Soldier thistle (Picnomon acarna), from family Asteraceae, is an erect annual plant (not a grass) growing to 1 m high. Yellow-green in appearance due to numerous yellow spines on the leaves. Stems of soldier thistle are ranched with spiny wings and dense white hairs. They are woolly in appearance. Soldier thistle leaves are covered with fine white hairs and yellow spines, 1-1.5 cm long. White hairs on the leaves give them a woolly appearance. Rosette leaves grow to 30 cm long, S. thistle has pink or purple flower heads, growing 2-4 cm long and can be solitary or in groups at the end of branches. Flowers are formed in late spring from September to November with the plant flowering in early summer.

The seeds of Soldier thistle are smooth and shiny with a detachable parachute of bristles on one end about 4-6 mm long. Seed germinates mainly in autumn, but has been seen to germinate after sufficient summer rainfall in disturbed areas. Wind is the main method of dispersal for this weed. Seeds can disperse over short distances and whole plants can also be blown around after breaking off at the base. Seeds can also be carried on animals, vehicles, machinery and in water. P. acarna grows rapidly in spring, particularly on heavy red-brown soil and spreads fast in degraded areas with little competition. A small percentage of seeds can remain viable in the soil for several years. Plants may appear on recently cultivated areas where there has been no seeding for several years.