Annual sowthistles
Annual sowthistle, Sonchus oleraceus L.
Spiny sowthistle, Sonchus asper (L.) Hill





Images above: Upper left: Annual sowthistle seedlings (Joseph DiTomaso, University of California, Davis). Upper right: Annual sowthistle rosette (Scott Morris, Cornell University). Middle: Annual sowthistle plants (Jack Clark, University of California). Bottom left: Annual sowthistle leaf base (Scott Morris, Cornell University). Bottom right: Annual sowthistle leaf (Scott Morris, Cornell University).



Images above: Upper left: Spiny sowthistle rosette (Scott Morris, Cornell University). Upper right: Spiny sowthistle leaf base (Scott Morris, Cornell University). Bottom: Spiny sowthistle flowers (Scott Morris, Cornell University).
Identification
Other common names:
- Annual sowthistle: sowthistle, smooth sowthistle, hare's lettuce, hare's colewort, milk-thistle
- Spiny sowthistle: spiny-leaved sowthistle, sharp-fringed sowthistle, prickly sowthistle, rough milk-thistle, rough sowthistle, chaudronnet (Quebec)
Family: aster family, Asteraceae
Habit: Erect summer or winter annual herbs.
Description: Seedlings form alternate-leaved rosettes before bolting. Cotyledons are hairless, have short stalks, and are circular to egg-shaped. The first true leaves are sparsely hairy, stalked and are circular to spatula-shaped. By the four-leaf stage seedlings exude a milky sap when damaged.
- Annual sowthistle: Cotyledons are 0.16-0.31” (0.4-0.8 cm) long by 0.06-0.16” (0.15-0.4 cm) wide. The first true leaves have irregularly toothed edges. Later leaves are oval to egg-shaped, irregularly lobed, and have winged stalks.
- Spiny sowthistle: Cotyledons are 0.14-0.24” (0.36-0.6 cm) long by 0.06-0.16 (0.15-0.4 cm) wide. The first true leaves have downward facing, spiny teeth along the edges. Subsequent leaves are oval to lance shaped with wavy and spiny edges and short stalks.
Mature plants reach 1-5 ft (30-152 cm) tall on upright, hairless, hollow stems which branch occasionally towards the top. Leaves are alternate, smooth, and clasp the stem at the base. All parts exude milky sap when damaged. The root system is a short taproot.
- Annual sowthistle: Leaves are dull green to blue-green, deeply lobed, 2.4-12” (6-30 cm) long by 0.4-6” (1-15 cm) wide and clasp the stem at the base with a pair of angular lobes. The leaves generally have a triangular to lance shaped lobe at the tip, and 3 or fewer paired lobes on the lower portion of the leaf. Lobes may be reduced or absent on the upper leaves. Leaf edges are irregularly toothed and may have soft spines. Leaves are deeply divided reaching almost to the midrib. Acute auricles are uneven and slanted.
- Spiny sowthistle: Leaves are glossy, green to purplish, oval to lance shaped or occasionally lobed, 2-10” (5-25 cm) long by up to 3” (8 cm) wide, and clasp the stem at the base with a pair of rounded, spiny lobes. Leaf edges are wavy and covered in sharp, prickly spines. Leaves are not as deeply divided as those of annual sowthistle. Leaves are lacking petioles, but have distinct large, “rams-horn” auricles clasping the stem.
Flowers are yellow and produced in clusters of 1-5 flowerheads at the ends of stems and branches. The flowerhead stalks may be smooth or covered in gland-tipped hairs. Each flowerhead is 0.4-1” (1-2.5 cm) wide and has up to 250 petals. At the base of each flowerhead there are 3-5 rows of green, overlapping bracts. Seeds are covered with a hard, dry layer of fruit tissue. Seeds are oval to oblong, flattened, and have 3-5 longitudinal ridges on each side. Each seed is attached to a feathery, white pappus up to 0.3” (0.8 cm) long.
