Fact Sheet No. 3.102
Natural Resources Series|Range
by K.G. Beck*
Musk thistle is an aggressive weed of
foreign origin that occurs in pastures,
rangeland, roadsides and non-crop areas
(Figure 1). It is a biennial weed, although
occasionally it is an annual. Because musk
thistle reproduces solely from seed, the key
for successful management is to prevent seed
production. Over 46,000 acres are infested
with musk thistle in Colorado (Figure 2).
Germination and seedling establishment
are correlated with moisture and light. Thus,
more seeds germinate and establish plants in
open pastures and other degraded areas.
Vigorously growing grass competes
with musk thistle, and fewer thistles occur
in pastures where grazing is deferred.
However, musk thistle also can become a
problem in pasture or rangeland that is in
good condition.
Phenology
Seedlings normally emerge early in
spring, develop into rosettes and spend the
first season in this growth stage. Seedling
emergence also can occur in fall. All seedlings
grow into rosettes and overwinter in that
stage. Rosettes are usually large and compact
with a large, corky taproot that is hollow near
the crown (Figure 3). Leaves have consistent
shape, sometimes expressing a frosted
appearance around the leaf margins, and
often have a cream-colored midrib (Figure 4).
Early in spring of the second year,
overwintered rosettes resume growth. Shoots
begin to elongate (bolt) in late March through
May, depending on weather and elevation
(Figure 5). Musk thistle flowers (Figure 6)
and starts to produce seed 45 to 55 days after
it bolts. Musk thistle has very large bracts
beneath flowers that are armed with sharp
spines and shoots beneath flowers are almost
devoid of leaves.
Quick Facts
• Musk thistle is a biennial
weed that reproduces only
from seed.
• The key to successful musk
thistle control is to prevent
seed production.
• Apply herbicides such as
Tordon, Milestone, Transline,
Perspective, Vanquish/Clarity
or 2,4-D to musk thistle
rosettes in spring or fall. Apply
Escort or Telar up to the early
flower growth stage.
• Combine control methods
into a management system
for best results.
*
Colorado State University Extension weed science
specialist and professor, bioagricultural sciences and
pest management. 11/2013
Musk Thistle
Musk thistle dies after it sets seed. It
spends approximately 90 percent of its
life cycle in a vegetative growth stage.
Musk thistle's tolerance to most herbicides
increases after it bolts.
Reproduction and Spread
Musk thistle is a prolific seed producer.
One plant can set up to 20,000 seeds.
However, only one-third of the seeds are
viable. Musk thistle produces many heads.
The terminal, or tallest, shoots flower first,
then lateral shoots develop in leaf axils.
A robust plant may produce 100 or more
flowering heads.
Musk thistle flowers over a seven- to
nine-week period. It begins to disseminate
seed from a head about two weeks after
it first blooms. It is common to observe
musk thistle with heads in several stages of
floral development and senescence. Thus,
musk thistle sets seed over an extended
time period.
Most seed is dispersed within the
immediate vicinity of the parent plant.
This leads to a clumped pattern of seedling
development and results in intraspecific
competition and mortality. Wind and water
are good dissemination methods and seeds
are also spread by animals, farm machinery
and other vehicles. Less than 5 percent of
seed remains attached to the pappus when it
breaks off the flowering head and floats away
on wind currents.
© Colorado State University
Extension. 9/98. Revised 11/13.
www.ext.colostate.edu
Figure 1: Musk thistle infestation in the
Colorado foothills.
Management
Cultural control.
Maintaining pastures
and rangeland in
good condition is
a primary factor
for musk thistle
management. To
favor pasture and
rangeland grass
growth, do not
overgraze. Fertilize
only when necessary
and according
to soil testing
recommendations. To
successfully manage
musk thistle, prevent
seed formation.
Mechanical control. Musk thistle will
not tolerate tillage and can be removed
easily by severing its root below ground
with a shovel or hoe. Mowing can
effectively reduce seed output if plants
are cut when the terminal head is in the
late-flowering stage. Gather and burn
mowed debris to destroy any seed that
has developed.
Chemical control. Several herbicides
are registered in pasture, rangeland and
noncrop areas to control musk thistle.
