Sonchus arvensis L.

Perennial Sowthistle, Sowthistle

Category
Forb/Herb

Description

Perennial plant that looks like a huge dandelion with 1 or more stalks up to 6 ft. (1.8 m) tall. Stems may branch near the top. The often hollow stems and the leaves contain a milky sap. Prickly lettuce looks very similar to sow thistle, but has prickles on the underside of the leaves along the midveins.

Stem

Stems are usually branched.

Leaves

Leaves are larger and more crowded at the base of the stems and sparser toward the upper part of the stem, arranged alternately. Leaves are generally oblong to lance shaped, often lobed, have prickly edges, clasp the stem, and do not have a leafstalk.

Flowers

Plants flower in mid summer forming clusters of yellow flower heads, each 1.5 to 2 in. across, at the ends of the stems. Under the flowers are green bracts with sticky hairs.

Fruit

Seeds are wind dispersed and sometimes animal dispersed with the aid of pappi, hooked bristles attached to the seeds.

Images

Photo: Tom Heutte, USDA Forest Service, Bugwood.org
More images of Sonchus arvensis

Life History

Habitat

Often associated with moist soils and can tolerate saline conditions. Found along riverbanks and lake edges, roadsides, disturbed ground, woods, and meadows. It grows in desert areas but mainly in depressions where moisture sometimes gathers.

Origin and Distribution

Occurs throughout Canada and the United States except in the most southeastern states.

h2. Management Recommendations

Control of sow thistle is difficult because plants have extensive root systems and are relatively resistant to herbicides. Tilling helps to control sow thistle in fields. 2,4-D amine and other herbicides that act on the plant hormone auxin show some control of sow thistle, especially if applied while plants are growing vigorously.

Mechanical control

Methods used to prevent vigorous vegetative growth also reduce seed production and, in the long-run, the soil seed bank becomes diminished. In areas where soil tillage can be used and is regularly carried out, S. arvensis can be controlled by soil tillage. Soil operations that break its fragile roots followed by deep ploughing can be very effective, and even repeated shallow tillage can be effective if repeated when the regrowth of aerial shoots indicates a new (weakened) stage of the plant (Håkansson, 1969). However, even in those temperate areas where soil erosion problems are less pronounced, intensified soil tillage as a routine measure for controlling the weed should be avoided. In order to control the weed by exploiting its weak features, soil tillage justified for purposes other than weed control can often be modified without intensification. If followed by a competitive crop, breakage of the roots even by shallow cultivation may have a growth-reducing effect, because the increase in shoot numbers caused by breakages is often outbalanced in terms of its controlling effect by the shoots having become weaker and less competitive. The weakening effect of breakage can be strongly increased by tillage which results in deeper burial of the broken roots. Breakage by stubble cultivation in late summer or early autumn in combination with ploughing followed by a competitive autumn- or spring-sown crop can keep populations of S. arvensis at low levels. The effect of such breakage brought about in the latter part of the growing season, when innate dormancy prevents immediate bud activation, becomes visible in the following spring and summer.

It is evident that competition from crops should be utilized as an important integrated means of controlling S. arvensis. In field experiments, competition from barley reduced the production of new regenerative roots of this species to 1-10% of production levels without competition (Håkansson and Wallgren, 1972b; Håkansson, 1986). Any mechanical means of weakening the plants, justifiable in the context of soil care and energy consumption along with any economically and ecologically sound method of strengthening the competitiveness of the crop stand, should be used in combinations adapted to local conditions.

Strong joint effects of cutting or grazing and competition can be obtained in perennial fodder crops, such as leys of grass and/or legumes. Control of S. arvensis is facilitated, or made unnecessary in annual crops, if they are alternated with such fodder crops or similar perennial crops (Håkansson, 1982, 1995a).

