Onopordum acaulon L.
Family Asteraceae, Tribe Cynareae
stemless thistle, horse thistle
Fruit an achene, obovoid, somewhat compressed, 4–5 mm long, 2–2.8 mm wide, 1.3–1.9 mm thick; cross-section rhombic; surface grayish brown to brown with black markings (splotches), wavy transverse ridges under numerous longitudinal ridges including 4 major ribs, glabrous. Scar basal, round to oval, tan; often with a hardened elaiosome attached at the edge. Pappus cream-colored, 2–3 cm long, deciduous in a ring and often missing, leaving a brown to white apical collar measuring 1.3–1.9 mm at its widest. Style base dull yellow to brown, typically ribbed. Embryo spatulate, cotyledons broad; endosperm absent.
Fruits of various species of this genus look similar. Outside of its native distribution, Onopordum acaulon is most likely to be confused with three other invasive weed species: O. illyricum, which has a shorter pappus (10–12 mm long); O. acanthium, which has a short pappus (7–9 mm long) that is pinkish instead of cream-colored; and O. tauricum, which also has a short pappus (8–10 mm long) that is pinkish as well as larger achenes (5–6 mm long).
Onopordum acanthium L. (non-FNW)
Onopordum tauricum Willd. (non-FNW)
native to Algeria, Morocco, and Tunisia in northern Africa as well as France and Spain in Europe
naturalized in Australia
steppes, stony slopes, fallow fields and any disturbed grounds; can tolerate full sun to part-shade, sandy to heavy soils and calcareous loams, and dry to moderately wet conditions
Onopordum acaulon is a biennial herb forming a rosette of deeply lobed leaves. Reproduction is sexual only; the plumed fruits are borne on the wind. Dispersal may also occur via vehicles, contaminated produce, animal feet or fur, or along water channels. There are two subspecies: subsp. acaulon and subsp. uniflorum (Cav.) Franco.