- Annual sowthistle: Seeds are brown to olive, finely wrinkled, never winged, and are 0.08-0.16” (0.2-0.4 cm) long by 0.04” (0.1 cm) wide.
- Spiny sowthistle: Seeds are light brown, smooth, often winged on the edges, and 0.08-0.1” (0.2-0.25 cm) long by 0.06” (0.15 cm) wide.
Similar species: Perennial sowthistle (Sonchus arvensis L.) and prickly lettuce (Lactuca serriola L.) both have milky sap and similar foliage and flowers to spiny and annual sowthistle. Perennial sowthistle has larger flowers [1-2” (2.5-5 cm) wide] than either species and has an extensive, spreading system of perennial storage roots rather than a taproot. Prickly lettuce leaves have prominent spines along the midrib, while the midribs of spiny and annual sowthistle leaves lack such spines. Oxtongue (Picris) species have similar flowers and habits to spiny and annual sowthistle. The stems and leaves of oxtongues are covered with short, stiff hairs, while those of spiny and annual sowthistle are smooth.
Management
Annual and spiny sowthistles can act as either spring or winter annuals making them adapted to both fall- and spring-sown crops. Thus, they are less easily controlled by rotation between crops with different growing seasons than most other weed species. They tend to be particularly abundant and prone to seed production in winter grains. Since germination is favored by light and seedling emergence is limited to the surface 0.4” (1 cm) of soil, they are particularly prevalent in no-tillage systems (Chauhan et al. 2006b, Widderick et al. 2004). A moderately long period in the rosette stage makes annual sowthistles susceptible to suppression by crops that cast heavy shade and by organic mulch materials. Grain crops planted at high density and narrow rows decrease growth potential of annual sowthistle (Widderick et al. 2004). The long rosette stage also makes them relatively easy to bury with aggressive tine weeding or by throwing soil into the crop row. Shallow cultivation either in a bare fallow or after planting the crop is effective for flushing out and destroying seedlings (Widderick et al., 2010). Since the seed bank is relatively short-lived, inversion tillage is useful for controlling these species, especially after a season with abundant seed production, provided subsequent tillage does not occur for 30 months (Widderick et al. 2004). Mowing field edges, cleared stream banks and other non-forested habitats can prevent spread into adjacent fields (Widderick et al. 2004).
Ecology
Origin and distribution: Annual and spiny sowthistle likely originated near the Mediterranean Sea (Hutchinson et al. 1984). Spiny sowthistle has been introduced into North and South America, Africa, Australia, New Zealand and south and east Asia (Holm et al. 1997). Annual sowthistle is similarly widespread. Both species occur throughout most of the U.S.A. and southern Canada, and spiny sowthistle occurs northward into central Alaska and the Yukon (Hutchinson et al. 1984).
Seed weight: Annual sowthistle, 0.27-0.32 mg (Chauhan et al. 2006b, Milberg et al. 2000), 0.42 mg (Lewin 1948); spiny sowthistle 0.28 mg (Gaba et al. 2019), 0.30 mg (Lewin 1948), 0.32 mg (EFBI), 0.41 mg (Milberg et al. 2000).
Dormancy and germination: A high percentage of annual sowthistle seeds will germinate immediately after dispersal if they receive light (Chauhan et al. 2006a, Lewin 1948, Widderick et al. 2010). They emerge considerably better in light than in dark or with only a pulse of light (Chauhan et al. 2006a, Milberg et al. 2000, Widderick et al. 2010). Germination was good at alternating day/night temperatures of 77/50 °F (25/10 °C), but neither species germinated well at any constant temperature according to one report (Hutchinson et al. 1984). However, non-dormant annual sowthistle germinated well at a wide range of constant temperatures from 41 to 95 °F (5 to 35 °C) in another report (Widderick et al. 2010). High moisture levels are required for optimum germination of annual sowthistle (Chauhan et al. 2006a, Widderick et al. 2010). Some germination can occur at moderate salt concentrations and pH levels greater than 8 (Chauhan et al. 2006a). Spiny sowthistle germinates well in light immediately after seed collection, but acquires a capacity to partially germinate in dark after six months (Wesson and Wareing 1969). Cold (43 °F or 6 °C), wet conditions decrease spiny sowthistle seed dormancy (Hutchinson et al. 1984) but germination after 18 weeks of cold stratification is still greater in light than in dark (Milberg et al. 2000).
