Sonchus arvensis
(perennial sowthistle)

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Identity

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Preferred Scientific Name

• Sonchus arvensis L. (1753)

Preferred Common Name

• perennial sowthistle

International Common Names

• English: field sowthistle
• Spanish: cerraja arvense; morraja
• French: laiteron des champs

Local Common Names

• Germany: Acker- Gaensedistel
• Italy: crespino dei campi
• Netherlands: akkermelkdistel
• Sweden: åkermolke; fettistel

EPPO code

• SONAR (Sonchus arvensis)

Taxonomic Tree

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• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Asterales
•                         Family: Asteraceae
•                             Genus: Sonchus
•                                 Species: Sonchus arvensis

Notes on Taxonomy and Nomenclature

Top of page Sonchus arvensis, the name given by Linnaeus, is the only name of the species that has ever been used with general acceptance. In Boulos (1976) two subspecies are distinguished, ssp. arvensis and ssp. uliginosus (Bieb.) Nyman. The ssp. arvensis corresponds to var. arvensis discussed by Pegtel (1972, 1973, 1974) and Alex and Switzer (1977). This biotype, which is the dominant biotype which occurs as a weed on arable land, is characterized by having hairs with yellow glandules on the upper parts of the peduncles and the involucral bracts. The ssp. uliginosus, a glabrous type, is largely the same as var. maritimus (Pegtel, 1973), var. glabrescens (Alex and Switzer, 1977) or the species S. uliginosus (Shumovich and Montgomery, 1955), which is seldom found on arable land, and mainly then on very moist ground. The glandular-hairy, arable weed type can also be found outside arable areas, on other frequently disturbed ground, in waste places, on maritime sands, etc (Jessen and Lind, 1922-23; Korsmo, 1930). Pegtel (1973) states that both the glandular and glabrous types are C3 plants, and that the former regenerates faster, favouring its persistence on arable land. Information on chromosome numbers for this species is conflicting. According to Boulos (1976) there seems to be no clear correlation between chromosome numbers, 2n = 36 or 2n = 54, and the 'subspecific differentiation' in S. arvensis. Shumovich and Montgomery (1955) state that the biotypes (distinguished as species, S. arvensis and S. uliginosus) readily hybridize, which complicates their taxonomy. The genetic relations between varieties or subspecies of S. arvensis (and S. uliginosus) and the perennials S. maritimus and S. palustris, both with 2n = 18 (Boulos, 1976) deserve further investigation. Boulos (1961) also discusses relations to another species, S. brachyotus, mistaken for S. arvensis in northern Asia.

The material in this datasheet relates to S. arvensis var. arvensis (ssp. arvensis) occurring as a weed on arable land.

Description

Top of page Morphological descriptions are found in Stevens (1924), Hegi (1929), Korsmo (1930, 1954), Håkansson (1969) and Boulos (1976).

S. arvensis var. arvensis is a far-creeping perennial, spreading vegetatively by thickened roots. Flowering shoots are erect, 30-150 cm long. Stems are usually branched. Leaves are glabrous with dentate margins, weakly spiny, the lower being entire to pinnatipartite with triangular lobes, the upper larger, pinnatipartite to pinnatisect, amplexicaul with rounded, often dentate auricles. Capitula, often numerous, are terminal and distinctly pedunculated. Receptacles are without scales. The glomerules are 4-5 cm in diameter with yellow ligules, which are about as long as the corolla-tube. The involucrel has 35-50 bracts, 14-17 mm in length. These, and the upper parts of the peduncles, have yellow glandular hairs.

Fruits are dark brown achenes, oblong, 2.4-3.4 mm x 0.8-1.4 mm, flattened, narrowed toward the base, with around 12 longitudinal ridges, crossed by wrinkles. The upper end is truncate with a pappus of white hairs.

Some of the originally slender roots thicken as a result of secondary growth and become regenerative. These roots are fleshy and rather fragile. Many of them, often the great majority, elongate horizontally before thickening. These creeping roots, which may reach lengths of several metres under favourable conditions, enable a vegetative spread over an extensive area. Most roots develop in the upper (10-15 cm) soil layer, although a few can sometimes be found at greater depths. Inclined and vertical thickened roots often reach depths of 25 cm, or more. The proportion of vertical thickened roots is sometimes higher than the type situation described. The mean and maximum depths reached by the reproductive roots can also be much greater (see Stevens, 1924). The thickened roots, covered by a cream-coloured (young) to yellowish brown or brown (old) cortex, become 3-6 (extremes: 1.5-8) mm in diameter. The cortex surface is more or less warty due to pressure from vegetative buds, initiated below the cortex and finally expanding through the cortex layer. Most of these buds become dormant at various stages of expansion, sometimes after emergence outside the cortex.

