APC (2020). Australian Plant Census, Council of Heads of Australasian Herbaria (CHAH). Available at: https://biodiversity.org.au/nsl/services/apc. [accessed 18/11/2020].
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Perennial Thistle (Cirsium arvense) is a perennial, herb, commonly to 300-900 mm tall, but occasionally grows to 1.5 metres tall. New seedlings produce a taproot and rosettes of leaves. Taproots produce an extensive lateral creeping roots (rhizomes) that produce vegetative buds, from which new erect shoots and rosetted plants develop. The rosette leaves are up to 100-150 mm long, with lobed and with spiny undulating margins, slightly hairy on top and white woolly on the underside although this is variable, oblong (length a few times greater than width, with sides almost parallel and ends rounded) to lanceolate (lance-shaped; about 4 times as long as broad, broadest in the lower half and tapering to the tip). Rosettes eventually produce an erect hairless or slightly hairy ridged stem, that branches in the upper part. The stem leaves and similar to rosette leaves, but can be up to 200 mm long, and more deeply lobed, and can be oblanceolate (lance-shaped; about 4 times as long as broad, broadest in the upper half and tapering to the base. Stem leaves are alternately placed on the stem with the base of the leaf scarcely forming a wing on the stem.
Flowers are in compound daisy-like flower-heads, 2-2.5 cm long and 1.5-2.5 cm diameter and form in clusters of 1-5 at the ends of the stem branches. Flower heads are spineless and small compared to other thistles (DPIRD 2020). Each flower-head has several rows of purple-tipped spiny floral bracts (modified leaves) which surround the numerous tubular pink or light purple florets which are longer than the bracts. Each flower head can produces many flowers, and flowering normally occurs from December to March. On some plants flower-heads are functionally female, ovoid in shape and have only seed producing perfumed florets. Each floret ultimately forming a single seed with many seeds produced on a flower-head. Other plants have only male flower-heads, globular in shape and with male florets producing pollen but not setting seed.
(Note that in Wales some flower-heads have been shown to be hermaphrodite producing pollen and setting viable seed but how widespread this is is not known).
The fruits or 'seeds' (cypselas) are brown, smooth, shiny and finely striate, slightly flattened and slightly curved, 2.5-4.5 mm long and 1 mm wide, with a pappus of silky white feathery hairs up to 3 cm long on top of the seed. The pappus is easily detached and does not help spread seed more than about 10 m from the parent (Harden 1992; Parsons & Cuthbertson 1992).
For further information and assistance with identification of Perennial Thistle contact the herbarium in your state or territory.
Pink or purple
Perennial Thistle is a species of cold to warm temperate areas with rainfall or more than about 700 mm per year, with fertile deep loamy soils, and plenty of sunshine. It occurs from sea level to subalpine areas. It can tolerate heavy frosts and likes soils high in nitrogen. Open areas are preferred for initial establishment, and the species is generally found in pastures, crops, roadsides and neglected areas (Parsons & Cuthbertson 1992).
Perennial Thistle (Cirsium arvense) can be distinguished from its sister species Spear Thistle (Cirsium vulgare) by its perennial rhizomatous root system and male and female flowerhead occurring on separate plants. Spear Thistle is not rhizomatous and dies after shedding seed, and the flowerheads are larger than those in Perennial Thistle and the florets produce both pollen and seeds. Perennial Thistle also lacks wings on the stems (Navie 2004; Slee 2007, pers. comm.).
The Slender Thistles (Carduus pycnocephalus and Carduus tenuiflorus) and Nodding Thistle (Carduus nutans) may also be confused with Perennial Thistle. The Slender Thistles are short-lived, have winged stems and generally smaller, stalkless (sessile) flowerheads. Nodding Thistle also has winged stems and larger, stalked (pedunculate) flowerheads with a characteristic drooping form (Navie 2004).
Perennial Thistle (Cirsium arvense) impacts on agricultural production, especially in degrading valuable grazing land on normally productive fertile soils. Its persistent root system and perennial habit make it particularly troublesome (Parsons & Cuthbertson 1992). Perennial thistle colonise disturbed areas. Perennial thistle forms large colonies with dense growth crowding out desirable plants. It is a weed of pastures, crops, roadsides and wasteland in higher rainfall areas.
Agriculture: Perennial Thistle competes with crops such as potatoes, asparagus and peas and can interfere with harvesting. The flower buds are similar in shape, size and texture to shelled peas creating problems in mechanically harvested pea crops (Parsons & Cuthbertson 1992). The spiny plants can form dense swards of stems to 2 m tall impeding movement of stock and humans. If grazed it can cause mouth ulceration in sheep (Parsons & Cuthbertson 1992; DAF undated). Once established on a bare area Perennial Thistle can shade out desirable species, or prevent growth of desirable species by allelopathic effects (which inhibit the growth of other species) on adjacent pasture plants. It can invade native grasslands through establishment on disturbed bare areas (Parsons & Cuthbertson 1992). It harbours insects that attack commercial crops and is an alternate host for certain plant pathogens (Parsons & Cuthbertson 1992).
Native ecosystems: Not widely known from native vegetation.
Urban areas: Potentially problematic in gardens, roadsides, and urban situations.
Most seed of Perennial Thistle falls near the parent plant. The seed has a pappus of silky bristles to aid dispersal but this is very easily dislodged from the top of the seed. There are records of seed being blown 10 m, and up to 247 m, but other studies suggest that that seed tends to stay in the flowerhead and is only released when the flowerhead rots. Most widespread dispersal seems to be from contaminated agricultural seeds, irrigation water, contaminated agricultural machinery etc. (Parsons & Cuthbertson 1992).
