Pest plant - Variegated thistle

Variegated thistle (Silybum marianum) is an annual herb also known as St Mary’s thistle, milk thistle or bull thistle. It is a competitive pasture weed.

Description

• erect annual or biennial herb to 2.5m with a thick taproot
• stems branch from the base, thick but hollow or full of pith, ribbed with small spines and often downy with cobwebby hairs
• leaves have a shiny, mottled green upper surface and a slightly hairy lower surface; they are deeply divided into toothed segments with short spines along the edges
• flowering heads are 4-8cm and round, packed with purple florets to 5cm long, protruding from the head and ending in spines
• seeds are long and shiny, 6-8 mm, mottled black and grey with many fine bristles, joined together in a ring attached to one end of the seed.

Why is it a problem?

• will eliminate most plants by shading them out, and via competition for moisture and nutrients
• unpalatable and not readily grazed by stock unless sprayed or cut
• of little nutritional value and becomes poisonous under certain conditions
• spines of the plant can cause physical injury and restrict stock movement.
• dense stands of variegated thistle may also harbour vermin.

Distribution

• Eyre Peninsula – isolated plants
• Northern pastoral – isolated patches in towns only
• Northern agricultural districts and Yorke Peninsula – isolated plants, more common in higher rainfall areas
• Murray Mallee – small isolated patches
• South East – scattered in upper south east, widespread in lower south east
• Central region – scattered patches in higher rainfall areas.

Ecology

• prefers heavy, fertile soils with high nitrogen, needs bare or disturbed ground to germinate
• no growth in winter, then grows rapidly in spring before producing flowering stems
• flowering starts in October, continuing into early summer
• plants die during summer, dry stems can remain standing for several months.

How it spreads

• reproduces only by seeds that germinate after autumn rain but can remain viable in the soil for up to 9 years.

Related links

• Pest plants (web page)