Distribution

Top of page S. arvensis is native to Europe and occurs in all European countries. It has been spread by humans to large parts of the temperate world, and also to subtropical areas in all continents. It has become naturalized in such areas in 59 countries. See further under 'Habitat' (Korsmo, 1930; Salisbury, 1964; Boulos, 1976; Holm et al., 1997).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Habitat

Top of page S. arvensis mainly occurs in temperate and subtropical areas with humid climates. It does not thrive in warm tropical climates. As a C3 plant, it may have difficulties in producing vigorous systems of underground reproductive roots under competitive conditions in a hot climate, like other C3 perennials with underground creeping organs, such as Elymus repens and Cirsium arvense (Håkansson, 1982, 1995c).

S. arvensis grows on most types of soil at a wide range of pHs, but may prefer rather moist mineral soils, preferably clay and loam soils rich in humus, and humus soils. It does not thrive well on coarse, dry soils. The species is regarded to be favoured by moderate to high levels of nitrogen and other nutrients, particularly potassium (Borg, 1964; Ellenberg, 1974; Zollinger and Kells, 1987). However, in competition from barley, even moderate nitrogen applications affect S. arvensis negatively (Håkansson and Wallgren, 1972b; Håkansson, 1986).

Hosts/Species Affected

Top of page As a weed on arable land, S. arvensis 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. Examples of such crops are small-grain cereals, maize, oil-seed crops, sugar-beet, potatoes and all kinds of vegetables. It also occurs in fields with perennial crops, particularly in orchards and vineyards, when competition from other herbaceous vegetation is at least partly controlled by soil cultivation or by herbicides to which S. arvensis is less sensitive (Håkansson, 1995b). In perennial fodder crops (leys) of grass and/or clover and/or lucerne, etc, with closed stands, mowing and competition in combination usually restrain its growth considerably. Its vigor therefore frequently decreases over years with increasing age of the crop, although it can often persist for many years (Håkansson, 1982, 1995a.).

Host Plants and Other Plants Affected

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Biology and Ecology

Top of page From a short-term perspective, the quantitative response of S. arvensis populations to management measures in cropping systems mainly depends on their regenerative roots. However, the rather extensive seed production capabilities of this species are of utmost importance for the survival and recovery of the weed population when the vegetative organs have been eliminated, or heavily damaged by cultural measures, natural enemies, etc. Over the long-term, reproduction by seeds is also very important because it enables populations to adapt rapidly to changed environmental conditions.

The aerial shoots are sensitive to frost and are therefore usually killed in early autumn in temperate climates. Roots transferred to the soil surface by tillage in autumn are, to a large extent, killed or severely damaged during winter, either by frost injury or by desiccation. It has been shown that roots in soil can to some extent survive temperatures as low as -20°C for 3 weeks (Schimming and Messersmith, 1988). Thickened roots can survive desiccation to 50% of their normal moisture levels (Gruzdev and Tulikov, 1966), but they may become too dry and die after a few days in open air and sunshine (Ravn, 1964).

The vegetative life cycle of S. arvensis in temperate areas may be described on the basis of Håkansson (1969) and Håkansson and Wallgren (1972a, 1972b) as follows. New primary shoots emerge in spring, usually somewhat later than shoots of perennial grasses such as Elymus repens. They usually originate from activated buds on last year's thickened roots, and to a lesser extent on roots of the year before and from buds on the underground bases of last year's aerial shoots, particularly when such bases are still attached to thickened roots. Even separate stem bases are regenerative to some extent. All, or nearly all, buds will have become dormant during the previous late summer or early autumn due to an innate dormancy induced in the vegetative system. During that period, dormancy cannot be broken by fragmentation of the roots, or can be broken only to a minor extent. Dormancy is gradually broken by low temperatures from mid autumn to early winter. New growth then frequently starts from activated buds at appropriate temperatures in the spring. In an intact vegetative system, this growth represents proportionally few buds. Most buds remain inactive due to the apical dominance of the shoots of the earliest activated buds. Fragmentation by soil tillage in the previous autumn or in the spring and early summer leads to activation of an increased proportion of buds. Regeneration is rapid where pre-formed buds are present. Where they are not, they can develop in isolated fragments, but quite slowly.