Within Australia seed was thought to be rarely produced as plants were uncommon and male and female plants needed to grow within 100 m of each other for successful pollination. But as plant numbers gradually increased also sexual reproduction increased (Parsons & Cuthbertson 1992).
At a more local scale individual plants spread by growing new shoots from vegetative buds on the roots. Cultivation of existing sites of infestation can fragment and distribute roots bearing buds which may then grow into "new" plants (clones). Small pieces of root can effectively emerge as new shoots from a depth of about 50 cm. A small piece of root bearing vegetative buds can produce plants covering 25 square metres in a year (Parsons & Cuthbertson 1992).
Perennial Thistle was first declared noxious in Victoria in 1885, where it remains a problem: mainly in wetter southern pastoral areas. Time of introduction into other areas of Australia is uncertain although a specimen was collected in Western Australia at Geraldton in 1933. Much of its spread corresponds with the increased sowing of oats in Australia in the 1920s and 1930s (Parsons & Cuthbertson 1992; Hussey et al. 1997).
For Perennial Thistle (Cirsium arvense), as with all thistles the best means of control is to prevent initial establishment. With Perennial Thistle this means maintaining good quality pasture free from bare areas as germination is always more successful on open sites. Once an infestation is established control is problematic because of the rhizomatous nature of the plant. An integrated approach using a combination of strategies is likely to be the most effective (Parsons & Cuthbertson 1992).
Chemical control: Although the top growth Perennial Thistle is susceptible to many herbicides, long term control is difficult to obtain, as it is with most deep rooted perennials. Herbicide applications can be effective if the right herbicide is chosen and applied at the right time. As with other thistle species, seedlings are the most susceptible growth stage and provided the plants are growing vigorously, treatment should be applied as soon as germination is complete. Care should be taken to differentiate between seedlings and rosettes that have regrown from root stock as the response to control methods are different. Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au.
Non-Chemical control: Cultivation of infested areas is not advised because this fragments the rhizome and can seriously spread the problem by moving roots with vegetative buds to new areas. The only exception to this is if the ground can be repeatedly cultivated every couple of weeks to totally prevent emergence of any shoots until the root system is exhausted. It is though that over about a year the root system will die. However, it generally at least two to three years are required. In the mean time the land is out of production and this method could also contribute to soil erosion and a breakdown of the soil structure (Parsons & Cuthbertson 1992). Pasture species which grow actively in summer, such as lucerne and phalaris, are good competitors and can out-compete Perennial Thistle especially when combined with regular mowing (Parsons & Cuthbertson 1992). Parsons & Cuthbertson (1992) also comment that studies in New Zealand have suggested that goat grazing can be very damaging to Perennial Thistle.
Biological Control: Perennial Thistle has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. However, Biological control involving a rust fungus and Puccinia punictiformis and native aphids have been known to restrict its spread but do not offer long term control (Parsons & Cuthbertson 1992).
For further information on control and management of Perennial Thistle see: DiTomaso & Kyser (2013); DPI NSW (2020) and DPIRD (2020); Parsons & Cuthbertson (1992); TPIPWE (2019).
New seedlings germinate in spring or autumn and produce a vertical taproot and a rosette of leaves. The vertical taproot after two months, then produces lateral roots that eventually arch down vertically again. These lateral roots produce with growing points (rhizomes) produce areal shots at the point of the arch, with these vegetative shoots (plants). New vegetative shoots develop in late winter and early spring (or summer in Tasmania) and plants develop a dense growth of rosettes during springs. The plants produces continues to produce extensive lateral creeping roots (rhizomes).
Flowering stems then develop from well established rosettes in later spring with flowering occurring in summer. Some plants flower in their first year but this is not common. Plants die off after flowering and seed is shed from the female plants, apparently affected by frost, but the roots bearing vegetative buds survive to re-infest the area as the weather starts to warm up (Parsons & Cuthbertson 1992).
Seed storage in the soil is reported to be for as long as 21 years for at least a small percentage of the seed produced. However, most seeds germinate within a short time after being shed (Parsons & Cuthbertson 1992)
NSW, SA, TAS, VIC, WA
Perennial Thistle is not common in New South Wales but has been recorded in the South Coast, Southern and Northern Tableland and Central Western Slopes regions (Harden 1992).
In South Australia Perennial Thistle has been recorded from the Southern Lofty region and the South-east regions in the past, but in 2005 was only recorded from a localised population in the south-east of the state (Government of South Australia 2015).
In Tasmania it is common in all areas including the drier eastern regions, but is a particular problem in the wetter north-west of the state (TPIPWE 2019)
It is a problem in wetter southern pastoral areas of Victoria. It is abundant in south Gippsland, on the Gippsland Plain from Bairnsdale to Rosedale, and sporadically in the Western District around Colac and west to Warrnambool. It also occurs on arable land around Ballarat. Isolated infestations occur around and north-west of Cann River, north of Buchan to Wulgulmerang, near Mansfield and east of Jamieson.
Infestations in Western Australia were small and were recorded from Geraldton, around Fremantle and at Walpole west of Albany. It has not been recorded in Western Australia for a number of years (DPIRD 2020).
Perennial Thistle is native in Europe, western Asia (i.e. Eurasia) and northern Africa (Navie 2004; GRIN 2007). It is now widely naturalised in climatically temperate parts of the world, and a serious and troublesome agricultural weed in many countries including many of those where it is a native species (Parsons & Cuthbertson 1992).
SA, TAS, VIC, WA
Canada Thistle, Creeping Thistle, Californian Thistle, Perennial Creeping Thistle.