Soldier Thistle

Picnomon acarna (L.) Cass.

Synonyms - Cirsium acarna, Carduus acarna.

Family: - Asteraceae.


Picnomon is from the Greek pikros meaning bitter or sharp and nomos meaning pasture and refers to the sharp spines of this plant that invades pastures.

Arcana is an old genus of thistles.

Soldier Thistle.

Other names:

Yellow Plumed Thistle



A spiny, stout, woolly, much branched, narrow leaved, annual thistle with purple flowers in sets of 1 to 4 at the ends of stems or in axils.



Two. Oval or round. Tip slightly indented. Base tapered. Short stalk. Hairless.

First leaves:

Oval, opposite. tip pointed. Edges spiny and toothed. Scattered long hairs that are denser towards the base.


Forms a rosette.

Petiole - none.

Blade - Oblong to lance shaped. Basal rosette leaves up to 300 mm long. Slightly lobed with long, yellow spines at the ends of the lobes and small hair like spines on the edges of the leaves. Dense woolly hairs underneath with less on top.

Stem leaves - Alternate. Long and narrow with long, 10-15 mm, yellow spines on small lobes and small hair like spines on the edges of the leaves. Prominently veined. Base of leaf continues down the stem as a wing. Densely woolly haired underneath with less on top.


Erect, much branched, stout, 500-1000 mm tall, spiny wings with yellowish spines. Woolly cobweb hairs. Woody at the base.

Flower head:

Single, terminal and axillary. Oblong and tapering near the top, 20-40 mm long by 10 mm diameter, with parallel sided bracts ending in a bent back yellow spine and with 2-8 side spines. Terminal spine on each bract is multi forked and yellow. Without stalks. Single or sets of 2-4 above floral leaves that are longer than the head. On the ends of branches or in leaf axils.


Pink to purple.

Bisexual. Purple to pink. Cylindrical, tapering at the top. 20-40 mm long.

Ovary -

'Petals' - Purple to pink.

Stamens -

Anthers -


Brown to black with lighter lengthwise markings, 40-60 mm long, smooth, shiny. Pappus of many feathery silver-white bristles, 10-20 mm long that are loosely attached to the seed.


Tan to black with white stripes, oval too cylindrical, 5-6 mm long by 2-3 mm wide. Enclosed in the fruit.


Slender, branched taproot with many laterals.

Key Characters:

Involucre bracts ending in a pinnatisect spine.


Life cycle:

Annual. Seeds germinate in autumn to winter and form small rosettes over winter. Rosettes rapidly increase in size in spring and form flowering stems. Flowers from December to February and the plant dies soon after.



By seed.

Flowering times:

December to February in SA.

Seed Biology and Germination:

Vegetative Propagules:




Population Dynamics and Dispersal:

Spread by seed. Wind is the main spreading agent blowing seeds or whole plants that break off at the base. Water, animals and machinery also account for a considerable amount of spread.

Origin and History:

Mediterranean and South Western Asia.





Semi-arid to sub-humid temperate regions. Annual rainfall areas of 300-600 mm with a long growing season.


Prefers sandy or stony soils or the heavy red-brown earths.

Plant Associations:




Spiny weed of crops, cereals, pastures, roadsides, watercourses and disturbed areas.

In crops it chokes harvesters and competes with the crop.

Stock avoid grazing it because it is so spiny.

Spines may injure sheep and dogs.


Not recorded as toxic.


Noxious weed of SA and VIC.

Management and Control:


Eradication strategies:

Manually remove isolated plants.

Cultivation, mowing and slashing before flowering is effective.

Spraying with hormone herbicides when the plants are young is effective. Spray Graze using 2,4-D amine is useful in clover pasture situations.

Small areas can be treated with clopyralid or Tordon 75-D for longer lasting control.

Herbicide resistance:

Biological Control:

Related plants:

None in the same Picnomon genus. The Carduus and Cirsium genus are closely related.

Plants of similar appearance:

Perennial Thistle, Spear Thistle, Nodding Thistle, Slender Thistle.


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P109. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P933. Diagram.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #974.1.

Moerkerk, M.R. and Barnett A.G. (1998) More Crop Weeds. (R.G and F.J. Richardson, Melbourne). P60. Photos. Diagrams.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P223-224. Photos.


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