Slender or June centaury

Erythraea tenuiflora Hoffmannsegg & Link, Fl. Portug. 1: 354, plate 67. 1820

Herbs annual, 2–75(–90) cm. Stems usually 1, simple or branching above or near middle or occasionally with few, slender branches from base. Leaves: rosette of basal leaves present or absent at flowering but not forming well-developed rosette; blade obovate to oblong, 15–25 × 3–15 mm, apex rounded to obtuse; cauline blades obovate to elliptic-oblong, ovate, or lanceolate (proximal) to nar­rowly lanceolate (distal), 10–30(–42) × 2–9(–18) mm, apex obtuse (proximal) to acute. Inflorescences dense, ± corymboid, dichasial cymes; flowers sessile or occa­sionally on pedicels to 2 mm. Flowers: calyx 5–9(–11) mm; corolla 8–14(–17) mm, lobes (1–)2–4.5 mm; anthers 0.7–1.7 mm; stigmas elliptic. Seeds yellowish to reddish brown. 2n = 40.

Flowering summer–fall. Along streams, edges of marshes, seasonally damp meadows, bluffs, and road­sides; 0–1800 m; introduced; Ala., Calif., La., Miss., Okla., Oreg., Tex.; Eurasia; n Africa; introduced also in West Indies, South America, Pacific Islands (New Zealand), Australia.

Pending the availability of a more satisfactory classifi­cation, the name Centaurium tenuiflorum is used here in a broad sense. According to G. Mansion et al. (2005), C. tenuiflorum in the broad sense includes a diploid entity, C. tenuiflorum subsp. acutiflorum (Schott) L. Zeltner [sometimes treated as C. acutiflorum (H. W. Schott) Druce]; a probable autotetraploid, C. tenuiflorum subsp. tenuiflorum; and an unnamed entity believed to be an allotetraploid derivative of diploid C. tenuiflorum × C. erythraea. The first two are native in Europe and not known from North America. The entity that has become naturalized outside its native range, including the flora area, is the allotetraploid.

Centaurium tenuiflorum has often been assumed to be native in California and has incorrectly been called C. floribundum (Bentham) B. L. Robinson or C. muehlenbergii (Grisebach) W. F. Wight ex Piper (J. S. Pringle 2010b). Those names are correctly associated with Zeltnera muehlenbergii.

Exceptional plants of Centaurium tenuiflorum are more diffusely branched and have longer primary bran­ches than is usual. In the most extreme cases, the plants may be branched from near the base, with large numbers of flowers borne in noncorymboid, witch’s-broomlike inflorescences. These plants may represent the results of injuries, virus infections, or unusual environmental conditions.