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Scotch Thistle

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Botanical Name: Onopordum acanthium
Other Common Names:

Declarations

 
Class Regions
Class 4Bland Shire Council,Cabonne Council,Cootamundra Shire Council,Cowra Council,Forbes Shire Council,Lachlan Shire Council,Mid-Western Regional Council,Orange City Council,Parkes Shire Council,Upper Macquarie County Council,Weddin Shire Council
Landholder Responsibilities: Growth and spread of the plant must be controlled according to the measures specified in a management plan published by the local control authority and the plant may not be sold, propagated or knowingly distributed
Why Is It Bad?: Scotch Thistle has the potential to reduce the viability and production of prime agricultural land in the Lachlan Valley, a critical part of the economy of the region. This species is undesirable in the Lachlan Valley region because it competes well with pastures and it can be difficult to control by traditional pasture improvement techniques, it is rejected by livestock because of dense spines, and if eaten the spines can cause damage to the eyes and mouths of animals. It forms dense stands that compete strongly with pastures and lucerne, and they also exclude livestock and reduce carrying capacity.

Identification

Habit: Herb
Leaves: Leaves and stems are woolly, whitish-grey, and divided with spiny margins. Leaf blades continue down stems in broad spiny wings. Leaves are 10-35cm long and 3-10cm wide.
Flowers: Flowers are purple, globular and have spiny bracts. Flowering occurs from August to December.
Fruit: Seeds are rectangular, 4-5mm long and greyish-brown in colour. A single plant can produce 40,000 seeds.
Roots: Taproot.

Control Methods

Manual Removal: Single plants can be grubbed, making sure to remove at least 50mm of the taproot.
Chemical Use: Chemical use must be applied when plants are actively growing. Difficulties often occur as different mixture rates are required for successful control of different maturity levels of the plant.
Fire:
Slashing & Cutting: These techniques are usually not effective because cut plants can regrow from the base and cut flowering stems may still produce fertile seed.
Biological Control: The Seed-head Weevil (Larinus latus), Stem-boring Weevil (Lixus cardui), and Crown Moth (Eublemma amoena) all affect competitiveness and seed production of Scotch Thistle.
Grazing: Scotch Thistle is palatable to goats and can be controlled but not eradicated through grazing pressure.
Cultivation & Scalping: Ploughing is effective in killing young plants providing they are completely turned over. This is best achieved with a mouldboard plough.
Smothering: Strong, competitive, well-managed pastures are effective in shading thistle seedlings, reducing establishment of the weed during the main germination periods. Careful grazing management is necessary to minimise bare ground, which assists thistle seedling establishment. Contact your local Catchment and Agriculture Services staff for pasture management advice.
Solarisation:
Competition: Establishment of a strong perennial grass-based pasture will allow sites to compete well with Scotch Thistle, reducing germinations.
Monitoring: Successful treatment programs rely on ongoing monitoring of sites. Regrowth and new seedlings can easily become larger infestations if follow-up treatments are not part of the management program.

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Image Credit: Megan Power





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