There are two Slender Thistle present in Tasmania which are difficult to destinguish, Winged Slender Thistle Carduus tenniflorus and Shore Slender Thistle Carduus pycnocephalus L., they are generally considered as one species.
Slender thistles are found in most areas of Tasmania, but are particularly numerous in the Midlands. They occur in pasture, waste land and arable crops. They are strongly competitive in pasture and can appear in significant numbers in cereals.
The main germination period is the Autumn, though Winter and Spring germination also occurs more particularly in the Northern Tasmania. A tall, slender flower stem is produced in early spring and flowers are formed from mid-September to December. Plants usually die in early summer. The life of the plant may be extended in northern Tasmania where it may even survive into autumn, particularly in the north west.
Control by competition and cropping
The best general control of slender thistles is obtained by encouraging competition from useful plants.
Vigorous pasture helps to restrict seedling establishment and growth, so management practices should be aimed at maintaining a vigorous and unbroken stand during the period when thistles are germinating.
Where possible block grazing should be used in preference to set stocking.
Failure to control attacks by pasture insects which can result in bare ground, may lead to a serious invasion by slender thistles because of the lack of competition at a critical period. The sowing of useful species on patches of land left bare by insect attacks is recommended. Establishing insect resistant grasses such as Phalarsis or Cocksfoot would be an advantage in areas regularly subject to attack.
In heavily infested arable areas growing a cereal crop for one or two consecutive seasons can reduce the thistle production provided a suitable spray program is used.
Cereals can be sprayed between the 5-leaf to full tillered stage, when the thistles are still in the young rosette or seedling stage, using MCPA, 2,4-D amine or an MCPA/dicamba mixure. It is not recommended that 2,4-D be used on oats.
Any thistles not completely killed by the herbicide will be suppressed by the crop provided it is vigorous.
Status under the Noxious Weeds Act
Slender thistles are declared a Secondary Weed under the Noxious Weeds act 1964. Landholders may be required to control slender thistle by an Enforcement Notice issued by a Weeds Inspector, or comply with prescribed measures contained in a regulation under the Noxious Weeds Act.
 
 
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