Seed longevity: In soil stirred three times a year, annual sowthistle seeds declined by 48-65% per year. For spiny sowthistle, seeds declined by 48-62% per year (Roberts and Neilson 1981). Mortality of annual sowthistle seeds was 82% when left on the soil surface for almost 8 months through the Australian winter, whereas mortality was 55% for seeds buried 2” (5 cm) (Chauhan et al. 2006a). In another Australian experiment, annual sowthistle seed mortality was greater than 99% after one year at the soil surface, but was 53% when buried 4” (10 cm) (Widderick et al. 2010). Some buried seeds of annual sowthistle lasted for at least 5 years (EFBI).
Season of emergence: In cooler climates, most emergence of both species takes place in mid- to late spring, but some seedlings emerge throughout the growing season whenever conditions are favorable for germination (Hutchinson et al. 1984). Emergence is greatest at day/night temperatures of 54/39 to 66/50 °F (12/4 to 19/10 °C) (Hutchinson et al. 1984). In warm climates, particularly Mediterranean climates with hot, dry summers, annual sowthistle can behave as a winter annual, with a flush of emergence following planting of fall-sown crops. Because annual sowthistles lack dormancy when exposed to light, and they germinate over a wide range of temperatures given sufficient soil moisture, emergence occurs throughout the spring and summer in moist climates, but only during the winter season in Mediterranean climates (Widderick et al. 2010).
Emergence depth: Emergence of annual sowthistle is best from 0 to 0.4” (0 to 1 cm), with little emergence from 0.8 to 1.2” (2 to 3 cm) (Chauhan et. al. 2006a, 2006b, Widderick et al. 2010). Up to 77% of annual sowthistle seeds will germinate on the soil surface (Sheldon 1974, Chauhan et. al. 2006a). Seeds probably emerge well from the soil surface because the flat shape of the seed allows for good soil contact and water uptake (Sheldon 1974).
Photosynthetic pathway: C3
Sensitivity to frost: Young plants of both species are highly frost tolerant. Fall emerging individuals overwinter as a rosette of leaves close to the ground and can withstand temperatures several degrees below 0 °F (-18 °C) (Lewin 1948). In the Canadian prairies and northward, however, the species overwinter only as seeds. Flower buds are killed by early frost (Lewin 1948), and flowering plants are partially killed by frost (Stevens 1924).
Drought tolerance: Annual sowthistle can survive short periods of drought that are sufficiently severe to cause wilting of the leaves (Lewin 1948). Spiny sowthistle is sensitive to drought stress. Plants watered three times per week grew only half as large as those that were watered daily, and plants watered only once per week failed to reproduce (Hutchinson et al. 1984).
Mycorrhiza: Annual sowthistle is strongly mycorrhizal (Peterson et al. 1985), but spiny sowthistle, although listed as mycorrhizal on disturbed sites in Utah (Pendleton and Smith 1983), has a low rate of mycorrhizal infection (Dhillion and Friese 1994).
Response to fertility: Annual sowthistle tolerates low N fertility well. It occurs on soils with pH from 6.5-9.0, and does best at pH over 8.0. Since P would have very low availability in the high pH conditions in which annual sowthistle thrives, it apparently also tolerates low P. Spiny sowthistle tolerates low fertility. In one experiment it actually produced more seeds when unfertilized than when given 10-10-10 fertilizer before planting. Nevertheless, another study found that this species had three times the concentration of K and twice the concentration of P and Ca as a barley crop in which it was growing (Holm et al. 1997).
