May 2014 AZ1628
Introduction
Many invasive, nonnative plants have become established
throughout Arizona. Of these, many are recognized as noxious
weeds: a plant species that has been designated by state,
federal or other agricultural or land management authority
as economically or ecologically harmful to agricultural or
horticultural crops, natural habitats or ecosystems, and
humans or livestock. Noxious weeds are regulated by law and
merit special attention and coordinated management efforts
to prevent their spread and contain existing infestations.
Preventing their spread through awareness, detection and
rapid removal is of key importance to minimize their real and
potential negative impacts.
By increasing our awareness, all of Arizona’s residents can
play an important role in controlling noxious weeds and those
with the potential to become harmful to our state’s economy
and ecology.
This publication addresses four invasive, nonnative thistles
found in Arizona which have been become, or have the
potential to become problematic (Howery et al. 2009):
• Musk thistle – Carduus nutans
• Canada thistle – Cirsium arvense
• Bull thistle – Cirsium vulgare
• Scotch thistle – Onopordum acanthium
These thistles are native plants of Europe and Eurasia, and
have no known natural enemies or control vectors in our state.
Each of these plants competes with and can displace native
plant species. They are particularly a concern in Arizona
rangelands. The spiny foliage and flower parts effectively
inhibit livestock from grazing the thistles and surrounding
plants, reduce available forage and discourage livestock
and wildlife from entering infested areas. Infestations also
limit outdoor recreation since the spiny thistles can form
impenetrable “fences,” making trails and riparian areas
inaccessible to people and livestock. With the increase in
the extent and severity of Arizona’s wildland fire activity,
an emerging concern is that noxious thistle infestations can
establish in forest and woodland areas after a wildfire.
Although similar in many ways, each plant listed above has
unique traits, making their impact and methods for control
different. The following sections address each species in more
detail. To conclude, a table of appropriate control measures
is provided.
Invas Ive , nonnat Ive th Istles of a r Izona
Christopher Jones
COLLEGE OF AGRI CUL TURE
AND
LIF E SCIENCES
COOPER ATIVE EXTENSION
Musk thistle – Carduus nutans
Habitat Musk thistle can establish on open disturbed
sites, overgrazed rangeland and pasture, forest clearings,
roadsides, stream banks and ditches. It tolerates a wide range
of environmental conditions and soil types. It can be found
at high elevations in rich soils as well as in saline soils at
lower elevations. Musk thistle is presently confined to spotty
infestations in Apache and Navajo counties.
Impacts A dense stand of musk thistle can reach six feet
tall and act as a natural barrier because of its spiny leaves.
Livestock and other animals avoid musk thistle because it is
poor forage, and wildlife have been known to leave areas that
have become heavily infested. Livestock will not graze near
musk thistle, and sheep will only eat the rosette stage of the
plant if there is nothing else to eat. When musk thistle invades
scenic areas, trails and waterways, the quality of outdoor
recreation opportunities can be downgraded.
Identification Musk thistle is a biennial plant that forms a
thick corky taproot and rosette in the first season, which may
occur with the summer monsoon rains or fall/early winter
rains in Arizona (Figure 1). The rosette can grow to 2 to 4 feet
in diameter. In the spring, one or multiple stalks emerge from
Figure 1. Musk thistle rosettes emerge with monsoons rains at higher ele\
vations
and with fall and winter rains at lower elevations. Image by Patti Fenne\
r.
2
the rosette, reaching 2 to 6 feet or more in height. The stalks
may exhibit some branching and are covered with dense,
short hairs (Figure 2). Leaves are hairless and have deep
lobes (Figure 3). They are usually dark green with a light
green midrib, and have a spiny margin. The leaves extend
beyond the stem, forming a “winged” stem. In the summer
and early fall months, a single plant can produce many large,
“powder puff” flower heads (1 ½” to 3” diameter) that contain
numerous individual disk flowers that are pink or purple, and
sometimes white. The flower head often bends downward
to portray a “nodding” characteristic. Below the flower are
numerous large spine-tipped bracts (Figure 4).
Reproduction Musk thistle’s only means of reproduction
is by seed. An “average” plant can produce 10,000 to 20,000
seeds of which one-third are viable. Most seeds scatter within
the immediate vicinity of the parent plant which can lead to
patterns of impenetrable clumps of infestation. Seeds are also
dispersed by wind and water, as well as animals, recreational
vehicles and farm equipment. They can remain viable in the
soil for over 10 years.
