What you need to know about variegated thistle
Variegated thistle (Silybum marianum) gets its scientific name from the Greek word, silybon, which is the name of a thistle-like plant. Variegated thistle is a member of the daisy family, Asteraceae; a very large family that includes many weeds such as thistles (Cirsium spp., Carduus spp., Onopordum spp. etc.), flatweed (Hypochaeris radicata), English dandelion (Taraxacum officinale), capeweed (Arctotheca calendula) and skeleton weed (Chondrilla juncea). The daisy family also includes native species such as the Swan River daisy (Brachysome iberidifolia), introduced garden plants such as marigolds (Calendula officinalis) and crop plants such as lettuce (Lactuca sativa) and sunflowers (Helianthus annuus).
Variegated thistle is sometimes incorrectly referred to as Scotch thistle. However, the true Scotch thistle (Onopordum acanthium) is not known to occur in Western Australia (WA). Variegated thistle is a native of Eurasia and the Mediterranean and was probably introduced to WA in the early days of settlement. It is a declared plant (noxious weed) throughout the State. It is a weed in New Zealand, North and South America, South Africa and Afghanistan as well as in the countries of its native range. It is also a weed in South Australia, Victoria, New South Wales, Tasmania and Queensland.
Why variegated thistle matters
Variegated thistle is an important weed of the high and medium rainfall districts of the south west of WA. It is well established in areas such as Donnybrook and Bridgetown and competes with useful pasture plants for light, moisture and nutrients. If left unchecked, it can form very large infestations, choking out valuable pasture species as well as weeds. In such dense infestations, it can form impenetrable stands and provide shelter for pest species such as rabbits.
Variegated thistle is poisonous under certain conditions. It can kill cattle and occasionally sheep, especially when hungry animals consume large quantities in the absence of alternative feed, although its menacing spines act as a deterrent. It becomes more palatable to stock and more toxic when it has wilted, for example, after cutting, ploughing or spraying. Its toxicity is due to high levels of nitrates, especially when growing on high-nitrogen soils such as sheep camps, when growing in shade or after prolonged dull weather. Symptoms start with trembling and staggering. Animals can be saved if they receive prompt veterinary care.
Its needle-like spines can cause injury to animals, including domestic dogs. Variegated thistle can be a serious contaminant of wool.
What you should look for
Variegated thistle forms a broad rosette of shiny pale or dark green leaves variegated by a network of white veins. The leaves are deeply lobed with a spine at the point of each lobe. Each plant may cover an area of over one metre in diameter. In spring a branched flowering stem without spines grows up to 300 centimetres high, though most plants are 90-180 centimetres high. The stems are finely ribbed and are not winged. The stems can be hollow or filled with pith. Flowering starts in October and continues until early summer. One large purple compound flower is produced at the end of each stem branch. A protective circle of spines up to 13 centimetres in diameter surrounds each flower. These spines are like needles and can easily pierce heavy leather gloves. It could be confused with other thistles, but it is the only species in WA that has variegated foliage. Stemless thistle (Onopordum acaulon) has grey foliage but never produces a stem.
Life history of variegated thistle
Variegated thistle is usually an annual plant but occasionally does not flower until the second year. It reproduces only from seed. The seeds are black or brown, about five millimetres long, and somewhat flattened with a smooth seed coat. Each seed has a parachute-like structure of hairs about two centimetres long; this is called a pappus. Seed set takes place in summer. The seeds are too heavy to be blown far by wind in spite of the pappus, therefore most seeds fall onto the bare ground at the base of the dead parent plant. Variegated thistle seed may remain dormant in the soil for up to nine years. As little as 50% of the surviving seed may germinate in any one year. Seeds may be carried by water, vehicles, machinery or stock and in hay, chaff or silage.
The worst infestations occur in years when the ground is bare in autumn as the weed seedlings face little competition. Most germination occurs after autumn rains, but can occur in winter and late spring. Control is more difficult when there are several germinations during the year. The rosettes grow rapidly during the winter and early spring, reaching up to a metre in diameter, then sending up a flowering stem. Flowering usually starts in October. A large plant can produce many flower heads, and each flower head can produce up to 200 seeds.
What you can do about variegated thistle
Practise good biosecurity because if you haven't got variegated thistle, you don't want it! Take particular care when purchasing fodder, soil, garden mulch and so on, to avoid buying anything contaminated with weed seeds or toxic plants. Ensure contractors entering your property have clean equipment to avoid the introduction of new weeds.
If you have variegated thistle on your property, take care to prevent it spreading to other properties. Control small infestations before they spread. Join forces with your neighbours and your local government authority to remove variegated thistle and other weeds. You can obtain control advice by telephoning the Pest and Disease Information Service.
Ploughed firebreaks provide an ideal seedbed for variegated thistle and other weeds such as narrow leaf cottonbush (Gomphocarpus fruticosus) so you may wish to install chemical firebreaks instead. Chemical firebreaks have the added advantage of providing a firm surface that can be used as an access track or an escape route in the event of fire.
Prompt action is necessary to eradicate new and/or isolated patches of variegated thistle and to prevent the weed becoming established in new areas. Many types of thistle, including variegated thistle, become established on bare or over-grazed patches of land, so practise good pasture and livestock management to avoid over-grazing. In other words 'Always live on the top half of your grass'. Good pasture management will also help to prevent soil erosion and other problems and to maintain your property's value.
If you cannot adequately manage livestock to control weeds and other excess vegetation, consider using slashing, mowing and/or spraying instead of grazing. Small landholders may find the combination of a backpack sprayer and a ride-on mower adequate to keep weeds under control and reduce the risk of fire.
To avoid nitrate poisoning, do not allow stock to access areas where variegated thistle has been slashed or has wilted due to ploughing or spraying. Please seek veterinary advice promptly if you suspect stock have been poisoned by variegated thistle (or other weeds), in order to minimise death and suffering.
Some weedy thistles are promoted as medicinal herbs, or as cottage garden plants. Avoid cultivating weedy thistles and thistle-like plants such as knapweeds in your garden. Do not order seeds from mail order catalogues or the internet unless you know they are species which are permitted entry to Western Australia. To find out whether a species is permitted entry to WA, search the Western Australian Organism List (WAOL).
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Where can I get more information about thistles?
The book Noxious Weeds of Australia, by W.T. Parsons and E.G. Cuthbertson (2001, CSIRO Publishing) has detailed information about all the thistle species that are declared noxious in every State.