Aerial shoots after emergence firstly develop leaf rosettes. If inter- or intraspecific competition is not too strong, some of the originally slender roots developed in the spring in connection with the primary shoots begin to thicken when these shoots have 5-7 well developed leaves (more than about 3 cm in length). The thickening roots become regenerative at a diameter of about 1.5 mm. From this stage, undisturbed plants develop an increasing amount of branched regenerative (reproductive) roots of increasing total length and thickness, up to their final thickness of 2.5-6 (extreme: 8) mm. The new roots develop both from the parent roots of the new shoots and from the below-ground stem bases of the shoots. Growth in thickness mainly occurs among roots of the latter category, but also among roots from the stem bases.

During some weeks of high summer, an additional cohort of aerial shoots usually emerge from some of the buds on the new thickened roots. This emergence gradually ceases as innate dormancy is induced during the latter part of the growing season. This dormancy does not prevent photosynthesis of emerged shoots, nor continued growth, nor increases in the reserves of the reproductive roots.

On the primary shoots of the spring cohort, the number of leaves in the rosettes rapidly increases in early summer. When there are 10-15 well developed leaves, stems elongate and branch. Cauline leaves develop at increasing distances from the rosette. Inflorescence buds are visible from the early period of stem elongation. Glomerules gradually open, starting in high summer and often continuing until autumn. Achenes rapidly develop after the opening of the glomerules, but yellow flowers and unripe achenes can still be seen when shoots are fading after autumn frosts.

The stage when roots first thicken, indicating early development of new regenerative roots, is a stage when minimum amounts of food reserves are present in the underground vegetative system. The optimal length of time between disturbance repeated by burial (or defoliation) for controlling S. arvensisis is the length of time needed by the new shoots, which have emerged after disturbance, to develop 6-8 leaves (Håkansson, 1969; Håkansson and Wallgren, 1972a).

Growth in S. arvensis is usually considered to be favoured by relatively large amounts of nitrogen in the soil (Ellenberg, 1974). However, this seems to be strongly dependent on local competitive conditions. When plants are weakened and shoot emergence is delayed by soil tillage, and when plants have to compete with a dense crop stand, an increased nitrogen application can have the opposite effect. The reason for this is certainly that the plants are sensitive to shading (see Habitat) and so the negative effects of nitrogen fertilization become apparent due to increased shading from the crop (Håkansson and Wallgren, 1972b).

Regenerative roots seldom live longer than 2 years. When new thickened roots develop, parts of the older roots grow thicker and survive for another season, whereas other parts decay.

The seeds (achenes) of S. arvensis have been studied by many authors. Seeds can survive several years in the soil and build up a seed bank (Brenchley and Warington, 1933, 1936; Chepil, 1946; Roberts and Neilson, 1981). Germination mainly occurs in spring, favoured by high temperatures (25-30°C) and fluctuating temperatures, e.g. 5-25°C, near the soil surface (Håkansson and Wallgren, 1972a; Pegtel, 1972), and by soil cultivation (Roberts and Neilson, 1981). In seedlings studied by Håkansson and Wallgren (1972a), some roots began to thicken and become regenerative when the shoots had 5-7 leaves in addition to the cotyledons. Thickened roots of about 1.5 mm then occurred and proved regenerative. Before this stage, seedlings respond to various disturbances like seedlings of annual plants. After that, they behave like plants from vegetative buds, although with a more restricted or delayed (cf. Stevens, 1924) production of flowering shoots.

Natural enemies

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Notes on Natural Enemies

Top of page Several insects attack S. arvensis, e.g. Tephritis dilacerata, which causes galls in flower heads, thereby reducing seed production (Schroeder, 1973; Shorthouse, 1980). The rust fungus Puccinia suaveolens can infect S. arvensis, resulting in desiccation of plants (Kiselev, 1971).

Schroder (1973) provides a comprehensive list of insects associated with Sonchus spp. in Europe of which those screened as potential biological control agents are listed in the table of Natural Enemies.