Chemical Control

Chemical control measures may be regarded as complementary to cultural measures when these are insufficient. Only systemic herbicides are sufficiently effective on a perennial weed with an extensive underground reproductive system such as S. arvensis. Early experiments indicated that the plant has a minimum tolerance to MCPA and 2,4-D when aerial shoots are in the 'late rosette to early bud stage' (Vidme, 1961). This minimum tolerance period logically occurs in later stages than the tolerance towards mechanical disturbance, because the downward assimilate streams are weak in earlier stages (Fykse, 1974). It is also susceptible to MCPB, 2,4-DB and glyphosate (Fryer and Makepeace, 1978), and to clopyralid, tribenuron and triflusulfuron (Mamarot and Rodriguez, 1997). Recent work in Canada (Darwent et al., 1998) in minimum and zero till systems has indicated that application of clopyralid to oilseed rape followed by annual applications of clopyralid + MCPA in the following two years in barley reduced sowthistle populations from 3.9 shoots/m² to 0.5 shoots/m². In unweeded, zero till control plots densities increased to ca 40 shoots/m². Applying metsulfuron in the second year and dicamba + potassium salt of MCPA in the third year in place of clopyralid + MCPA was equally effective.

Biological control

The possibilities of using the natural enemies of S. arvensis for biological control have been studied (see Natural enemies), especially in Europe (Schroeder, 1973). Introductions were made into Canada, starting in 1979. Tephrititis dilacerata did not become established despite an extensive release programme. Cystiphora sonchi is established but suffers heavy parasitism and is not effective. Liriomyza sonchi was established in Nova Scotia in 1987 and was under evaluation in 1990 (Julien, 1992).

Bibliography

Kaufman, Sylvan Ramsey and Kaufman, Wallace. Invasive Plants: Guide to Identification and the Imapacts and Control of Common North American Species. Mechanicsburg, PA: Stackpole Books, 2007.

Vidme T, 1961. Control of Sonchus arvensis L. with chemicals. Weed Research, 1:275-288.

Fryer JD, Makepeace RJ, 1978. Weed control handbook. Volume 2. Recommendations including plant growth regulators. Weed control handbook. Volume 2. Recommendations including plant growth regulators., Ed. 8:xv + 532 pp.; [220 X 140 mm].

Håkansson S, 1969. Experiments with Sonchus arvensis L. I. Development and growth, and the response to burial and defoliation in different developmental stages. Annals of the Agricultural College of Sweden, 35:989-1030.

Håkansson S, Wallgren B, 1972b. Experiments with Sonchus arvensis L. III. The development from reproductive roots cut into different lengths and planted at different depths, with and without competition from barley. Swedish Journal of Agricultural Research, 2:15-26.

Håkansson S, 1986. Competition between crops and weeds - influencing factors, experimental methods and research needs. Proceedings of the EWRS Symposium 1986, Economic Weed Control, 49-60.

Fryer JD, Makepeace RJ, 1978. Weed control handbook. Volume 2. Recommendations including plant growth regulators. Weed control handbook. Volume 2. Recommendations including plant growth regulators., Ed. 8:xv + 532 pp.; [220 X 140 mm].

Darwent AL, Harker KN, Clayton GW, 1998. Perennial sowthistle control with sequential herbicide treatments applied under minimum and zero tillage systems. Canadian Journal of Plant Science, 78(3):505-511; 20 ref.

Schroeder D, 1973. The phytophagous insects attacking Sonchus spp. (Compositae) in Europe. Proc. 3rd Int. Symp. Biol Control Weeds, Monpellier.

Fykse H, 1974. Research on Sonchus arvensis L. 1. Translocation of 14C-labelled assimilates. Weed Research, 14(5):305-312

Mamarot J, Rodriguez A, 1997. Sensibilité des Mauvaises Herbes aux Herbicides. 4th edition. Paris, France: Association de Coordination Technique Agricole.

Julien MH, 1992. Biological Control of Weeds: a World Catalogue of Agents and their Target Weeds. Wallingford, UK: CAB International.