Fact Sheet No. 102 Natural Resources Series

  

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. Herbicide 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.