Impact

Top of page According to Holm et al. (1997) S. 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. Shashkov et al. (1977) found that 3-15 shoots per square metre of this species reduced wheat yields by 4.5-7%. Yields of oats in Canada can be reduced by 58% (Holm et al., 1997). In forage crops it is harmful mainly because it is not palatable (Marten et al., 1987). In crops sown at wide row spacings (which are therefore weakly competitive) it severely reduces yields by competition if not effectively controlled.

Uses

Top of page Leaves of S. arvensis, which are high in minerals and vitamin C, have been used for salad in Europe for centuries (Holm et al., 1997). Duke and Ayensu (1985) report that the plant is used for insecticidal and medical purposes in China. It is considered as a potential source of natural rubber (Buchanan et al., 1978).

Uses List

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Human food and beverage

• Vegetable

Materials

• Pesticide
• Rubber/latex

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical

Similarities to Other Species/Conditions

Top of page Although the taxonomy is debated and the classification of S. arvensis and related Sonchus plants into species, subspecies and varieties differs between authors and floras (see Notes on Taxonomy and Nomenclature), the arable weed type classified as S. arvensis var. arvensis, or S. arvensis ssp. arvensis, is rather easy to distinguish from other taxa on the variety, subspecies and/or species level by using floras with good identification keys.

Prevention and Control

Top of page Introduction

Combinations of direct control measures, i.e. measures for killing growing plants, and management measures with the objective of reducing the growth and reproductive ability of the weed are desirable. Early comparisons of systems of integrated cultural, mechanical and chemical measures were reported by Derscheid et al. (1961). For literature reviews, see Holm et al. (1997). Based on studies of the characteristics of S. arvensis presented by Håkansson (1969, 1982) and Håkansson and Wallgren (1972a, 1972b), management and control methods are discussed below.

Cultural Methods

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).

References

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Alex JF; Switzer CM, 1977. Ontario Weeds. Descriptions, Illustrations and Keys to their Identification. Ontario, Canada: Ministry of Agriculture and Food College, University of Guelph, Publication 505:200 pp.

Borg P, 1964. On the relationship of arable weeds to some ecological factors of soil in the Helsinki district. Annales Botanici Fennici, 1:146-160.

Boulos L, 1961. Cytotaxonomy studies in the genus Sonchus: 3. On cytotaxonomy and distribution of Sonchus arvensis L. Botaniska Notiser, 114:57-64.

Boulos L, 1976. Sonchus L. In: Flora Europaea, Volume 4 Plantaginaceae to Compositae (and Rubiaceae). Cambridge, England: Cambridge University Press, 327-328.

Brenchley W; Warington K, 1933. The weed seed population of arable soil: II. Influence of crop, soil and methods of cultivation upon the relative abundance of viable seeds. Journal of Ecology, 21:103-127.

Brenchley W; Warington K, 1936. The weed seed population of arable soil: III. The reestablishment of weed species after reduction by fallowing. Journal of Ecology, 24:479-501.

Buchanan R; Cull I; Otey F; Russel C, 1978. Hydrocarbon producing and rubber producing crops: Evaluation of United States plant species. Economic Botany, 32:131-145.

Chepil W, 1946. Germination of weed seeds: I. Longevity, periodicity of germination and vitality of seeds in cultivated soil. Scientific Agriculture, 26:307-346.

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.

Derscheid L; Nash R; Wicks G, 1961. Thistle control with cultivation, cropping and chemicals. Weeds, 9:90-102.

Duke J; Ayensu E, 1985. Medical Plants of China. Algonac, Michigan, USA: Reference Publications Inc.

Ellenberg H, 1974. Zeigerwerte der Gefasspflanzen Mitteleuropas. Scripta Geobotanica 9. Gottingen: Goeltze.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

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].

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

Gruzdev G; Tulikov A, 1966. Peculiarities of vegetative reproduction of yellow sowthistle, Sonchus arvensis, and creeping thistle, Cirsium setosum M.B. Izvestiia Timiriazevskaia Sel'skokhoziaistvennaia Akademia, 1966(6):83-95.

Hakansson S, 1982. Multiplication, growth and persistence of perennial weeds. Biology and ecology of weeds [Holzner, W. and Numata, M. (Editors)] Dr. W. Junk The Hague Netherlands, 123-135

Hanf M, 1982. AckerunkrSuter Europas mit ihren Keimlingen und Samen. Ludwigshafen, Germany: BASF Aktiengesellschaf.

Hegi G, 1929. Illustrierte Flora von Mittel-Europa. VI/2. Munich, Germany: J.F. Lehmanns Verlag.

