Perennial sowthistle [Sonchus arvensis L.][SONAR][CDFA list: A] Photographs
Map of Distribution
thistle, field sow-thistle, swine-thistle, milk thistle, field
milk thistle, corn sow-thistle, tree sow-thistle, dindle, gutweed
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.
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.
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.
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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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.
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
CHARACTERISTICS:Aerial stems die back after first frost.
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.
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.).
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
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.
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.
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