Growth Habit: Tasmania has two species of thistle commonly referred to as Sow Thistle. Both are very similar in habit, distribution and appearance, with the main difference being leaf shape and the presence of stiffer spines on Prickly Sow Thistle.
Both plants form a rosette to approximately 250mm in diameter whilst young, then produce an upright flowering stem which may grow to 1.5m. 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: Annual.
Flowers: Several small flowers, approximately 20 to 25mm long and 10 to 15mm 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 4mm long and 1 to 3mm 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.
- Both sow thistle species are widespread weeds in Tasmania, however they pose nuisance value more than a major economic threat.
- 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.