Arizona or marsh centaury

Erythraea calycosa Buckley var. arizonica A. Gray in A. Gray et al., Syn. Fl. N. Amer. 2: 113. 1878; Centaurium arizonicum (A. Gray) A. Heller; C. calycosum (Buckley) Fernald var. arizonicum (A. Gray) Tidestrom

Herbs annual or biennial, (10–)20–50(–60) cm. Stems 1–10, usu­ally branching ± sparsely near or above middle. Leaves: basal usually present at flowering, sometimes numerous; blade oblanceolate to lanceolate, (7–)15–70 × 4–10 mm; cauline blades lanceolate to oblanceolate, (13–)25–70 × (2–)5–8(–13) mm, apex obtuse to acute. Inflorescences predominantly dichasial or distally monochasial cymes; pedicels 4–40(–60) mm. Flowers 5-merous; calyx 7–12 mm; corolla (15–)18–25 mm, lobes (lin­ear to) lanceolate to lanceolate-ovate or ellip­tic, 7–12 × 1–5 mm, apex acute; anthers 2.5–3.5 mm; stigmas 2, fan-shaped. Seeds dark reddish brown. 2n = 24, 40.

Flowering spring–fall. Stream banks, marshes, other moist, open habitats; 50–2800 m; Ariz., Calif., Colo., Nev., N.Mex., Tex., Utah; Mexico (Chihuahua, Coahuila, Nuevo León, San Luis Potosí, Sonora).

Zeltnera arizonica and Z. calycosa appear to inter­grade in western Texas and Coahuila, Mexico, as noted by C. R. Broome (1973), and have often been treated as varieties of a single species. Zeltnera arizonica was sub­sumed in undivided Centaurium calycosum by N. H. Holmgren (1984b), who attributed its allegedly distinguishing features largely to environmental effects, whereas B. L. Turner (1993d) considered the resemblance between these taxa to be superficial and Z. arizonica (as Centaurium) appropriately recognized at species rank. From studies for this flora, acceptance of this species seems warranted.

In Zeltnera arizonica, the relatively sparse branches generally spread at 10–20°, whereas in Z. calycosa the usually denser branches spread at 20–60°.

Zeltnera arizonica is highly variable in the propor­tionate width of its corolla lobes. Some plants in the western part of its range resemble Z. exaltata vegeta­tively but differ in having corolla lobes much longer in proportion to the tube.