Canada thistle – Cirsium arvense
Habitat Canada thistle can establish on open disturbed
sites, rangelands and pastures, forest clearings, roadsides,
crop fields, gardens, stream banks and ditches. It can
tolerate a variety of soil types and some salinity, but is
most competitive in deep, well-drained soils with adequate
moisture availability. Canada thistle is known to be present
in Apache, Coconino and Yavapai counties. It is not known
to occur in the Sonoran and Mojave deserts.
Impacts If left unmanaged, Canada thistle can outcompete
crops and native plants and develop into large infestations.
It appears to have allelopathic qualities, which inhibit
other plants from thriving near its roots. The spiny foliage
discourages livestock and wild animals from grazing the
thistles and surrounding plants, which reduces available
forage and can restrict the use of recreational areas. It can
cause infections in livestock due to abrasions. Canada thistle
is also a host species for several agricultural insect pests and
diseases, including sod web-worm, bean aphid, stalk borer
and cucumber mosaic virus.
Identification Canada thistle is an erect perennial flowering
plant. From seed, it forms a rosette in its first season (Figure
5). In the spring, slender green stalks grow from 1 ½ to 4 feet
tall. Its ridged stems become hairy and branching as they
mature (Figure 6). Leaves are 2” – 8” long, alternate, oblong
to lance-shaped and irregularly to deeply lobed (Figure 7).
The leaf margins are toothed with prickly yellowish spines.
Flower heads are ½” wide to 1 ½” long, are unisexual and
form clusters of up to five flower heads at the ends of branches
(Figure 8). Each flower head contains numerous individual
disk flowers that may be pink or purple, and sometimes white.
Bracts are dark-tipped. Both flowers and bracts are spineless.
Female flowers are fragrant.
Figure 2. Musk thistle stalks are typically multi-branched. Image by Eri\
c
Coombs, Oregon Department of Agriculture, bugwood.org.
Figure 3. Musk thistle leaves are hairless and have deep lobes. Image by\
Eric
Coombs, Oregon Department of Agriculture, bugwood.org.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
3
Figure 4. Musk thistle blooms are “powder puff” like with numerous spine-tipped bracts below, and may exhibit a nodding characteristic. Image by Mary Ellen (Mel) Harte, bugwood.org.
Reproduction Unlike most thistles, Canada thistle is a
creeping perennial (rhizomatous) plant that spreads through
vegetative buds in its root system as well as by seed. New
plants can sprout from rhizomes and generate rosette-type
shoots throughout the growing season. Horizontal roots
may extend 15 feet or more and vertical roots may grow 6
to 15 feet deep. The roots can extend in the spring and fall
and whenever soil moisture is adequate. Male and female
flowers develop on separate plants (dioecious) and often
Figure 6. Canada thistle stalks are slender and green, and branch as the\
y ma - ture. Image by Jeff Schalau.
grow in circular patches that are one clone and sex. They are
pollinated by insects. An “average” plant can produce 1,500
seeds per flowering shoot. Most of the seeds fall near the plant,
but local spread is primarily due to vegetative reproduction.
Seeds can be dispersed long distances by wind and water,
as well as animals, recreational vehicles, shoes, clothing and
farm equipment. They can remain viable in the soil for up
to 20 years.
Figure 7. Canada thistle leaves are alternate, oblong to lance-shaped an\
d ir - regularly to deeply lobed. Image by Mary Ellen (Mel) Harte, bugwood.or\
g
Figure 8. Canada thistle flowers grow in clusters of up to five. Both the flower and bracts are spineless. Image by Jeff Schalau.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
4
Figure 9. Bull thistle rosettes can grow up to three feet in diameter. Image by
Patti Fenner.
Figure 10. Bull thistle stalks. Image by Joseph M. DiTomaso, University of Cali -
fornia - Davis, bugwood.org
Figure 11. Bull thistle leaves are dark green, alternate, with wavy margins and
deeply lobed. Image by John Cardina, Ohio State University, bugwood.org.
Figure 12. Bull thistle flowers have spiny bracts that taper up toward the bloom.
