Growth Habit: Variegated thistle is a widespread weed in Tasmania and generally occurs in pasture, crops, roadsides and neglected areas. In soils of high fertility it can build up toxic levels of nitrates. Seeds generally germinate in autumn, depending upon moisture levels, however germination at low levels may occur at other times of the year.
The young plant develops into a rosette, usually up to 1 metre in diameter, but plants are capable of growing larger under ideal conditions. The leaves exhibit striking white variegations, hence the plant's name. A large specimen in flower is a very striking plant.
The tall upright flower stem comprises many branches and is produced in late winter. This usually reaches up to 1 metre but may grow considerably larger.
Flowers: Variegated thistle produces one flower head on each stem to 60mm in diameter. Colour is bright purple and it is surrounded by a series of spiny bracts. The flowers are quite attractive.
Fruit/Seed: Two types of seeds are produced, typical of many members of the daisy (Asteraceae) family. All seeds are approximately 5 to 8mm long, 2-4mm wide, angular and protected by a hard coat. Seed colours range from dark brown to black. A single variegated thistle may produce several hundred seeds.
Dispersal: Variegated thistles are spread entirely by seed. The seeds are equipped with a small pappas, or parachute of hairs, however they are not disbursed over long distances by wind. Most seed falls within a few metres of the parent. Livestock, particularly sheep, also spread seed in their wool.
Status: Silybum marianum is declared as Secondary Weeds under the Noxious Weeds Act 1964.
Variegated thistle is a serious weed in Tasmania, particularly in the lower rainfall areas of the midlands.
Infestations can be very dense and may totally dominate pasture or a crop. It is also a widespread and troublesome weed of roadsides.
Any removal of variegated thistles must be followed up by establishment of competition to prevent thistles from reinfesting. In particular bare areas left in pasture after thistle removal should be oversown with competitive grass seed.
All control methods employed should aim to attack the thistles in autumn or early winter before plants become established and are therefore much more likely to recover.
Small patches of variegated thistle can be chipped with a hoe, provided most of the taproot is also removed. Slashing or mowing may reduce seed set, but is ineffective at killing variegated thistles.
Grazing with sheep or cattle will not usually kill variegated thistles, however quick grazing with high stocking rates of sheep in autumn just after thistles have germinated will greatly reduce seedling numbers. Grazing must be managed carefully however as overgrazing may weaken the pasture, allowing the thistles to colonise bare areas and successfully compete with the weakened grass.
Goats can be used effectively to eat variegated thistles, however unless their grazing is regular, plants may survive.
Several herbicides are registered for use on variegated thistles in Tasmania for both spot and boom spraying. These include glyphosate, dicamba, MCPA, bromoxynil and 2,4-D.
Any herbicide spraying should be targeted at the young plants in autumn and winter, as the thistles become quite tough and resistant to most herbicides as they commence flowering.