APC (2020). The Australian Plant Census, Council of Heads of Australasian Herbaria, Avalible at: https://biodiversity.org.au/nsl/services/apc , [accessed 24/11/2020].
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Perennial Sow-thistle (Sonchus arvensis) is a perennial (long lived) herb with long creeping roots (rhizomes) or creeping stems (stolons), spreading vegetatively, with other plants shooting and growing from the creeping roots and stems. The entire plant is filled with a milky white sap. The stems are upright, simple or branched (mainly near the top), finely grooved or ribbed, hollow and 50-150 cm high, with two to many stems per plant. The leaves are alternate, variable in size, and variably divided, with coarsely toothed edges and pointed lobes which are bent downward. They are firm and dark green, paler on the lower surface, hairless and lance-shaped. The rosette and lower stem leaves are 15-35 cm long and 3-10 cm wide, divided about halfway to the midrib. The upper leaves are less lobed, and are narrow-triangular to awl shaped. The stem leaves clasp the stem and do not have stalks (Cooke 1986; Garnock-Jones 1988; Stace 1997).
The inflorescence (flower structure) is branched, and consists of numerous flower-heads. The flower stalks are 1-5 cm long, hairless or densely glandular hairy. Each flower-head is surrounded by a graded series of 25-45 bracts (modified leaves), which are hairless or are covered with tangled short, white branching hairs, and long yellow glandular hairs. The flower head consists entirely of ligulate florets (petal-like flowers), and has both male and female parts. There are no tubular florets. The flower heads are 40-50 mm across. The blades of the ligulate florets are yellow, spreading, and up to 3 cm long (Cooke 1986; Garnock-Jones 1988; Stace 1997). Each flower produces a seed with many seeds produced on a flower-head.
The fruits or 'seeds' (cypselas) are dark brown, not winged, and are narrowly oval in shape, weakly flattened, 2.5-3.8 mm long, and 1.0-1.5 mm wide, with 4-6 lengthwise ribs on each face. The ribs and the spaces between them are conspicuously transversely wrinkled. The top of the fruit has a tuft (pappus) of numerous silky, white, parachute-like hairs, 10-14 mm long, that are not shed (Cooke 1986; Garnock-Jones 1988; Stace 1997).
For further information and assistance with identification of Perennial Sow-thistle, contact the herbarium in your state or territory.
Perennial Sow-thistle grows best in poorly drained, moist sites that are rich in organic matter, usually on fine-textured soils, especially loams. It prefers slightly alkaline to neutral soils and does not do as well in acidic or highly alkaline soils (Clapham et al. 1962; Lemna & Messersmith 1990; Kravchencko 2006; Washington State Noxious Weed Control Board 2007). In North America it can also establish on dry sites, and sites with little disturbance (McWilliams 2004). In Canada, Perennial Sow-thistle occurs in areas that receive average annual precipitation of 300-3 000 mm (McWilliams 2004). It is also tolerates low levels of salinity (Kravchenko 2006). In North America, it is typically found in cultivated fields, waste areas, moist meadows, woodlands, shores of lakes and rivers, beaches, ditches, lawns, and roadsides (Lemna & Messersmith 1990).
In the United Kingdom, it grows in arable and waste land, is common in abandoned pastures, waysides, dunes and shingle by the sea, ditches and riverbanks (Stace 1997; Bond et al. 2007).
Perennial Sow-thistle is also naturalised in New Zealand, and has been recorded from roadsides, hedges, swamps, and cultivated land (Garnock-Jones 1988).
According to Holm et al. (1997) Sonchus arvensis is reported as a weed in 59 countries, and is most often mentioned as a serious weed in cereal crops. It is listed as a serious or principle weed in 15 countries.
There are three species of Sonchus in Australia: the alien Rough Sow-thistle (Sonchus asper) and Milk Thistle (Sonchus oleraceus), and Native Sow-thistle, (Sonchus hydrophilus), which is believed to be native. Perennial Sow-thistle differs from these other species in its perennial habit (the others are either annual or biennial), in having creeping roots (the other three species are tap-rooted), and in having narrow oval fruits, which are transversely wrinkled (Thompson 2007).
