Key for the desctiption of plants from Centaurea stoebe/maculosa/diffusa group

Mostly from J. Ochsmann (2000-onwards). The plants from Centaurea arenaria group were not included.

The biennial or perennial taproot plants with pinky violet or creamy colored flowers. The rosette leaves pinnatifid, stems acutely scabrous. The involucrae bracts 8-12 mm (0.3-0.5 in) in length, with 5-7 veins, on the top with relatively big, rigid and often spiny appendages without auricles at the base. Achenes 2.5-3.5 mm (0.1-0.15 in) in length. Plants of steppes.

1. Ray florets missing, flowers cream-coloured, rarely purple, capitula (widest lower part) ca 2-6 mm wide, phyllary appendages always with distinct terminal spine, achenes small, 2-3 mm, pappus 0 (at most rudimentary) ... C. diffusa

– Ray florets present, capitula ca 3.5-11 mm wide ... 2

2. Flowers whitish or purple (intermediate colours occur), capitula ca 3.5-8 mm wide, phyllary appendages light to dark, very variable, terminal spine 0 to well developed, length of achenes variable, 2.5-3.5 mm, pappus missing or up to 0.6 mm ... C. x psammogena (C. diffusa x C. stoebe)

– Flowers purple, capitula over 5 mm wide, terminal spine 0 ... 3

3. Pappus ca 0.5-1 mm (sometimes 0?), phyllary appendages big, black, with 8-11 cilia per side (endemic in SE Europe) ... C. stoebe subsp. serbica (= C. affinis, C. tartarea)

– Pappus ca 1-2.5 mm, appendages dark brown to black, with 4-10 cilia per side ... 4

4. Plants normally single-stemmed, monocarpic, capitula ca 6.5-11 mm wide, phyllary appendages with 6-10 cilia per side, diploid (2n = 18), Europe, absent in North America ... C. stoebe subsp. stoebe (= C. rhenana, C. paniculata, C. maculosa)

– Plants normally many-stemmed, perennial, polycarpic, capitula ca 5-8 mm wide, phyllary appendages with ca 4-7 cilia per side, bracts often tinged with dark violet, tetraploid (2n = 36), invasive in Europe and North America ... C. stoebe subsp. micranthos (= C. micrantha, C. biebersteinii)