Image by Loke T. Kok, Virginia Polytechnic Institute and State University, bug -
wood.org.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
5
Bull thistle – Cirsium vulgare
Habitat Bull thistle can establish on open disturbed sites,
overgrazed rangelands and pastures, irrigated pastures,
forest clearings, roadsides, fence lines, riparian areas, stream
banks and ditches. It tolerates a wide range of soil textures,
but does best in soils that are nitrogen rich, have a neutral pH
and adequate soil moisture. It primarily occurs in scattered
populations at higher, moister elevations above 5,000 feet and
irrigated pasture at lower elevations in Arizona. Bull thistle
is known to be present in Apache, Cochise, Coconino, Gila,
Greenlee and Yavapai counties.
Impacts Bull thistle can invade and colonize rapidly in
disturbed areas, and is reported as troublesome in heavily
grazed pastures fertilized with nitrogen. It outcompetes
native plant species and becomes well established. The spiny
foliage discourages livestock and wild animals from grazing
the thistles and surrounding plants, which reduces an area’s
available forage. It is commonly found to be a contaminant
of hay (alfalfa and/or grass), which decreases the market
value of the forage.
Identification Bull thistle is a biennial plant that forms a short
fleshy taproot with several primary roots extending from the
root crown. Seed leaves are smooth; their shape varies from
round to narrow-based with broad rounded ends. It begins as
a rosette in the first season, which may occur with the summer
monsoon rains or fall/early winter rains in Arizona (Figure
9). The rosette can grow up to 3 feet in diameter. In the spring,
multiple (and occasionally single) stalks bolt from the rosette,
reaching 1 to 6 feet in height (Figure 10). Stems are hairy, have
spiny wings, and are often branched and have dark purple
veins. Leaves are 3” to 12” long, dark green, alternate, have
wavy margins and deeply lobed with coarse prickly hairs on
the top and wooly hairs underneath (Figure 11). The spines
occur on the margins of the leaf, typically on the ends of the
lobes, along the length of the leaf and at the tip of the terminal
lobe. Flower heads are 1” to 2” in diameter and are often
gumdrop-shaped (Figure 12). They usually grow singly at
the ends of shoots, but may also in clusters. The flower heads
contain numerous individual disk flowers that are typically
pink to purple and sometimes magenta-red, but rarely white.
They are fragrant. Spiny bracts surround the seed head, and
appear tapered and narrow.
Reproduction Bull thistle’s only means of reproduction is
by seed. Seed production can vary from 100 to 300 seeds per
flower and one to over 400 flowers per plan, and may average
over 4,000 seeds per plant. Its seeds are readily dispersed by
water, wind, animals, human activities, and contaminated hay.
Most seeds fall close the parent plant. They may germinate
throughout the growing season depending on soil moisture.
Most seeds germinate or die within the first year, but may
remain viable for up to 3 years or more if buried deeply.
Scotch thistle – Onopordum acanthium
Habitat Scotch thistle can establish on open disturbed
sites, rangelands and pastures, forest clearings, crop fields,
roadsides, riparian areas, stream banks and ditches. It grows
best in disturbed soils with high soil moisture, but it is also
considered drought tolerant and can invade drier, nutrient-
deficient sites as well. It is present in every northern county
in Arizona.
Impacts Bull thistle can invade and colonize rapidly in
disturbed areas, and is reported as troublesome in heavily
grazed pastures fertilized with nitrogen. It outcompetes
native plant species and becomes well established. The spiny
foliage discourages livestock and wild animals from grazing
the thistles and surrounding plants, which reduces an area’s
available forage. It is commonly found to be a contaminant
of hay (alfalfa and/or grass), which decreases the market
value of the forage.
Identification Scotch thistle is a biennial plant that forms
a stout corky taproot. It develops a large rosette with spiny
leaves that can grow over 2 feet long and a foot wide (Figure
13). Seedlings can emerge throughout the growing season
and establish rosettes. In the spring and summer, stems
grow up to 4 to 12 feet tall (Figure 14). They are typically
numerous and branched, with broad, spiny wings. Leaves
are alternate, oblong and large, with margins that are toothed
or coarsely lobed with yellow, green or white spines that are
sharp and stiff. Both leaves and stems are covered with fine
grayish-blue “wooly” hairs (Figure 15). Flower heads contain
numerous individual disk flowers, are round and grow up to
2” in diameter, with a “shaving brush” appearance (Figure
16). Flower colors are vibrant purple, violet or reddish,
and sometimes white. A single Scotch thistle can produce
numerous flower heads (70 – 300 per plant). They grow at
the stalk ends as a single flower heads or in clusters, and are
surrounded by long spine-tipped bracts.
