Variegated Thistle
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(Silybum marianum) 
What is variegated thistle?
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Variegated thistle is a pasture and crop weed.
How to identify variegated thistle
- Variegated thistle is a large biennial plant (living for two years) forming rosettes of a metre or more in diameter and a flowering stem up to 2 metres in height.
- Rosette leaves are lobed and with spiny margins, and the upper surface is a bright, shining green with distinct white variegations. The flower heads are large and showy, bright purple in colour, and surrounded by long, stiff, spiny bracts.
- Most seed germination takes place in early autumn but can also occur in spring and summer in irrigated areas. Variegated thistle normally over-winters as a rosette but in spring-sown crops it can establish as a seedling then flower and produce seed within the life of the crop.
Image top: Variegated thistle flower, © M. Baker.
Images above, L-R: Variegated thistle infestation, & variegated thistle leaves, © M. Baker.
Variegated thistle in Tasmania
- Variegated thistle occurs in most agricultural areas of Tasmania. Infestations are most common in the Midlands and in the south, lighter in the north-east and north-west, and very light in the far north-west.
- Variegated thistle is found on roadsides and in waste areas, in pastures and occasionally in crops. Variegated thistle grows best in areas of high fertility such as sheep camps and stock-yards.
- The rosettes are strongly competitive in pastures and crops causing severe yield losses. Dense infestations kill other vegetation, leaving large bare patches when the thistles die off, which are then vulnerable for re-infestation by weeds.
- Some livestock (especially cattle) can suffer from nitrate poisoning from grazing variegated thistle. Grazing of variegated thistle can lead to the rapid onset of toxicity symptoms and death, or to chronic symptoms of ill-thrift depending on nitrate concentrations in the plant and the amount of material consumed. Variegated Thistles may be consumed in large quantities where little alternative feed is available or palatability is increased by cutting or by herbicide application.
Detailed management and control guidelines for variegated thistle can be found in the Variegated thistle Control Guide. Refer also to
Herbicides for Variegated thistle Control. For further information see Weed Links and Resources.
See also:
Weed Links and Resources
Other useful links:
Pest GenieAPVMA
Variegated Thistle Control Guide
Do
- Plan your control program, this will save time and money in the long-run;
- Consider the impact of your control methods on off-target species, especially if herbicides are used;
- Ensure machinery and equipment is washed down between sites or prior to contractors leaving site;
- Get in early - For new infestations, eradicate before the plants reach the flowering stage: once plants begin seeding, control becomes more difficult and expensive;
- Carefully time your use of herbicide for best results (see
Herbicides for variegated thistle control for more information);
- Coordinate your control program with neighbouring landholders where your weed problem crosses property boundaries; and
- Revisit and regularly inspect the site and ensure follow-up is undertaken.
Don't
- Don't introduce thistles to thistle-free areas (e.g. by failing to wash down machinery and equipment between sites);
- Don't start your control program without first planning your approach;
- Don't allow variegated thistle to flower and set seed before treatment; and
- Don't rely on one attempt at removal - follow-up is essential.
Spread of variegated thistle
- Variegated thistle is spread by seed. Variegated thistle seed has only a rudimentary 'pappus' or parachute of hairs and is not spread by wind.
- Variegated thistle seed is commonly spread in cereal seed. Seed is also spread by humans and livestock, in the fleece of sheep, on the wheels of vehicles, and by run off during heavy rain.
Avoid the introduction of variegated thistle
- Avoid introducing variegated thistle seed into clean areas, or into areas from which the weed is being eradicated.
- Implements and vehicles which have been used on infested areas should be thoroughly cleaned on leaving. See the
Washdown Guidelines for Weed and Disease Control for detailed information on how to wash-down equipment and personnel to reduce the chance of spreading variegated thistle.
- All seed, feed grains or hay should be free of variegated thistle seed.
- Any livestock suspected of carrying seed on their bodies or in their digestive system should be held in a suitable area for approximately two weeks before being put on clean paddocks.
- Special care should be exercised when buying sheep from other properties as seed is readily carried in wool.
Physical removal
- Individual plants and small patches can be removed using hand-hoeing. Ensure that the growing point and the top 20 to 40 mm of the taproot are removed.
Cultivation
- Variegated thistle does not readily invade healthy and vigorous pasture, and seedlings establish mainly when pasture opens up due to over-grazing or insect damage.
- Aim to maintain a vigorous pasture. Heavily infested areas may need to be re-sown with perennial pasture grasses.
Chemical control
- A number of herbicides are registered for use on variegated thistle in Tasmania. See
Herbicides for Variegated Thistle Control for more information.
- To avoid poisoning, stock should be removed from infested areas before any herbicide application and kept off the area until the thistles are completely dead. This may take 2-3 weeks after spraying.
Herbicides for Variegated Thistle Control