Holm L; Doll J; Holm E; Pancho J; Herberger J, 1997. World Weeds. Natural Histories and Distribution. New York, USA: John Wiley and Sons, Inc.

Hskansson 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.

Hskansson 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.

Hskansson S, 1995. Weeds in agricultural crops. 1. Life-forms and occurrence under Swedish conditions. Swedish Journal of Agricultural Research, 25:143-154.

Hskansson S, 1995. Weeds in agricultural crops. 2. Life-forms and occurrence in a European perspective. Swedish Journal of Agricultural Research, 25:155-161.

Hskansson S, 1995. Weeds in agricultural crops. 3. Life-forms, C3 and C4 photosynthesis and plant families in a global perspective. Swedish Journal of Agricultural Research, 25:163-171.

Hskansson S; Wallgren B, 1972a. Experiments with Sonchus arvensis L. II. Reproduction, plant development and response to mechanical disturbance. Swedish Journal of Agricultural Research, 2:3-14.

Hskansson 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.

Hultén E, 1950. Atlas of the Distribution of Vascular Plants in North-West Europe. Stockholm, Sweden: Esselte AB.

Jessen K; Lind J, 1922-23. Det danske Markukrudts Historie. Det Kongelige Danske VidenskabsSelskabs Skrifter, Nat. and Mathem. Afdeling 8. Ræcke VIII. Copenhagen.

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

Kiselev A, 1971. Biological control of weeds. In: Sornye Rasteniya i Mery Borba s-Nimi. Moscow, 159-161.

Korsmo E, 1930. UnkrSuter im Ackerbau der Neuzeit. Berlin, Germany: Verlag Julius Springer.

Korsmo E, 1954. Anatomy of weeds. Oslo, Norway: Grohndal & Sons, 172-175.

Lousley J, 1968. A Glabrous Perennial Sonchus in Britain. Proceedings of the Botanical Society of the British Isles, 7(2):151-157.

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

Marten GC; Sheaffer CC; Wyse DL, 1987. Forage nutritive value and palatability of perennial weeds. Agronomy Journal, 79(6):980-986; 25 ref.

Pegtel D, 1973. Aspects of ecotype differentiation in the perennial sowthistle. Technical Committee of International Society for Horticultural Science, 32:55-72.

Pegtel D, 1974. Effect of crop rotation on the distribution of two ecotypes of Sonchus arvensis L. in the Netherlands. Acta Botanica Neerlandica, 23:349-350.

Pegtel DM, 1972. Effects of temperature and moisture on the germination of two ecotypes of Sonchus arvensis. Acta Botanica Neerlandica, 21(1):48-53

Ravn K, 1964. Kvikbekæmpelse on eftersret. Tolvmandsbladet, 36:457-459.

Roberts HA; Neilson JE, 1981. Seed survival and periodicity of seedling emergence in twelve weedy species of Compositae. Annals of Applied Biology, 97(3):325-334

Salisbury E, 1964. Weeds and Aliens. 2nd Edition. London, UK: Collins.

Schimming WK; Messersmith CG, 1988. Freezing resistance of overwintering buds of four perennial weeds. Weed Science, 36(5):568-573

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

Shashkov VP; Kolmakov PP; Volkov ED; Trifonova LF, 1977. The influence of rhizomatous weeds in spring wheat crops on the utilization of nitrogen, phosphorus and potassium. Agrokhimiya, 14(3):57-59

Shorthouse JD, 1980. Modification of the flower heads of Sonchus arvensis (family Compositp) by the gall former Tephritis dilacerata (order Diptera, family Tephritidae). Canadian Journal of Botany, 58(14):1534-1540

Shumovich W; Montgomery F, 1955. The perennial sowthistle in northeastern North America. Canadian Journal of Agricultural Science, 35:601-605.

Stevens OA, 1924. Perennial sow thistle. Growth and reproduction. North Dakota Agricultural College. Agricultural Experiment Station, Bulletin 181.

USDA, 1970. Selected Weeds of the United States. Agriculture Handbook No. 366. Washington DC, USA: United States Department of Agriculture, 324-325.

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

Zollinger R; Kells J, 1987. Edaphic and environmental factors affecting the growth and development of perennial sowthistle (Sonchus arvensis). Proc. North Central Weed Control Conf. (U.S.), 42:1.

Distribution Maps

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