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Invasive Species - Bull Thistle - Cirsium vulgare (Savi) Ten.
Sunflower family: Asteraceae
Bull Thistle is a widespread weed that can grow in a wide range of
environments but is most troublesome in recently or repeatedly disturbed
areas such as pastures, overgrazed rangelands, recently burned forests,
forest clear-cuts, and along roads, ditches, and fences.
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Common Names: bull thistle, common
thistle, spear thistle
Native Origin: Europe, western Asia, and North Africa
Description: Bull thistle is a
biennial, and sometimes annual or monocarpic perennial
in the sunflower family (Asteraceae). In the juvenile
phase, individual bull thistle plants form a single
rosette with a taproot up to 28 inches (70 cm) long.
Rosettes may develop up to 3.3 feet in diameter. The
taproot does not spread, but develops several smaller
lateral roots. Stems have spiny wings and grow 1 to 6.6
feet tall, with many spreading branches, and sometimes a
single stem. Bull thistle stem leaves are more or less
lance-shaped and 3 to 12 inches long, prickly hairy on
the top and very hairy underneath. Lobes on leaves are
tipped with stout spines. Bull thistle flowerheads are
1.5 to 2 inches in diameter, 1 to 2 inches long, usually
solitary, and more or less clustered at the ends of
shoots and branches. Flowers are subtended by narrow,
spine-tipped bracts.
Distribution: This species is
reported from states shaded on Plants Database map. It
is reported invasive in AK, AZ, CA, CO, HI, ID, MA, MD,
MI, MN, MO, NC, NJ, OH, OR, PA, SD, TN, UT, VA, WA, WI,
WV, and WY.
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Bull thistle fruits are achenes, 1/16
inch long, with a long, hairy plume that is easily
detached.
Habitat: It is a widespread weed that
can grow in a wide range of environments but is most
troublesome in recently or repeatedly disturbed areas
such as pastures, overgrazed rangelands, recently burned
forests, forest clear-cuts, and along roads, ditches,
and fences. It is found on dry and wet soils, but is
most common on soils with intermediate moisture.
Bull thistle can invade almost any
type of disturbed area, from forest clearcuts to
riparian areas to pastures where it can form dense
thickets displacing other vegetation. The spiny nature
of the plant renders it unpalatable to wildlife and
livestock and reduces the forage potential of pastures.
Bull thistle is native to Europe, western Asia, and
northern Africa. Bull thistle was introduced into the
eastern United States several times during the 19th
century. It is now established in all 48 contiguous
states as well as Alaska and Hawaii (USDA, NRCS, 1999).
It has been designated as a noxious weed in Maryland,
Pennsylvania, Minnesota, Iowa, Oregon, and Colorado.
Photo: Bruce Marlin
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Ecological
Impacts: Although bull thistle
is a problem predominantly in
disturbed areas, it also can be
found in natural areas. The
basal rosette may grow to over 3
feet in diameter before bolting.
Once established, bull thistle
out-competes native plant
species for space, water, and
nutrients.
Control and
Management:
• Manual- Mow to prevent
seeding.
• Chemical- It can be
effectively controlled using any
of several readily available
general use herbicides such as
glyphosate, triclopyr, or
dicamba. Follow label and state
requirements.
• Biocontrol: The seed-feeding
fly, Urophora stylata
Fabricius, has been selected and
released for biological control
of bull thistle.
Bull thistle
differs from Canada thistle,
Cirsium avense (L.) Scop.,
in that leaves are pubescent on
both sides, while those of
Canada thistle are not pubescent
on top, and may or may not be so
on the underside. Flower bracts
of bull thistle have spines, in
contrast to those of Canada
thistle. Leaves are covered with
coarse hairs on the upper
surface of the leaf blade, and
are woolly below. Long spines
extend from the leaf blade at
the midrib and at each lobe. The
leaf bases extend downward on
the stem forming long wings.
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Bull thistle is a biennial species
that reproduces by seed. The root system consists of
several primary roots each with several smaller lateral
roots. It does not reproduce by vegetative means. Bull
thistle is erect and bushy in appearance, up to 2 m
high, and has many spreading branches (Fig. 1). Stems
are erect, stout, often branched, and hairy. Leaves are
green on the upper side, with woolly gray hairs on the
underside, and end in long, pointed, yellow spines. The
compact large purple flower heads (2.5 to 5.0 cm in
diameter) are borne singly at the tip of a stem (Fig.
2), each producing up to 250 light straw-colored seeds.
Mature plants can produce up to 4,000 seeds per plant.
Bull thistle grows best on nitrogen-rich, neutral soils
with moderate moisture (Klinkhamer and de Jong, 1993).
It is not typically found on sand or on soils with high
humus content and is absent from pure clay soils.
Establishment is promoted by soil disturbance, which
increases nutrient, water, and light availability to
seedlings and reduces the vigor of competing vegetation
(Randall, 1994). Bull thistle does not grow well in
shade and drought. Phenolic acids inhibit competing
plants through allelopathic effects or serve as a
defense, coupled with spines, against herbivory
(Klinkhamer and de Jong, 1993).
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Bull Thistle Seed
Photo: Forest & Kim Starr, United States Geological
Survey, Bugwood.org
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