Soldier thistle

Common name:

  • Soldier thistle

Scientific name:

  • Picnomon acarna (L.) Cass.

Plant status

Catchment management authority boundaries

Regionally prohibited in the North Central, Port Phillip and Western Port catchments.

Regionally controlled in the Goulburn Broken and North East catchments.

Restricted in the Mallee, Wimmera, Corangamite, Glenelg Hopkins, West Gippsland and East Gippsland catchments.

Plant biology


Herbaceous plant — Forb (flowering herbaceous plant — not a grass)

Growing soldier thistle


Soldier thistle is an erect annual herb growing to 1m 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 to 1.5cm long. White hairs on the leaves give them a woolly appearance.

Rosette leaves grow to 30cm long while stem leaves alternate and are long and narrow, about 1 to 1.5cm in length.

Soldier thistle behind wire fence


Soldier thistle has pink or purple flower heads, growing 2 to 4cm 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 to 6mm long.

Seed germinates mainly in autumn, but has been seen to germinate after sufficient summer rainfall in disturbed areas.

Green leaves of soldier thistle

Growth and lifecycle

Method of reproduction and dispersal

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.

Rate of growth and spread

Soldier thistle grows rapidly in spring, particularly on heavy red-brown soil and spreads fast in degraded areas with little competition. It can compete with crops in favourable conditions.

Seedbank propagule persistence

A small percentage of solder thistle 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.

Preferred habitat

Soldier thistle prefers semi-arid to sub humid areas of temperate regions on dry sandy or stony soils. It is a weed of roadsides, channel banks, neglected areas and occasionally cereal crops and pastures in areas with an annual rainfall of 300 to 600mm.


In Victoria, soldier thistle is found in the Mallee and northern regions of the state.

Growth calendar

The icons on the following table represent the times of year for flowering, seeding, germination, the dormancy period of soldier thistle and also the optimum time for treatment.

Flowering        Flowering iconFlowering iconFlowering iconFlowering icon
 Seeding icon          Seeding icon
Germination  Germination iconGermination iconGermination icon       
DormancyDormancy iconDormancy iconDormancy icon         
Treatment     Treatment iconTreatment iconTreatment iconTreatment iconTreatment icon  


Agricultural and economic impacts

Well-established infestations of soldier thistle can compete with crops and choke harvesting equipment. The sharp spines of the plant can injure animals and are generally avoided by stock.


Prescribed measures for the control of noxious weeds:

  • application of a registered herbicide
  • physical removal

Read about prescribed measures for the control of noxious weeds.

Other management techniques

Changes in land use practices and spread prevention may also support soldier thistle management after implementing the prescribed measures.

Page last updated: 29 Jun 2020