Dune Thistle (Actites megalocarpus) can often be confused with species of Sonchus. It is perhaps most likely to be confused with the Perennial Sow-thistle on account of its perennial, rhizomatous (creeping underground stem) habit. Dune Thistle differs in having smaller flower-heads than Perennial Sow-thistle, 10-20 mm across (compared to 40-50 mm in Perennial Sow-thistle), and larger fruits, 4-8 mm long (compared to 2.5-3.8 mm in Perennial Sow-thistle). The margins of its achenes (fruits) are winged, unlike Perennial Sow-thistle (Jeanes 1999).
In Australia, other alien Sow-thistle species, such as Milk Thistle (Sonchus oleraceus), are major weeds of cropping systems, especially in south-eastern Queensland and northern New South Wales, and the importance of Milk Thistle as a weed appears to be increasing (Widderick et al. 1999; Chauhan et al. 2006). Given the problems caused by other Sow-thistle species in Australia, and the weed history of the Perennial Sow-thistle (Sonchus arvensis) in North America, it is a potentially serious threat to agriculture in climatically suitable areas of Australia (Stajsic pers. comm. 2008).
Agriculture: Perennial Sow-thistle (Sonchus arvensis) is a weed on arable land, and produces competitive plants particularly in annual, agricultural and horticultural crops in regularly cultivated fields. It can cause weed problems in any annual crop in temperate areas. (CABI 2020). Perennial Sow-thistle (Sonchus arvensis) is one of Russia's worst weeds. It infests all types of crops, and also occurs in fallows, vegetable and fruit gardens (Kravchenko 2006). In the United States of America and Canada, where it is naturalised, Perennial Sow-thistle is often an invader of cultivated areas, especially of small grain and row crops (McWilliams 2004; Washington State Noxious Weed Control Board 2007). In Canada, it has been reported as the fifteenth most abundant weed in cereal and oilseed crops surveyed, where it occurred in 39% of rapeseed fields (Peschken et al. 1983). It causes economic loss by reducing crop yields, and increasing cultivation and herbicide expenses. It has reduced spring wheat yields by up to 45% in North Dakota (Washington State Noxious Weed Control Board 2007). Perennial Sow-thistle is a host of several important agricultural pests and diseases, such as the potato tuber rot nematode (Ditylenchus destructor) (Lemna & Messersmith 1990; Bond et al. 2007).
Native ecosystems: Sow-thistles are also environmental weeds in Australia. Both the Milk Thistle and the Rough Sow-thistle (Sonchus asper) are considered to be serious environmental weeds in Victoria, where they have invaded at least 13 types of Victorian native vegetation (Carr et al. 1992). Perennial Sow-thistle is a potentially serious environmental weed in climatically suitable areas of Australia because it is long lived, spreads rapidly due to its creeping roots and has a broad ecological range (Stajsic pers. comm. 2008).
Urban areas: Perennial Sow-thistle (Sonchus arvensis)b is a weed of gardens in other parts of the world and could be come problematic in Australia, similar to other Sow-thistles spreading to roadsides, road reserves, neglected and abandoned areas.
Perennial Sow-thistle reproduces by seeds, and vegetatively by its long creeping roots. At sites where the species is established, the creeping roots allow the plants to produce new shoots to colonise new sites. The species may also be dispersed from site to site by the transport of root fragments in soil (Washington State Noxious Weed Control Board 2007; Federal Noxious Weed Disseminules of the U.S. undated).
The fruits are dispersed by wind and water, and as the pappus has hooked hairs, they are able to cling to the fur of animals, clothes, vehicles, farm machinery and tools. They can also be dispersed as contaminants of agricultural products such as commercial seed and hay (Lemna & Messersmith 1990; Washington State Noxious Weed Control Board 2007; Federal Noxious Weed Disseminules of the U.S. undated).
