Weeds can be a burden on any property, but for livestock producers in particular, they present a significant economic risk. Research conducted by the Centre for Invasive Species Solutions in 2018 showed that weed management costs Australia over $5 billion annually – and one type of weed that has been found to cause significant damage, due to its quickly-spreading nature, is the thistle.
Thistles are a prickly annual weed that have caused major disruptions to farming practices all over Australia, including large sections of New South Wales, Victoria, South Australia, and Western Australia. So, to avoid economic distress and ensure the health of your pastures and subsequently, your stock, it’s crucial to know how to manage thistle growth and prevent these pesky plants from irreversibly taking over your land.
Keep reading as we learn how to identify various types of thistle, and explore different techniques for managing the spread of thistles with herbicides.
What are thistles?
Thistles are a type of flowering broadleaf weed characterised by their sharp leaves and prickles, which usually occur on the margins of the leaf, but can also be found on the stem or flat parts of the leaf. Most thistles fall under the family Asteraceae – common species in Australia include the scotch thistle, Illyrian thistle, stemless thistle, variegated thistle, and the spear thistle.
Why is eradicating thistles so important?
It is paramount that livestock producers take precautions to prevent and eradicate thistles on their property, as they can compete with pasture growth and in turn, have a negative effect on the carrying capacity of your property. This is because thistles are prolific seeders, meaning they grow quickly and often uncontrollably.
Once established, thistles can be very difficult and expensive to control, meaning you’ll have to funnel more time, energy, and money into keeping them in check – resources that could have been allocated elsewhere.
Thistles can be detrimental to your pastures and animals in a number of ways. These weeds smother pastures in spring, which inhibits their growth in those crucial early stages. As these thistles mature and cover more of your property, they can create barriers that prevent livestock movement, which consequently affects weight gain and overall animal health.
Parts of the thistle plant can also break off, catching in the wool of sheep or other wool-producing animals, impacting the value of your wool clip.
Finally, thistles pose a risk of injury to both livestock and those responsible for handling them, as they are spiky and often hard to avoid even when properly protected.
It’s clear from the outset that because thistles can create a major risk to the overall livelihood of your farming business, prevention is the best medicine. Eradicating thistles is an ongoing job, but with the use of the correct herbicides at the right time, coupled with key land management and biosecurity measures, you will be able to keep this weed burden under control or prevent it from escalating, saving you time, money, and most importantly, ensuring peace of mind.
What are the main types of thistles found in Australia?
There are five major thistle weeds found in pastures in Australia, each with a unique appearance and differences in the way they grow and spread.
The scotch thistle is native to Europe and Western and Central Asia, and was first introduced to Australia as an ornamental plant (a type of plant known for its beauty and aesthetic characteristics). Scotch thistle grows in large areas of New South Wales, including across the Tablelands in their entirety and in the North-West, Central, and South-West Slopes. It is also found throughout Victoria and in southern parts of South Australia and Western Australia. Scotch thistle thrives in well-drained soils with moderate to high fertility, but is common, particularly in high-fertility areas with perennial pastures that get heavy winter rainfall.
The scotch thistle is characterised by an erect single stem with many branches, which are covered with thick woolly hairs of a white-grey colour. It has broad, spiny wings along its stems. As thistles are a flowering plant, a good way to tell species apart is through their blooms – the scotch thistle has a round purple flower with spiny, needle-like bracts that are yellow and around 3mm long at the base.
The scotch thistle spreads by seed, which can easily attach to stock and clothing due to its tiny, fine hairs. This means they are often spread in hay, or via machinery and vehicles moving throughout yours and other properties. The seed can also survive the digestive tract of animals like sheep, which means it can also be spread via their droppings as a result.
Grown in areas similar to the scotch thistle, you’ll find Illyrian thistle in Southern New South Wales, and throughout Victoria and South Australia. This weed flowers from approximately December to February, but as an annual, is present year-round. Illyrian thistles spread just like scotch thistles, and they are often mistaken for one another.
From a physical perspective, the Illyrian thistle is erect and can grow to 1.5 to 2m high. It has the same white, woolly appearance as the scotch thistle, however, the key difference between these two weeds is their flowers. The Illyrian thistle forms a rosette in its first year of growth, which then develops into an erect stem with a flower at the tip. You’ll notice that the leaves of the Illyrian thistle rosettes are deeply lobed with a different figurehead to the scotch thistle. Its flowers are also purple, however, they protrude more than the scotch thistle’s, and the Illyrian thistle’s bracts are reddish-purple in colour, not yellow.
