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Variegated thistle

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Variegated thistle

Also known as silybum marianum.Variegated thistle

Variegated thistle looks like

Variegated thistle is a robust erect annual, growing up to 2.5 metres with stout flowering stems that are hollow and branched from the base.  The glossy rosette leaves have white veins and blotches giving it a variegated look.  The large purple flower is surrounded by many sharp spines.

Impact on the environment

Variegated thistles are short lived, flowering and seeding in the summer following germination.  Up to 6,000 seeds per plant can be produced and remain viable for more than 9 years.  Seeds can be dispersed by water movement, contaminated hay and machinery, by stock and birds.

Plants are found in overgrazed pasture, wasteland, along roadsides and drought prone areas.  It also grows well on high fertility soils.

How to control variegated thistle

Good pasture management is the key to control, as young seedlings are susceptible to pasture competition.  Stock management should aim to prevent overgrazing especially in summer.  For scatter plants, chipping or grubbing out is an effective control method if done before the plants flower.

Spot spraying with a herbicide is also effective on scattered plants before they flower.  For scattered dense infestations over a large area, aerial spraying should be considered.  Follow-up with spot spraying and chipping to control new seedlings or regrowth.

Regional Pest Management Plan

In the area north of the Karakatuwhero River to Potaka, variegated thistle is listed as a 'progressive containment' plant pest.  The long term objective is removal of all infestations and the short-term objective is no increase in distribution and a reduction in density.

In the rest of the district, variegated thistle is a sustained control plant pest.  This means plant pests abundant in suitable habitats in particular areas or across the district unlikely to be eradicated but able to be suppressed.

Land occupier's responsibility

Any land occupier shall before the production of hard seed, destroy all variegated thistle plants within 50m of the boundary of adjoining land, or if the adjoining property is being cleared of variegated thistle. 

It's the landowner's responsibility to control variegated thistle.  Where landowners fail to carry out proper control measures to the required standards, Council can use regulatory measures to achieve compliance.

Our plant biosecurity staff can give advice on the best control methods.

No person shall knowingly sell, propagate, breed, release or otherwise spread variegated thistle, under Sections 52 and 53 of the Biosecurity Act.

How to control variegated thistle

Good pasture management is the key to control, as young seedlings are susceptible to pasture competition. Here's some advice:

Stock management should aim to prevent overgrazing especially in summer.

For scatter plants - chipping or grubbing out before the plants flower.

Spot spraying with a herbicide before the plants flower.

Aerial spraying for scattered dense infestations over a large area. 

Check infested areas to make sure the plants don't seed.

Grub or spray plants before they flower.

Don't buy uncertified seed or hay from an unknown source.

Make sure any machinery entering your property has been thoroughly cleaned.

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