98
|--C. hadimensis Wagenitz, Ertugrul & Dural 1998WED98
|--C. holtziiWED98
|--C. pyrrhoblepharaWED98
`--C. taochiaWED98

Centaurea incertae sedis:
*C. paniculataBGS05
C. acaulis L. 1753 (see below for synonymy)AESPUU21
C. asperaH93
C. biebersteiniiWS96
C. calcitrapaBR65
C. cinerariaH93
C. cyanusC55
C. depressaT-W89
C. diffusaH93
C. dilutaH93
C. divergensW00
C. ebenoides Heldreich ex Moore 1878PL04
C. eriophoraH93
C. ibericaH93
C. jaceaBR65
C. laureotica Heldreich ex Halácsy 1898PL04
C. litardiereiAESPUU21
C. maculosaCLB99
|--C. m. ssp. maculosaCLB99
`--C. m. ssp. rhenanaCLB99
C. melitensisBR65
C. montanaC55
C. moschataH93
C. muricataH93
C. nigraBR65
C. nigrescensBR65
C. ornataAESPUU21
C. pinnatifidaH09
C. × pouzinii [C. calcitrapa × C. aspera]H93
C. × pratensis [C. jacea × C. nigra]BR65
C. pumilioAESPUU21
C. pungensD17
C. raphanina Sibthorp & Smith 1813PL04
|--C. r. ssp. raphaninaPL04
`--C. r. ssp. mixta (de Candolle) Runemark 1867PL04
C. salmanticaH93
C. seridisB28
C. solstitialisBR65
C. squarrosa [=C. virgata var. squarrosa]H93
C. sulphureaH93
C. tougourensisD17
C. triumfetti [incl. C. variegata]H91

Centaurea acaulis L. 1753 [=Colymbada acaulis; incl. Ce. balansae Boiss. & Reuter 1856, Ce. acaulis ssp. balansae (Boiss. & Reuter) Murb. 1897, Ce. acaulis ssp. boissieri Maire 1934, Ce. oranensis Greuter & Agab. 2005, Ce. pharaonis]AESPUU21

*Type species of generic name indicated

References

[AESPUU21] Agut Escrig, A., J. P. Solís Parejo & P. Urrutia Uriarte. 2021. Noticias sobre la presencia de Centaurea acaulis L. (Asteraceae) en la Península Ibérica. Flora Montiberica 81: 51–54.

[BGS05] Banfi, E., G. Galasso & A. Soldano. 2005. Notes on systematics and taxonomy for the Italian vascular flora. 1. Atti Soc. It. Sci. Nat. Museo Civ. Stor. Nat. Milano 146 (2): 219–244.

[B28] Betrem, J. G. 1928. Monographie der Indo-Australischen Scoliiden mit zoogeographischen Betrachtungen. H. Veenman & Zonen: Wageningen.

[BR65] Black, J. M., & E. L. Robertson. 1965. Flora of South Australia. Part IV. OleaceaeCompositae. W. L. Hawes, Government Printer: Adelaide.

Buttigieg, R., & E. Lanfranco. 2001. New records for the Maltese flora: Centaurea acaulis L. (family: Asteraceae). Central Mediterranean Naturalist 3 (3): 147–148.

[CLB99] Callaway, R. M., T. H. DeLuca & W. M. Belliveau. 1999. Biological control herbivores may increase competitive ability of the noxious weed Centaurea maculosa. Ecology 80: 1196–1201.

[C55] Candolle, A. de. 1855. Géographie Botanique Raisonée: Ou exposition des faits principaux et des lois concernant la distribution géographique des plantes de l’époque actuelle vol. 2. Librairie de Victor Masson: Paris.

[D17] Diels, L. 1917. Beiträge zur Flora der Zentral-Sahara und ihrer Pflanzengeographie. Nach der Sammelausbeute des Freiherrn Hans Geyr von Schweppenburg. Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie 54 (Beiblatt 120): 51–155.

Garcia-Jacas, N., A. Susanna, T. Garnatje & R. Vilatersana. 2001. Generic delimitation and phylogeny of the subtribe Centaureinae (Asteraceae): a combined nuclear and chloroplast DNA analysis. Annals of Botany 87: 503–515.

Greuter, W., & E. von Raab-Straube (eds) 2005. Euro+Med notulae, 1. Willdenowia 35: 223–239.

Greuter, W., G. Wagenitz, M. Agababian & F. H. Hellwig. 2001. (1509) Proposal to conserve the name Centaurea (Compositae) with a conserved type. Taxon 50: 1201–1205.

[H09] Heltmann, H. 2009. Der Königstein (Piatra Craiului), die Perle der Burzenländer Gebirge. Mauritiana 20 (3): 515–527.

[H93] Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley (California).

[H91] Hubálek, Z. 1991. Biogeographic indication of natural foci of tick-borne infections. In: Dusbábek, F., & V. Bukva (eds) Modern Acarology: Proceedings of the VIII International Congress of Acarology, held in České Budĕjovice, Czechoslovakia, 6–11 August 1990 vol. 1 pp. 255–260. SPB Academic Publishing: The Hague.

[PL04] Pohl, G., & I. Lenski. 2004. Zur Verbreitung und Vergesellschaftung von Pennisetum orientale Rich. in Nordeuböa (Griechenland) (Poaceae, Paniceae). Senckenbergiana Biologica 83 (2): 209–223.

[T-W89] Tenison-Woods, J. E. 1889. On the vegetation of Malaysia. Proceedings of the Linnean Society of New South Wales, series 2, 4 (1): 9–106, pls 1–9.

Wagenitz, G. 1986. Centaurea in south-west Asia: patterns of distribution and diversity. Proceedings of the Royal Society of Edinburgh, Section B, Biological Sciences 89: 11–21.

[WED98] Wagenitz, G., K. Ertugrul & H. Dural. 1998. A new species of Centaurea sect. Psephelloideae (Compositae) from SW Turkey. Willdenowia 28: 157–162.

[W00] Walsingham, L. 1900. Asiatic Tortricidae (continued). Annals and Magazine of Natural History, series 7, 5: 481–490.

[WS96] Wheeler, A. G., Jr & C. A. Stoops. 1996. Establishment of Urophora affinis on spotted knapweed in Pennsylvania, with new eastern U.S. records of U. quadrifasciata (Diptera: Tephritidae). Proceedings of the Entomological Society of Washington 98 (1): 93–99.

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