Like other thistles, illyrian thistle can have a major impact on pastures. The plant’s rosette-like leaves can smother pastures in early spring, affecting their growth in this important phase. Illyrian thistle spreads through livestock, machinery and vehicle movement, hay production, and sheep droppings.
Found in the south-west plains of New South Wales, as well as in Victoria, South Australia, and Western Australia, the stemless thistle is one to watch out for. As a prolific breeder, it can grow very quickly if not contained properly. Its seed is spread predominantly by wind dispersal, but it can also spread through livestock and machinery movements and hay production, similarly to other common types of thistle. The stemless thistles thrives in warm, temperate regions with high rainfall and sandy soils. It is drought-tolerant, making it even more difficult to control, and can have a high impact on pasture growth.
This type of thistle is easy to identify as, because the name suggests, it is stemless. It has a wreath-like appearance with flower heads in the centre of the plant. Like other kinds of thistle, such as the illyrian thistle and scotch thistle, it has a whitish, woolly appearance with purple flowers.
Also known as the milk thistle, the variegated thistle is a member of the daisy family. It’s found most commonly in New South Wales, Victoria, South Australia, and Western Australia, and prefers areas with medium to high rainfall. Like all thistles, it competes for the moisture, light, and nutrients that pastures and grasses need to be hospitable for livestock, which means that in large quantities, this thistle can quickly overrun pastures and prevent livestock movement and inhibit their health.
The variegated thistle can be identified by its broad rosette of large, shiny, dark green leaves with white variegated veins through the leaf. At the point of each lobe, the leaf has a visible spine. Each plant can cover an area of approximately 1m, and can grow up to 1.8m high.
Under certain conditions, variegated thistles can be poisonous to cattle and sheep, especially when consumed in large quantities and not alongside other grasses. While the spines of the variegated thistle can often deter animals from consuming it, when wilted, the plant is more palatable to livestock. Like all other thistles, the variegated thistle reproduces through seed which can be spread manually by livestock movements, machinery use, and more. The worst infestations of the variegated thistle usually occur when the ground is bare in autumn and the weed seed is facing very low competition from other plants. Most germination then happens after autumn rain, however this can also occur during winter and spring.
Finally, the spear thistle is common throughout Australia, especially along the coast and in the tablelands. You’ll undoubtedly find it in the western plains of New South Wales, throughout Victoria, and in southern parts of Western Australia. It prefers cool temperatures, full sun, and heavy clay soils.
The spear thistle is an upright plant that starts in a rosette shape, and then grows from a central point, eventually forming a purple flower at the top. As the plant develops, it grows upright branches with spiny stems. Its leaves are dark green, rough, and hairy on top, and white and woolly underneath. The seeds are noticeably surrounded by tufts of white hairs.
This weed mainly grows on agricultural lands, where it competes aggressively with pastures. Each plant can produce up to 8,000 seeds per year that can be spread by wind, moving water, livestock, machinery and vehicle movements, and hay production.
How to eradicate thistles
Now that you know how to identify thistles, let’s talk about eradicating them from your property to avoid any lasting damage to your livestock production. The best way to eradicate thistles is by using herbicides that kill the plant in spring and early summer before they have a chance to take hold and begin to impact pastures and livestock. Allowing thistles to develop and flower will only exacerbate the problem as, over time, the seeds will spread, leading to germination in the following spring.
There are a number of excellent herbicides that can be used to effectively treat thistles. Some of the best products pair active ingredients with combination products – these include bromoxynil and MCPA, and aminopyralid and fluroxypyr. You might also select a product with a single active ingredient such as MCPA 750, dicamba, glyphosate, fluroxypyr, 2,4-D ester, and clopyralid.
Many of these products can also be used to treat other broadleaf weeds in pastures and crop settings, which makes them multi-purpose when attempting to tackle noxious weed growth on your property. Over time, this saves labour and money, and ensures better pasture quality and carrying capacity for your property. Improving pasture quality, paddock rotation, and implementing biosecurity measures when moving machinery and bailing hay can also help to reduce thistle growth on your property, however, these should always be employed in conjunction with an effective herbicide of your choice.
While thistles can be extremely challenging to control, with the right knowledge and a herbicide tailored to your specific circumstances, there is a fantastic chance of successful eradication. At Specialist Sales, we offer a range of herbicide solutions, no matter your budget or the area you are treating. If you need expert advice and assistance with eliminating thistles, please get in touch with our experienced customer care team.