Centaurea acaulis, Stemless Star-thistle

In an earlier post, 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.

REFERENCES

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.

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.

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

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