Soil physical requirements: Both species occur on a wide range of soil textures from clay to sand. Both species tolerate salt and alkaline soils provided moisture is available (Hutchinson et al. 1984, Lewin 1948).
Response to shade: Annual and spiny sowthistle do not tolerate more than light shade (Lewin 1948).
Sensitivity to disturbance: Since the leaves of the young plants remain flat on the soil in a rosette for several weeks, they are highly susceptible to burial, as with aggressive tine weeding or when soil is thrown into the crop row. They do not tolerate trampling, but can grow new stems if the first stems are broken or cut (Lewin 1948). Plants cannot regrow from the taproot alone. Plants of annual sowthistle cut at the flowering stage are still able to produce viable seeds (Gill 1938).
Time from emergence to reproduction: Spring emerging spiny sowthistle flowers about 9 weeks after emergence with the seeds ripening about one week later (Hutchinson et al 1984). Annual sowthistle emerging in early spring will flower in June, but overwintering rosettes will begin producing flower stems in March (Lewin 1948). In Queensland, Australia, annual sowthistle flowered 11 weeks after it emerged in late fall (Manalil et al. 2020). In warm weather on poor soil, annual sowthistle can flower in as little as 6 weeks (Lewin 1948).
Pollination: Both species self-pollinate, but are probably also cross-pollinated to some extent by small bees and flies (Lewin 1948).
Reproduction: A sample of 65 British annual sowthistles produced an average of 140 seeds per head and 44 heads per plant, for a total of 6,100 seeds per plant (Hutchinson et al. 1984). Annual sowthistle produced 3,000 to 4,000 seeds per plant in competition with a winter wheat crop (Manalil et al. 2020). In this experiment, plants dispersed 95-97% of seeds by wheat harvest, suggesting seed capture technology would not be effective against this species. A sample of 25 spiny sowthistles averaged 200 seeds per head and 105 heads per plant for a total of 23,000 seeds per plant (Hutchinson et al. 1984).
Dispersal: Tufts of hairs help the seeds to disperse long distances on air currents (Hutchinson et al. 1984). These hairs also tangle in fur, feathers and clothing, causing the seeds to be carried by animals and humans (Hutchinson et al. 1984). Seeds of annual sowthistle have been found adhering to feet and feathers of waterfowl (Vivian-Smith and Stiles 1994). The seeds of spiny sowthistle (and probably annual sowthistle as well) pass through cattle and are spread with the manure (Hutchinson et al. 1984). The seeds also float well and disperse along streams and lakeshores (Holm et al. 1997). Annual and spiny sowthistle seeds found in irrigation water had germination rates of 22 and 57%, respectively (Kelley and Bruns 1975).
Common natural enemies: Sowthistles are attacked by several species of leaf mining fly larvae. They are susceptible to downy mildew caused by Bremia lactucae which also infects commercial lettuce (Vieira and Barreto 2006).
Palatability: European peoples have used sowthistles as a potherb since ancient times (Lewin 1948). The content of minerals, vitamins, and anti-oxidants in these species is comparable to those of leafy vegetables (Guil-Guerrero et al. 1998, Jimoh et al. 2011). The foliage is palatable to livestock, though most animals have difficulty eating immature plants because the leaves lie flat on the ground.
References:
- Chauhan, B. S., G. Gill, and C. Preston. 2006a. Factors affecting seed germination of annual sowthistle (Sonchus oleraceus) in southern Australia. Weed Science 54:854-860.
- Chauhan, B. S., G. Gill, and C. Preston. 2006b. Seedling recruitment pattern and depth of recruitment of 10 weed species in minimum tillage and no-tillage systems. Weed Science 54:658-668.
- Dhillion, S. S., and C. F. Friese. 1994. The occurrence of mycorrhizas in prairies: Applications to ecological restoration. Thirteenth North American Prairie Conference 13:103-114.
