Sonchus asper
(spiny sow-thistle)

Sonchus asper is an annual herb considered native to Europe, Africa and Asia that has been introduced to a wide range of countries around the world, where it frequently becomes an environmental and agricultural weed. The species grows in a wide range of habitats and climates, and produces large numbers of seeds (>20,000 seeds), which are easily dispersed by wind and water, but also as contaminants. Because S. asper is very successful colonising disturbed sites, as well as natural habitats at early successional stages, it has the potential to outcompete native plant species, inhibit the establishment of other native pioneer species and thus alter natural successional processes. It is also regarded as a noxious species due to hosting diseases and pests that affect crops.

The native distribution range of S. asper is still uncertain, but the species is considered native to Europe, Africa and temperate Asia. It has been introduced and become naturalized across parts of Asia, Scandinavia, Canada, USA, Mexico, Central and South America, the Caribbean, southern Africa, Australia, New Zealand, the Pacific Region and some sub-Antarctic islands, such as Gough Island, Auckland Islands and Campbell Island (DAISIE, 2018; Flora Mesoamericana, 2018; GISD, 2018; PIER, 2018; USDA-ARS, 2018).

No economic uses are known for S. asper, making the risk of intentional introductions low. However, this cosmopolitan weed is a prolific seed producer and its tiny seeds can be easily dispersed by human activities, as a contaminant in crop and grass seed, a hitchhiker on footwear, or attached to animals, which increases the risk of spread (Hutchinson et al., 1984; GISD, 2018; PROTA, 2018).

S. asper is a pioneer species that often invades sites disturbed by human activities, such as roadsides, cultivated fields, wastelands, gardens, gravel pits, mines and logged areas. It also occurs in areas naturally disturbed by grazing, landslides or fire. It can also be found in mountain slopes, forest and field margins, ruderal areas, grasslands, along lakeshores and streams, and on muddy sites, at elevations from 750 m to 3700 m (Flora Mesoamericana, 2018; Flora of China Editorial Committee, 2018; Flora of North America Editorial Committee, 2018; PROTA, 2018). Across Canada and the USA, this species is fairly common in riparian and coastal habitats, particularly on disturbed sites (Hutchinson et al., 1984; ANHP, 2010; GISD, 2018).

S. asper has been listed as a weed of alfalfa, cotton, coffee, beans, garbanzo beans, tomato and maize plantations (Villaseñor and Espinosa, 1998; Vibrans, 2009).

Genetics

The chromosome number reported for S. asper is 2n = 18 (Hutchinson et al., 1984; Flora of China Editorial Committee, 2018). Crossings between S. asper and S. oleraceus may occur, but the hybrids are sterile (Flora of North America Editorial Committee, 2018).

Reproductive Biology

S. asper has bisexual self-compatible flowers. Flowers are pollinated by insects, including solitary bees and flies, but also by wind. Seeds are produced both through selfing and outcrossing (Hutchinson et al., 1984).

Physiology and Phenology

In North America, seeds of S. asper germinate from spring through fall. Seeds normally germinate 2-3 weeks after planting. The seedlings attain the rosette stage at 6 weeks of age and start producing flowers after 9 weeks. Mature achenes are produced about a week after flowering (Hutchinson et al., 1984). Plants that germinate in the fall overwinter as rosettes (Hutchinson et al., 1984; Flora of North America Editorial Committee, 2018).

In China, S. asper has been recorded flowering and fruiting from May to October (Flora of China Editorial Committee, 2018). In the USA, it produces flowers and fruits from March to November in the north and year-round in the south (Flora of North America Editorial Committee, 2018). In Central America, it has been recorded with flowers and fruits throughout the year (Flora Mesoamericana, 2018; Flora of Nicaragua, 2018).

Longevity

S. asper is an annual or biennial herb (Flora of North America Editorial Committee, 2018; USDA-NRSC, 2018). It overwinters in the vegetative phase as rosettes or seeds. Seeds may remain viable in the soil for 1 to 3 years (Hutchinson et al., 1984; DiTomaso and Healy, 2007).

