Sonchus arvensis
(perennial sowthistle)

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).

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).

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.).

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.

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.

Human food and beverage

• Vegetable

Materials

• Pesticide
• Rubber/latex

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical

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.

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.

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).