Perennial sowthistle [Sonchus arvensis L.][SONAR][CDFA list: A] Photographs Map of Distribution

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SYNONYMS:creeping sow thistle, field sow-thistle, swine-thistle, milk thistle, field milk thistle, corn sow-thistle, tree sow-thistle, dindle, gutweed

GENERAL DESCRIPTION:Vigorous herbaceous perennial, with milky sap and creeping roots that produce new shoots, to 1.8 m tall. Noxious. Plants are highly competitive, persistent, and can rapidly colonize new sites by vegetative reproduction. Introduced from Europe.

SEEDLINGS:Cotyledons obovate, 4-8 mm long, 1-4.5 mm wide. Leaves obovate to oblanceolate, bluish-green, dull, glabrous, form a basal rosette. Margins wavy to lobed, with backwards pointing spiny teeth. Lower surfaces often have a powdery white or purplish film. Newly initiated leaves sometimes pubescent. Seldom flower the first year.

MATURE PLANT:Foliage bluish-green. Stems erect, hollow, ridged, branched only in the upper portion of the plant. Lower stems leafy. Upper stems can be glabrous or glandular-hairy. Leaves alternate, highly variable, entire to deeply pinnate-lobed, 5-30 cm long, 2-10 cm wide, clasping the stem at the base with rounded basal lobes (auricles). Lobes +/- triangular, often curved backwards, usually 2-5(7) per side. Terminal lobe typically longer, broader than lateral lobes. Margins with small spiny teeth. Upper leaves sessile, often unlobed. Lower leaves short-petioled, lobed.

ROOTS and UNDERGROUND STRUCTURES:True roots (often described as rhizomes) produce new shoots and fragment easily. Horizontal roots long, creeping, < 1 cm in diameter, typically 5-12 cm below soil surface, can grow to 2 m long or more in a season. Vertical roots can penetrate soil to a depth of 2 m and produce new shoot buds to a depth of 0.5 m. Overwintering roots can survive temperatures to -16º C.

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FLOWERS:Summer. Heads 3-5 cm wide, consist of numerous bright yellow to orange-yellow 5-lobed ray (ligulate) flowers at the stem tips. Pappus bristles fine, soft, white, numerous, ~ 8-12 mm long. Flower head stalks and phyllaries typically covered with stiff glandular hairs. Self-incompatible. Insect pollinated. Heads open ~ 2-3 hours after sunrise and close ~ noon.

FRUITS and SEEDS:Achenes +/- oblong, flattened, 3-4 angled with 2 minutely wrinkled longitudinal ridges between angles, 2.5-3.5 mm long excluding pappus, +/- 1 mm wide, light to dark brown.

POSTSENESCENCE CHARACTERISTICS:Aerial stems die back after first frost.

HABITAT:Disturbed sites with damp soils. Thrives on non-compacted, fine, rich, slightly alkaline to neutral soils. Tolerates some salinity. Seedlings are typically found along pond and river margins and in lawns, moist meadows, and uncultivated fields.

DISTRIBUTION:Uncommon. Northwestern Modoc Plateau, northeastern Cascade Ranges (ne Siskiyou, nw Modoc cos.). To 1800 m (5900 ft). Past infestations (now eradicated) occurred in Sacramento Valley (e Sacramento Co.), northern South Coast Ranges (nc Santa Clara Co.), South Coast (w & se Santa Barbara, cw Orange cos.).

PROPAGATION/PHENOLOGY:Reproduces by seed and vegetative shoots from roots. Root sections 1 cm long or more can produce new shoots from previously formed buds or develop adventitious buds. Seed disperses with wind, water, and by clinging to fur or feathers of animals and clothing of people. Flower heads produce viable seed within 5-6 days and disperse seed in ~ 10 days after opening. Some immature seed can continue to mature on cut stems. Isolated plants or clonal patches produce little seed because of self-incompatibility. Newly matured seed lacks a dormancy period. Most seed germinates in spring after soil has warmed to ~ 20º C. Light is not required. Seed can remain viable under field conditions for 3 or more years, but decomposes in water within ~ 3 months. Seedlings emerge from soil depths to 3 cm (optimal 0.5 cm), but survival is typically low, especially on bare soils. Seedling establishment increases on sites with high moisture and protective plant cover or litter.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL:Grazing and repeated cultivation can help reduce perennial sowthistle stands by depleting root energy reserves. Cultivation is most efficient when emerging shoots are at the 6-leaf rosette stage.

SIMILAR SPECIES:Refer to the Comparison of sowthistles and lettuces table.


Prevention: Perennial sowthistle generally spreads via wind dispersed seeds. However, the achenes also have a hooked pappus that readily attaches to clothes, shoes, hair, or fur. Additionally, small root fragments may regenerate new plants. While wind dispersed seed is difficult to prevent, it is important to be aware of neighboring infestations that may be a seed source.

Mechanical: Tillage implements that either deeply bury root fragments below 30 cm or leave them on the soil surface to dessicate may reduce infestations. The optimal timing for cultivation to reduce root energy reserves is when plants are in the 6-9 leaf rosette stage. Repeated cultivation will be necessary for most infestations.

Biological: There are currently no labeled biocontrol agents for use on perennial sowthistle in California. Cattle and sheep have been observed to readily graze the weed and reduce infestations in irrigated pastures. However, stocking rate information to accomplish this is uncertain.

Chemical: Auxin type herbicides such as 2,4-D, dicamba, MCPA, and clopyralid have been effective with repeated applications when plants are in the seedling or late rosette to early bud stages. Glyphosate may also be applied as an effective spot treatment.

Bell, A. R., J. D. Nalewaja, and A. B. Schooner. 1973. Response of perennial sowthistle
selections to herbicides. Crop Science 13: 191-194.
Declerck, R A and T. A. Steeves 1988. Oviposition of the gall midge Cystipora sonchi (Bremi) (Diptera: Cecidomyiidae) via the stomata of perennial sowthistle (Sonchus arvensis L.). Canadian Entomologist 120:189-194.
Devine, M D and W. H. Vanden Born. 1985. Absorption, translocation and foliar activity of clopyralid and chlorsulfuron in Canada thistle (Cirsium arvense) and perennial sowthistle (Sonchus arvensis). Weed Science 33:524-530.
Lemna, W. K. and C. G. Messersmith. 1990. The biology of Canadian weeds. 94. Sonchus arvensis L. Canadian Journal of Plant Science 70:509-532.
Peschken, D. P., A. S. Mcclay, J. L. Derby and R. Declerck. 1989. Cystiphora sonchi (Bremi) (Diptera: Cecidomyiidae), a new biological control agent established on the weed perennial sow-thistle (Sonchus arvensis L.) (Compositae) in Canada. Canadian Entomologist 121:781-792.
Zollinger, R. K. and J. J. Kells. 1993. Perennial sowthistle (Sonchus arvensis) interference in soybean (Glycine max) and dry edible bean (Phaseolus vulgaris). Weed Technology 7:52-57.
Zollinger, R. K. and J. J. Kells. 1991. Effect of soil pH, soil water, light intensity, and temperature on perennial sowthistle (Sonchus arvensis L.). Weed Science 39:376-384.
Zollinger, R K; Penner, D; Kells, J J. Absorption translocation and foliar activity of clopyralid and tribenuron in perennial sowthistle (Sonchus arvensis). Weed Science 40:528-533.

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