- EFBI. Ecological Flora of the British Isles. http://ecoflora.org.uk
- Gaba, S., P. Deroulers, F. Bretagnolle, and V. Bretagnolle. 2019. Lipid content drives weed seed consumption by ground beetles (Coleoptera, Carabidae) within the smallest seeds. Weed Research 59:170-179.
- Gill, N. T. 1938. The viability of weed seeds at various stages of maturity. Annals of Applied Biology 25:447-455.
- Guil-Guerrero, J. L., A. Giménez-Giménez, I. Rodríguez-García, and M. E. Torija-Isasa. 1998. Nutritional composition of Sonchus species (S. asper L., S. oleraceus L. and S. tenerrimus L.). Journal of the Science of Food and Agriculture 76:628-632.
- Holm, L. M., J. Doll, E. Holm, J. Pancho, and J. Herberger. 1997. World Weeds: Natural Histories and Distribution. John Wiley & Sons: New York.
- Hutchinson, I, J. Colosi, and R. A. Lewin. 1984. The biology of Canadian weeds. 63. Sonchus asper (L.) Hill and S. oleraceus L. Canadian Journal of Plant Science 64:731-744.
- Jimoh, F. O., A. A. Adedapo, and A. J. Afolayan. 2011. Comparison of the nutritive value, antioxidant and antibacterial activities of Sonchus asper and Sonchus oleraceus. Records of Natural Products 5:29-42.
- Kelley, A. D., and V. F. Bruns. 1975. Dissemination of weed seeds by irrigation water. Weed Science 23:486-493.
- Lewin, R. A. 1948. Biological flora of the British Isles. Sonchus L. (Sonchus oleraceus L. and S. asper (L.) Hill). Journal of Ecology 36:203-223.
- Manalil S., H. H. Ali, and B. S. Chauhan. 2020. Interference of annual sowthistle (Sonchus oleraceus) in wheat. Weed Science 68:98–103.
- Milberg, P., L. Andersson, and K. Thompson. 2000. Large-seeded species are less dependent on light for germination than small-seeded ones. Seed Science Research 10:99-104.
- Pendleton, R. L., and B. N. Smith. 1983. Vesicular-arbuscular mycorrhizae of weedy and colonizer plant species at disturbed sites in Utah. Oecologia (Berlin) 59:296-301.
- Peterson, R. L., A. F. Ashford, and W. G. Allaway. 1985. Vesicular-arbuscular mycorrhizal associations of vascular plants on Heron Island, a Great Barrier Reef coral cay. Australian Journal of Botany 33:669-676.
- Roberts, H. A., and J. E. Neilson. 1981. Seed survival and periodicity of seedling emergence in twelve weedy species of Compositae. Annals of Applied Biology 97:325-334.
- Sheldon, J. C. 1974. The behaviour of seeds in soil. III. The influence of seed morphology and the behaviour of seedlings on the establishment of plants from surface-lying seeds. Journal of Ecology 62:47-66.
- Stevens, O. A. 1924. Some effects of the first fall freeze. The American Midland Naturalist 9:14-17.
- Vieira, B. S., and R. W. Barreto. 2006. First record of Bremia lactucae infecting Sonchus oleraceus and Sonchus asper in Brazil and its infectivity to lettuce. Journal of Phytopathology 154:84-87.
- Vivian-Smith, G., and E. W. Stiles. 1994. Dispersal of salt marsh seeds on the feet and feathers of waterfowl. Wetlands 14:316-319.
- Wesson, G., and P. F. Wareing. 1969. The induction of light sensitivity in weed seeds by burial. Journal of Experimental Botany 20:414-425.
- Widderick, M., S. Walker, and B. Sindel. 2004. Better management of Sonchus oleraceus L. (common sowthistle) based on the weed’s ecology. Proceedings of the 14th Australian Weeds Conference, pp. 535-537.
- Widderick, M. J., S. R. Walker, B. M. Sindel, and K. L. Bell. 2010. Germination, emergence, and persistence of Sonchus oleraceus, a major crop weed in subtropical Australia. Weed Biology and Management 10:102-112.