The species has been recorded only once in Australia, at Clare in South Australia in 1960, and there have been no subsequent collections or reports of the species in Australia. It is unclear as to whether the Perennial Sow-thistle died out naturally or was extirpated. Its origin at Clare is unknown. It may have been introduced as a seed contaminant (Barker et al. 2005; Thompson 2007).
The control of Perennial Sow-thistle (Sonchus arvensis) in Australia should aim to contain and eradicate populations. Ideally, control or eradication should be aimed no later than the rosette stage, prior to the development of the creeping root system, to prevent spread by root fragmentation (Kravchenko 2006). As with Perennial Thistle (Cirsium arvense), another perennial herb in the same family, that also has rhizomes with similar life cycle and requirements as Perennial Sow-thistle, the same or similar control methods should apply. The best means of control is to prevent initial establishment, or maintaining good quality pasture free from bare areas as germination is always more successful on open sites. If 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.
Chemical control: Herbicides probably provide the most effective means of controlling Perennial Sow-thistle, and various types are used in North America (Cowbrough 2005). The same methods and consideration could be employed for Perennial Sow-thistle (Sonchus arvensis) that are for used for Perennial Thistle (Cirsium arvense). Herbicide applications should 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, and treatment is 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. Although the top growth may be susceptible to many herbicides, long term control is difficult to obtain, as it is with most deep rooted perennials.
Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au
Non-chemical control: Physical control: Isolated plants and small infestations can be physically hand pulled, hoed or grubbed. It is important that all root fragments are collected to prevent any new plants re-establishing from root fragments left buried in soil. The site may require monitoring over several years to ensure that any re-sprouting plants are destroyed.
Competition and management: Tillage and cultivation of land has varying levels of effectiveness depending on the stage of the life cycle, the type of tillage, and the frequency. It is most effective in the early stages of growth, ideally at the rosette stage, prior to the formation of creeping roots. Fragmentation of the roots may, however, exacerbate the infestation, as root fragments left buried in the soil may develop into new plants. Cultivation of infested areas with mature plants is not advised because broken 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 thought that over about a year the root system will die. However, it may be at least two to three years. 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. Pasture species which grow actively in summer, such as Lucerne and Phalaris species, are good competitors and may out compete Perennial Sow-thistle (Sonchus arvensis), as they do for Perennial Thistle (Sonchus arvensis), especially when combined with regular mowing (Parsons & Cuthbertson 1992). Tillage is inappropriate in areas of high quality vegetation (Kravchenko 2006; Bond et al. 2007).
Grazing: In situations where Perennial Sow-thistle is found to be palatable to stock, grazing can be used to reduce flowering and seed set (Stajsic, 2008 pers.comm.).
Also see CABI (2020) for control methods used around the world.
Perennial Sow-thistle, as the name suggests, is long lived. In New Zealand the species flowers in January and February, and it sets seed in February and March (Garnock-Jones 1988). The seeds produced by self-pollination are rarely viable (Derscheid & Schultz 1960).
In the United States of America, the seeds of Perennial Sow-thistle germinate in spring, and the plants often do not flower during the first year (Lemna & Messersmith 1990; Washington State Noxious Weed Control Board 2007).
The seeds of Perennial Sow-thistle can tolerate being submerged to some degree, and viable seeds have been found in irrigation water. However, the seeds are not adapted for long periods of submersion (Bond et al. 2007).
Not known to be present in Australia
Perennial Sow-thistle has only been recorded once in Australia, at Clare in the Northern Lofty Region of South Australia, in 1960, but is now presumed to be eradicated from this location (Cooke 1986; Barker et al. 2005; Thompson 2007).
Perennial Sow-thistle is native to Europe, Northern Africa and Asia, and is widely naturalised elsewhere (GRIN 2008).
Corn Sow-thistle, Field Sow-thistle, Field Milk-thistle, Creeping Sow-thistle, Dindle, Gutweed, Swine Thistle.