Reproduction Scotch thistle’s only means of reproduction is
by seed. An “average” plant can produce up to 14,000 seeds
or more. They can germinate throughout the growing season
depending on soil moisture, and set flowers throughout much
of the season too. Most seeds fall close to the parent plant.
Wind, water, wildlife, livestock and human activities aid
dispersal. Seeds can remain viable in the soil for seven years
and up to 39 years.
Control measures
As illustrated by descriptions, these thistles pose serious real
and potential economic and ecological problems in Arizona
that merit special coordinated management efforts to prevent
their spread and contain existing infestations. Arizona’s harsh
desert environment should limit the establishment of these
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
6
Figure 13. Scotch thistle rosettes can emerge throughout the growing sea\
son.
Image by Patti Fenner.
Figure 14. Scotch thistle plants can grow to an impressive 12 feet in he\
ight.
Image by Jeff Schalau.
Figure 15. Scotch thistle leaves and stems are covered with fine grayish-blue
“wooly” hairs. Image by Bonnie Million, National Park Service, Bugwood.jpg
Figure 16. Scotch thistle flowers are considered to have a “shaving brush”
appearance and have long sharp bracts. Image by Jeff Schalau.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
7
thistles in many areas. However, their presence makes the
odds of establishment in richer soils and at higher elevations
more likely. Lessons learned from other states with these
thistles have demonstrated that prevention and control of
their spread is imperative to minimize their negative impacts.
If you see any of these thistles, please contact your local
county Cooperative Extension office to get in touch with
the appropriate personnel or agency to examine the weed
infestation’s impact and determine a course of action.
Integrated Weed Management (IWM) is the most effective
way to address invasive and noxious weed issues. IWM
is a strategy that is developed in partnership with several
concerned agencies and stakeholders, using a combination
of preventative, cultural, biological and mechanical control
practices, including chemical (herbicide) treatments as
necessary to treat any specific weed problem. Through IWM,
herbicides can be used in the most efficient way possible
to minimize their environmental and health effects while
reducing and controlling the targeted weed’s population.
As a general strategy, preventing the production and
spread of seeds is the key method for controlling musk, bull
and Scotch thistles. Because Canada thistle is a rhizomatous
perennial, however, the key is to continually cause stress on
the plant in order to force it to use up its stored nutrients and
energy in the roots, a technique referred to as “carbohydrate
starvation.” Stress can also be induced by tillage or shoot
removal. Alternatively, treating plants at the appropriate
time using a systemic, translocated herbicide that kills the
roots and rhizomes is usually a more effective for control of
Canada thistle. Success often requires a sound management
plan implemented over several years.
Two relatively recently registered herbicides,
aminocyclopyrachlor and aminopyralid, have been
demonstrated to be more effective on thistles than other
recommended herbicides. And although the introduction of
insects or fungal agents may assist as part of an IWM strategy,
they have not been found to be particularly successful in the
control of invasive, nonnative thistles in the Southwest to
date. Before proceeding with any biological control practice
or use of a registered herbicide, consult with an Extension
specialist or the proper state or federal agency to assure their
use is appropriate and environmental impacts are minimized.
The following table highlights recommended treatments
for the four thistles. For detailed herbicide treatments, please
refer to the publications listed below.
List of References
Beck, K. B. 2008. Canada thistle. Colorado State University
Extension. Fact Sheet No. 3.108.
Beck, K. B. 2008. Musk thistle. Colorado State University
Extension. Fact Sheet No. 3.102.
Colorado Department of Agriculture. 2008. List B Species.
Colorado Department of Agriculture, Lakewood, CO.
DiTomaso, J.M., and G.B. Kyser et al. 2013. Weed control in
natural areas in the western United States. Weed Research
and Information Center, University of California, 544pp.
