What Does It Look Like?
What is it?
Soldier Thistle (Picnomon acarna) is a prickly, upright, annual thistle usually growing to 500-750 mm tall, but occasionally reaching 1 metre in height growing in semi-arid to sub-humid areas with rainfall of 300-600 mm. It forms a basal rosette (cluster) of leaves during the early stages of growth. The long thin spiny rosette leaves are oblanceolate (lance shaped, about 4 times as long as broad, broadest in the upper half, tapering to a narrow base) and grow to 300 mm long and 30 mm wide, slightly lobed, with many teeth with many distinct short yellow spines, covered in cobwebbed white hairs. The stems and leaves are densely covered in white hairs that give them a cobwebby or woolly appearance. The stems are many branched and have spiny 'wings' that extend downwards from the base of the leaves. The stem-leaves are mostly smaller 40-100 mm long and 4-20 mm wide, lanceolate (about 4 times as long as broad, broadest in the lower half and tapering to the tip ), deeply lobed, with long yellow coloured spines on their teeth or lobes, cobwebby, and prominently veined.
The pink or purple flower-heads are almost stalkless (sessile), cylindrical (20-40 mm x 10 mm in size), and borne singly or in small groups. They consist of numerous pink or purple coloured 'florets' (small individual flowers) surrounded by several layers of bracts (modified leaves) 20-30 mm long, that are tipped with sharp, multi-forked, yellow spines.
The fruits or 'seeds' (cypselas) are 4-6 mm long, brown to black in colour and topped with a ring (pappus) of feathery white bristles (10-20 mm long) (Parson & Cuthbertson 2001; Navie 2004).
For further information and assistance with identification of Soldier Thistle, contact the herbarium in your state or territory.
Flower colour
Purple or Pink
Growth form (weed type/habit)
Herb
Where it currently grows? Preferred habitat
Soldier Thistle (Picnomon acarna) is a weed of roadsides, waste areas, channel banks, crops and pastures in temperate, semi-arid and sub-humid environments. It grows on dry sandy or stony soils in areas with 300 to 600 mm of rainfall and a relatively long growing season (Parsons & Cuthbertson 2001; Navie 2004).
Are there similar species?
Soldier Thistle (Picnomon acarna) is relatively similar to Star Thistle (Centaurea calcitrapa), Spotted Knapweed (Centaurea maculosa), Black Knapweed (Centaurea nigra) and Creeping Knapweed (Rhaponticum repens) [as Acroptilon repens]. These species can be distinguished by the following differences:
- Soldier Thistle has spiny, winged, stems and a covering of short white hairs on the undersides of its toothed or slightly lobed, spiny, leaves. Its pinkish or purplish flower-heads are enclosed in several rows of spine-tipped bracts.
- Star Thistle has spineless stems and a covering of short hairs on the undersides of its lobed or deeply-divided, spineless leaves. Its purplish flower-heads are enclosed in several rows of spine-tipped bracts.
- Black Knapweed has spineless stems and a sparse covering of rough hairs on the undersides of its mostly entire (without teeth or lobes), spineless leaves. Its purplish flower-heads are enclosed in several rows of spineless bracts.
- Spotted Knapweed has spineless stems and a sparse covering of short hairs on the undersides of its entire, lobed or deeply-divided, spineless leaves. Its purplish flower-heads are enclosed in several rows of spineless bracts.
- Creeping Knapweed has spineless stems and a covering of short hairs on the undersides of its entire, toothed or deeply-divided, spineless, leaves. Its purplish flower-heads are enclosed in several rows of spineless bracts (Navie 2004).
