Sow and Prickly Sow Thistles (Sonchus oleraceus and S. asper ssp. glaucescens)

Sonches Olearaceus Photo: (c) Harry Rose CC BY 2.0 license

Family: Asteraceae

Growth Habit: Tasmania has one native and two species of introduced Sow Thistles:  Sow Thistle (Sonchus oleraceus) and  Prickly Sow Thistle (S. asper).

S. asper. Photos: (c) Oliver Pichard CC BY-SA 3.0 license

Both are very similar in habit, distribution and appearance.  S. oleraceus can be distinguished from S. asper by stiff and leathery leaves and the presence of distinct, stiffer spines, mostly smooth ‘fruit’ with ribs that are scabrous (rough to the touch) or with hairs. S. oleraceus however, has soft, thin leaves and wrinkled ‘fruit’, with ribs but without hooks.

Both plants form a rosette to approximately 250 mm in diameter whilst young, then produce an upright flowering stem which may grow to 1.5 m that arises from a deep taproot. The stems are a 5 angled hollow stem that is dark green (sometimes tinted with a reddish-purple tinge). Cut stems exude a milky sap.

Both species are common weeds throughout the State and most frequently occur on roadsides and neglected areas. They often appear in pasture and crops, however they rarely cause significant problems as they are readily grazed in pasture and out-competed by most crops.

Seeds generally germinate in autumn or spring.

Type of Plant: Erect annual herb.

S. oleraceus. Photo: (c) lxitixel CC BY-SA 3.0 license

Flowers: Several small flowers, approximately 20 to 25 mm long and 10 to 15 mm in diameter, are borne at the top of the flower stem. The florets are yellow, turning to a dense tuft of white hairs as seeds develop and are ready to be released.

Fruit/Seed: Most seeds are released throughout late autumn and late spring to early summer, depending upon the germination time of the plant and climatic conditions. They are approximately 2 to 4 mm long and 1 to 3 mm wide.

Dispersal: Sow Thistles are spread entirely by seed. The seeds are equipped with a small pappas, or parachute of hairs, which may carry the seed over large distances in strong winds. Seeds lying on the ground may also be transported in moving water.

Status: Sonchus oleraceus and Sonchus asper ssp. glaucescens are not declared under the Noxious Weeds Act 1964.

Weed Impact:

  • Both Sow Thistle species are widespread weeds in Tasmania, however they pose nuisance value more than a major economic threat.

Control Methods:

  • As Sow Thistles generally occur in degraded or poorly managed areas, their eradication can often be achieved by improving land management practices. In particular the establishment of a competitive ground cover which provides strong competition will greatly inhibit sow thistle seedlings from establishing.
  • Plants may be easily removed by hand tools or machinery.
  • Grazing is also useful as the thistles, particularly whilst young, are quite palatable to stock.
  • Several herbicides are registered for use on Sow Thistles in Tasmania for both spot and boom spraying, including glyphosate, metsulfuron methyl, dicamba, metribuzin, simazine and MCPA.

N.B. Always check the herbicide label before use.