Sow Thistle Management in Southern New South Wales

Why is Sow Thistle a weed of concern?

Sow thistle can germinate all year round
Produce lots of seeds
Readily use soil moisture, especially in stubbles and fallows
Can contaminate crops at harvest with green matter
Will develop resistance to commonly used herbicides such as Glyphosate and 2,4-D

Plant Characteristics

A single sow thistle plant can produce up to 25,000 seeds. The seeds have no dormancy meaning they can germinate straight after dispersal
The majority of the seeds fall within 2-3 metres of the parent plant, however a small number can travel a greater distance
Seeds will germinate all year round (5-35°C) but more commonly after a significant rainfall event
The majority of the seeds germinate from a depth of 1 cm or less, a small number from 2 cm and none from 5 cm. Hence, sow thistles are a problem in no-till farming situations
Burying seeds will reduce emergence but increases persistence with buried seeds surviving for up to 3 years in the soil
Resistance to Group B herbicides is widespread in northern New South Wales as well as numerous confirmed cases of Group M (Glyphosate) resistance. Resistance status in southern New South Wales is unknown however resistance to Group I (2,4-D) has been confirmed in South Australia

Management

Competitive crops can effectively suppress sow thistle from seeding in-crop and thick stubbles can reduce germination after harvest
Control late germinations in crop rather than wait until after harvest
Manage the seed bank by preventing plants setting seed in crop or fallow situations
Graze stubbles or fallows with sheep to manage sparse populations
Cultivation can manage a major weed flush
Avoid using a single mode of action when spraying with a herbicide ie: Glyphosate alone
Consider Starane Advance as a mixing partner with Glyphosate
Target small weeds when spraying
Consider using a double knock with Gramoxone if population is high, plants are mature or herbicide resistance is suspected
Residual chemicals such as Atrazine can help with knockdown and provide short term residual control

For additional information refer to the following links

Acknowledgement and Reference: Qld DPI various publications, GRDC various publications