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
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.
• Musk thistle is a biennial
weed that reproduces only
• The key to successful musk
thistle control is to prevent
• Apply herbicides such as
Tordon, Milestone, Transline,
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.
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
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
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.
and rangeland in
good condition is
a primary factor
for musk thistle
favor pasture and
growth, do not
only when necessary
to soil testing
musk thistle, prevent
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
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 +
(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
colored mid-rib and frosted appearance around
Figure 5: Musk thistle in bud growth stage; note
large bracts below developing flower.
Spring at rosette growth
stage; or in fall
Use higher rates for older
or dense stands
3 to 5 fl oz
Use higher rate for older
or dense stands; may be
used to edge ponds or
Spring at rosette to early
bolting growth stages; or
or dense stands
1 to 2 pints
Spring rosette growth stage;
or in fall
3 to 4.5 oz
Use higher rate for older or
Spring rosette to early bud
at 0.25% v/v
bud growth stages; or to fall
Add non-ionic surfactant
at 0.25% v/v
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
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
To combine chemical and biological
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
Cultural methods that favor desirable
plant growth can be combined with
chemical or biological control by
superimposing proper grazing management
Figure 6: Musk thistle flower; note large bracts
and lack of leaves on shoot below flower.