Environmental Requirements

The cosmopolitan range of S. asper suggests a broad tolerance of climatic variation and great ability to thrive in many different environments. However, light and disturbance seem to be the major requirements for the germination and establishment of this species. Currently, S. asper grows mostly in disturbed sites extending from 70°N to 50°S, in areas with annual rainfall ranging form 300 mm to 3000 mm and daily temperature above 5°C. It can grow in clay, loam and sandy soils with pH between 6.5 and 9. Plants tolerate saline soils and calcium carbonate content up to 55% (Hutchinson et al., 1984; Holm et al., 1997; ANHP, 2010).

S. asper is an important alternative host of a number of crop pests and diseases, including (Hutchinson et al. 1984; Vieira and Barreto, 2006; GISD, 2018):

• aphids (Aphididae) Aphis fabae, Hyperomyzus lactucae, H. pallidus and Uroleucon sonchi;

• the larva of the Cecidomyiid fly Aphidoletes aphidimyza, which was originally described from S. asper in Europe;

• leaf miners Ophiomyia pulicaria, O. coniceps, Phytomyza syngenesiae [Chromatomyia syngenesiae], Phytoliriomyza arctica and Calycomyza sonchi;

• the moth (Tortricidae) Cnephasia longana;

• nematodes Meloidogyne spp. and Pratylenchus penetrans;

• fungal infections Bremia lactucae (downy mildew), Sphaerotheca fuliginea [Podosphaera fuliginea] (a powdery mildew) and Alternaria sonchi; 

• viruses: beet western yellows virus, lettuce necrotic yellows virus, beet yellow stunt virus and sowthistle yellow vein virus.

Human food and beverage

• Vegetable

Medicinal, pharmaceutical

• Traditional/folklore

S. asper looks very similar to the close relative S. oleraceus, a species that is also a cosmopolitan weed. However, S. oleraceus has flatter, less spiky leaf margins and sharply acute, downward pointing auricles in leaves and bracts. In addition, in S. oleraceus, the achenes are narrower, never winged at the margins, transversely rugulose and scabrous at maturity. Most of them are brown, but those of the outermost whorl are paler and more olive in colour (Hutchinson et al., 1984; ANHP, 2010).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control                     

Physical/mechanical control

Small or isolated populations of S. asper can be manually removed while plants are young, prior to seed set. The taproot must be removed, as resprouting can occur if left in the soil.

Grazing of S. asper by cattle and sheep could be used to suppress infestations in pastures (GISD, 2018).

Biological control

In Australia, surveys searching for biological control agents of S. asper have identified the rust fungus Miyagia pseudosphaeria and an unidentified eriophyid mite species as potential candidates. However, studies determining the impact of these potential control agents on related native Australian species are still in progress (CSIRO, 2007; GISD, 2018).

Chemical control

Herbicides such as soxaben, 2,4-Dichlorophenoxyacetic acid, clopyralid, dicamba, glyphosate, picloram, Dimethyl tetrachloroterephthalate (DCPA), diethatyl-ethyl, MCPA, amitrole, atrazine, bromoxynil, chlorsulfuron, dichlorprop, isoxaben, norflurazon, oryzalin, oxyfluorfen and paraquat have been used to control infestations of S. asper in agricultural lands. However, S. asper has developed resistance to chlorsulfuron, atrazine and the herbicide metsulfuron-methyl (Rashid et al., 2003; GISD, 2018; Weeds of Australia, 2018). Biotypes resistant to the herbicides thifensulfuron, metsulfuron, prosulfuron, imazamox and imazethapyr have been found across North America, Australia and Europe (Park et al., 2012).

07/08/18 Original text by:

Julissa Rojas-Sandoval, Department of Botany-Smithsonian NMNH, Smithsonian Institution, Washington DC, USA