Onopordum acaulon

Name and classification

Onopordum acaulon L.
Family Asteraceae, Tribe Cynareae

Common names

stemless thistle, horse thistle

Disseminule

fruit

Description

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.

Identification considerations

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).

Similar species

Onopordum acanthium L. (non-FNW)

Onopordum illyricum L.

Onopordum tauricum Willd. (non-FNW)

Distribution

native to Algeria, Morocco, and Tunisia in northern Africa as well as France and Spain in Europe

naturalized in Australia

Habitat

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

General information

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.

achenes without pappus, at the base of each achene is a light-colored, shiny appendage that is the elaiosome (on left on first two fruits, on right on third fruit)

achenes without pappus, at the base of each achene is a light-colored, shiny appendage that is the elaiosome (on left on first two fruits, on right on third fruit)

achene with detached pappus, hairs of pappus have been broken and were originally longer

achene with detached pappus, hairs of pappus have been broken and were originally longer

apex

apex

attachment scar (no elaiosome attached)

attachment scar (no elaiosome attached)