Tordon 22K (picloram), Milestone
(aminopyralid), Transline (clopyralid),
Perspective (aminocyclopyrachlor +
chlorsulfuron), Banvel/Vanquish/Clarity
(dicamba), 2,4-D, or Banvel/Vanquish/
Clarity plus 2,4-D are commonly used.
Apply these herbicides in spring or fall
to musk thistle rosettes. Refer to Table
1 for rates and application timings and
always read the herbicide label before
using the product. Applications during
the reproductive growth stages with these
herbicides (bud through flowering) will not
eliminate viable seed development.
Escort (metsulfuron) or Cimarron X-tra
(metsulfuron + chlorsulfuron) also can
be used in pastures, rangeland, and non-
crop areas. Research from Colorado State
University and the University of Nebraska
shows that chlorsulfuron or metsulfuron
prevents or dramatically reduces viable
seed formation when applied in spring,
up to early flower growth stages. The latest
time to apply these herbicides is when
developed terminal flowers have opened
up to the size of a dime. Add a good
agricultural surfactant at 0.25 percent v/v
to Escort or Cimarron X-tra treatments
or control is inadequate (equivalent to
1 quart of surfactant per 100 gallons of
spray solution).
Figure 2: Musk thistle distribution in
Colorado, 2009.
Figure 3: Musk thistle rosettes.
Figure 4: Musk thistle leaves; note cream-
colored mid-rib and frosted appearance around
leaf margins.
Figure 5: Musk thistle in bud growth stage; note
large bracts below developing flower.
Table 1. rates and application timings to control musk thistle.
Herbicide
Rate
(Product/A)
Application
timing
Comments
Tordon
0.5 to 1 pint
Spring at rosette growth
stage; or in fall
Use higher rates for older
or dense stands
Milestone
3 to 5 fl oz
Spring at rosette growth
stage; or in fall
Use higher rate for older
or dense stands; may be
used to edge ponds or
streams
Transline
0.67 to 1.33 pints
Spring at rosette to early
bolting growth stages; or
in fall
Use higher rate for older
or dense stands
Banvel,
Vanquish, or
Clarity
(dicamba)
1 to 2 pints
Spring rosette growth stage;
or in fall
Use higher rate for older
or dense stands
Perspective
3 to 4.5 oz
Spring rosette growth stage;
or in fall
Use higher rate for older or
dense stands
Cimarrron X-tra
0.5 oz
Spring rosette to early bud
growth stages; or to fall
rosettes
Add non-ionic surfactant
at 0.25% v/v
Escort
0.5 oz
Spring to rosette to early
bud growth stages; or to fall
rosettes
Add non-ionic surfactant
at 0.25% v/v
Colorado State University, U.S. Department of
Agriculture and Colorado counties cooperating.
CSU Extension programs are available to all without
discrimination. No endorsement of products mentioned
is intended nor is criticism implied of products not
mentioned.
Biological control. The Colorado
Department of Agriculture has established
a weevil, Trichosirocalus horridus. This
weevil attacks the crown area of musk
thistle rosettes and kills or weakens the
plant before it bolts. This weevil is being
distributed throughout Colorado by the
Department of Agriculture. It tends to be
more effective than the seed head weevil.
The musk thistle seed head weevil,
Rhinocyllus conicus, can be found
throughout Colorado. The female deposits
her eggs on the back of developing flowers
and covers them with chewed leaf tissue.
After eggs hatch, larvae bore into the flower
and destroy developing seed. The seed
head weevil reduces seed production by
50 percent on the average. If used alone,
however, it is not an effective management
tool. Certain herbicides or mowing can
be combined with the seed head weevil if
these are used during late flowering stages.
This allows the weevils to complete their
life cycle and ensures their presence in
subsequent growing seasons. The musk
thistle seed head weevil is not being
redistributed anymore because it attacks
many different species of thistles, including
native thistles.
Integrating Control
Methods
To combine chemical and biological
control methods, apply herbicides
when they won't interfere with insect
development. That is, allow the control
insects to complete their life cycle. Or use
herbicides in areas that aren't sensitive
to their use and biological control in
areas where herbicides are impractical or
environmentally unsafe.
Cultural methods that favor desirable
plant growth can be combined with
chemical or biological control by
superimposing proper grazing management
and seeding.
Figure 6: Musk thistle flower; note large bracts
and lack of leaves on shoot below flower.