Graham, J., W.S. Johnson and K. McAdoo. 2007. Identification
and management of bull thistle. University of Nevada
Cooperative Extension. Fact Sheet 05-03.
Graham, J., and W.S. Johnson. 2007. Managing Canada thistle.
University of Nevada Cooperative Extension. Fact Sheet
03-43.
Howery, L.D., E. Northam, W. Meyer, J. Arnold-Musa, E.
Carrillo, K. Egen and M Hershdorfer. 2009. Non-native
invasive plants of Arizona. USDA and University of
Arizona Cooperative Extension. Publication #AZ1482.
Kadrmas, T., and W.S. Johnson. 2007. Managing musk thistle.
University of Nevada Cooperative Extension. Fact Sheet
02-55.
Kadrmas, T., and W.S. Johnson. 2007. Managing Scotch thistle.
University of Nevada Cooperative Extension. Fact Sheet
02-57.
Whitson, T.D., L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E.
Nelson, R.D Lee and R. Parker. 2006. Weeds of the West,
9th Edition. Western Society of Weed Science, University
of Wyoming, Jackson. ISBN 978-0-941570-13-8, 630pp.
Any products, services or organizations that are mentioned, shown or indirectly implied in this publication do not imply endorsement by The University of Arizona.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Jeffrey C. Silvertooth, Associate Dean & Director, Extension & Economic Development, College of Agriculture Life Sciences, The University of Arizona.The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
The U niversiTy of A rizonA
College of A griCUlTUre And life s CienCes
TUCson , A rizonA 85721
ChrisTopher Jones Associate Agent, Agriculture & Natural Resources
ConTACT : Chris Topher Jones ckjones@cals.arizona.edu
This information has been reviewed by University faculty. extension.arizona.edu/pubs/az1628-2014.pdf
Other titles from Arizona Cooperative Exten sion can be found at:
extension.arizona.edu/pubs
COLLEGE OF AGRICULTURE
AND LI FE SCIENCES
COOPERATIVE EXTENSION
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
8
Species Prevention Cultural Mechanical Biological Chemical
Musk thistle
Carduus nutans
• Prevent the
introduction and
production of seed.
• Use certified weed-
free seed and hay.
• Ensure vehicles
and equipment
are cleaned
before leaving
contaminated
areas.
• Monitor property
and adjacent land
and eradicate new
plants whenever
they appear.
• Develop a grazing
management
plan to prevent
overgrazing.
• Practice
good forage
management
in pastures and
rangelands.
• Revegetate after
tilling or fire to
outcompete thistle
reestablishment.
• Encourage the
growth of perennial
grasses as
appropriate.
• Grazing by sheep
to eat the rosettes
and by goats to eat
the flower heads.
• Prescribed fire may
be used at part of
IWM plan.
• Till, hoe, and
hand pull rosettes
and stalks before
flower and seed
development,
especially smaller
and isolated
infestations.
• Sever taproot 2”
below ground with
a sharpened hoe or
shovel.
• Mow just before
first flower buds
appear or when
blooms first appear,
then gather and
destroy mowed
debris.
• The following
insects may
be available:
Trichosirocalus
horridus (crown
weevil), Cassida
rubiginosa (tortoise
beetle), Rhinocyllus
conicus (head
weevil) and
Cheilosia
corydon (crown
fly). Puccinia
carduorum (rust
fungus) may also
be available.
• As with chemical
treatments, consult
with an Extension
specialist or
state or federal
agency to assure
environmental
impacts are
minimized.
• Biological control
vectors have not
been particularly
successful
for controlling
nonnative thistles
in the Southwest to
date.
• Many selective
broadleaf
herbicides are
registered and
some are available,
over-the-counter,
and can be used
as part of an IWM
strategy.
• Read labels, follow
directions and use
precautions.
• Products
containing amino-
cyclopyrachlor,
aminopyralid,
dicamba, picloram,
2, 4-D, clopyralid,
fluroxypyr, and
triclopyr are most
effective at the
rosette stage.
• Products
containing
metsulfuron,
chlorsulfuron,
imazapic, imazapyr
and sulfometuron
can be used
before flowering
and offer some
preemergence
control.
Table of control measures
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
9
Species Prevention Cultural Mechanical Biological Chemical
Canada thistle
Cirsium arvense
• Prevent the
introduction of
seed.
• Use certified weed-
free seed and hay.
• Ensure vehicles
and equipment
are cleaned
before leaving
contaminated
areas.
• Purge livestock by
feeding clean hay
and allowing 3 to 5
days pass before
entering a clean
pasture.
• Monitor property
and adjacent land;
eradicate new
plants as they
appear.
• Revisit infestations
to assure
eradication.
• IWM strategies
require developing
a long-term
management plan
for sustainable
control.
• In crops and
irrigated pastures,
grasses and alfalfa
compete well with
good management
and adequate soil
moisture.
• Alfalfa is only
competitive after
establishment.
• Encourage the
growth of forage or
perennial grasses.
• Competition
alone is seldom
effective.
• Grazing and
prescribed burning
are not effective.
• Frequent grubbing
of young and
small infestations
throughout the
growing season
to exhaust
underground
energy reserves.
• Repeated cutting of
alfalfa and forage
crops over several
growing seasons.
• Mowing within four
days following
intense rotational
grazing.
• Tillage can break
and spread roots,
and encourage
new plant growth,
so repeat every
3 weeks to
be effective;
otherwise, it is not
recommended.
• The following
insects may
be available:
Ceutorynchus
litura (stem weevil),
Larinus planus
(bud weevil)
and Urophora
cardui (stem gall
fly). Puccinia
punctiformis (rust
fungus) may also
be available.
• Before introduction
or application,
consult with
an Extension
specialist or
appropriate
state or federal
agency to assure
environmental
impacts are
minimized.
• Biological control
vectors have not
been found to
be particularly
successful
for controlling
nonnative thistles
in the Southwest to
date.
• Because of the
extensive the root
system, systemic
herbicides are
recommended as
part of an IWM
strategy.
• Read labels, follow
directions and use
precautions.
• Products
containing amino-
cyclopyrachlor,
aminopyralid,
2, 4-D, dicamba,
picloram,
clopyralid,
chlorsulfuron,
imazapyr,
sulfometuron and
glyphosate are
commonly used
under different
prescriptions and
times of year.
• Mowing combined
with herbicidal
treatments can be
effective.
• Re-treatment for 1
to 3 or more years
is common.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
10
Species Prevention Cultural Mechanical Biological Chemical
Bull thistle
Cirsium vulgare
• Prevent the
introduction and
production of seed.
• Use certified weed-
free seed and hay.
• Ensure vehicles
and equipment
are cleaned
before leaving
contaminated
areas.
• Monitor property
and adjacent land
and eradicate new
plants whenever
they appear.
• Develop a grazing
management
plan to prevent
overgrazing.
• Practice
good forage
management
in pastures and
rangelands.
• Revegetate after
tilling to compete
with thistle
reestablishment.
• Encourage the
growth of perennial
grasses as
appropriate.
• Sheep, goats and
horses will eat the
rosettes; goats
will eat the flower
heads.
• Prescribed fire may
be used at part of
IWM plan.
• Till, hoe and hand
grub rosettes
and stalks before
flowers develop,
especially smaller
and isolated
infestations.
• Sever taproot 2”
below ground with
a sharpened hoe or
shovel.
• As part of an IWM
strategy, mow just
before first flower
buds appear; or
mow as blooms
first appear, and
gather & destroy
mowed debris.
• The following
insects may
be available:
Rhinocyllus
conicus (head
weevil), Urophora
stylata (gall fly),
and Trichosirocalus
horridus (crown
weevil).
• Before introduction
or application,
consult with
an Extension
specialist or
appropriate
state or federal
agency to assure
environmental
impacts are
minimized.
• Biological control
vectors have not
been found to
be particularly
successful
for controlling
nonnative thistles
in the Southwest to
date.
• Many broadleaf
selective herbicides
are available and
can used as part of
an IWM strategy.
• Read labels, follow
directions and use
precautions.
• Products
containing amino-
cyclopyrachlor,
aminopyralid,
dicamba, picloram,
triclopyr, 2,
4-D, clopyralid,
aminopyralid,
chlorsulfuron,
imazapyr, and
metsulfuron are
most effective at
the rosette stage
or when the weed
is most actively
growing.
The University of Arizona - College of Agriculture and Life Sciences - Cooperative Extension
