Musk Thistle
Carduus nutans

Musk thistle is considered a nonnative plant species that grows from sea level to about 8,000 ft in elevation, in neutral to acidic soils, soils with high sand content. Musk thistle may be restricted by a low tolerance of extremes in soil water content, or nutrient deficiency. Musk thistle occurs in temperate regions and is not known to have a specific climate requirement. Musk thistle is most often described as occurring on disturbed sites and waste areas, and along roadsides. It will also occur on rangelands, pastures, open woodlands, and fertile lowlands. However, musk thistle does not grow well in excessively wet, dry or shady conditions.

Musk thistle is an herbaceous tap rooted, invasive thistle, is usually biennial or under Oklahoma climate conditions develops as an annual plant species. Musk thistle, also classified as a forbs or dicot, will germinate in late summer through fall. After germination the plant develops into the rosette stage. The following spring, usually May through June, the rosette shoots vertically developing a violet or purple flower seed head, heads are usually drooping. The stems are stout, erect, and covered with spiny lobed wings.

Reproduction will usually occur from June through July. The fruit that is produced is a one-seeded achene usually yellow-brown with one straight edge and one curved. Seed dispersal occurs within 7-10 days of flowering. Earlier maturing seeds have a higher viability than late maturing seeds from secondary branches. Seed dispersal is mainly due to wind and transport in thistle infested hay. Seeds have also been known to remain viable in the soil for up to 10 years.

Musk thistle, an aggressive biennial herb in the aster family (Asteraceae) has showy red-purple flowers about two inches in diameter that are surrounded by broad, spiny-tipped bracts. Flower heads, one plant can produce as many as 50 flowering heads, are large and disk shaped with tiny individual flowers within the head which can produce thousands of seeds. Painful spiny stems and dark green leaves coarsely lobed with a yellow-white spine at the end.  Mature plants can reach heights of 6 feet and have multi-branched stems.

Figure 1. Shows the distribution of musk thistle in the U.S.

Note: Light pink shows exotic distribution. Dark pink shows areas that are not ranked and are under review. White shows areas with no known invasions.

Figure 2. Shows the distribution of musk thistle on the state level in Oklahoma.

Note: Data is based on literature, herbarium specimens, and confirmed observations, however significant gaps may not be real. Only native and naturalized populations were mapped. Counties in green are invaded.

Musk thistle is an aggressive species that can form dense stands very rapidly. Infestations of these plants have the ability to reduce productivity of pasture and rangeland by suppressing growth of desirable plant species, including native species, and forage for livestock. Livestock won’t graze around musk thistle plants or in heavily infested areas.

The economic impact of musk thistle is greatest in pastures and rangeland. Moderate infestations of musk thistle have been reported to reduce pasture yields an average of 23 percent. The invasive nature of the plant can also lead to severe degradation of native grasslands and meadows. Grazing animals will focus on native vegetation giving the thistles a competitive advantage.

Thistles have long been associated with humans, and have been used for both food and medicine. Invasive thistles that are not native to North America will fill specific ecological niches and have traits useful to humans.

Management objectives for successful musk thistle control should be to prevent seed production. Since the seeds of musk thistle can remain viable in soil up to 10 years, infestations should be closely monitored to prevent re-establishment. Reducing or preventing seed production and dispersal can help decrease the spread of the plant. Healthy stands of desirable vegetation are required since seedlings cannot tolerate intense competition.

Seedling and rosette growth are usually the best times to implement control measures. 

There are 3 main control measures that can be implemented.

Manual

• For small populations hand pulling can be done throughout the year, but is most effective prior to the development of seeds.

• Minimizing disturbance to soils reduces the chance of germination of seeds stored in the soil.

Chemical

• Can be effective by using available herbicides.

• Using herbicides that can kill non target plants should be avoided.

• Spraying with herbicides in late fall or early spring have been successful.

Biological Control

• Two weevils have been introduced and released into the U.S. from Europe.

• Rhinocyllus conicus thistlehead-feeding weevil and Trichosirocalus horridus rosette weevil.

• These weevils have shown some notable success, but they may harm native thistles.
  
  
  Above: Thistle head-feeding weevil
  Below: Rosette weevil
  
  Fire

• Fire has not been effective as a method for direct control of musk thistle.
• For fire to have a direct effect it would have to reach high temperatures hot enough to destroy the root crown.
• Fire can create favorable conditions for musk thistle by allowing more light to penetrate the soil giving rise to more germination.
• Long term secondary effects have fire have not been documented.

It is highly suggested that control methods be combined into an integrated system for the best long term control of the plant. Management techniques should also be evaluated at each specific site by land use objectives, degree of infestations, and effectiveness of control measures

http://www.fs.fed.us/database/feis/plants/forb/carnut/all.html#INTRODUCTORY

http://www.invasive.org/browse/subject.cfm?sub=3011

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http://www.natureserve.org/explorer/servlet/NatureServe?searchName=
Carduus%20nutans

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http://www.fs.fed.us/database/feis/plants/forb/carnut/all.html

http://www.na.fs.fed.us/fhp/invasive_plants/weeds/musk-thistle.pdf

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http://www.agry.purdue.edu/Ext/forages/publications/WS-19.htm

http://www.na.fs.fed.us/fhp/invasive_plants/weeds/musk-thistle.pdf

http://www.natureserve.org/explorer/servlet/NatureServe?searchName=
Carduus%20nutans

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http://plants.usda.gov/java/profile?symbol=CANU4

http://www.ok.gov/~okag/forms/law/noxweedlaw.htm

http://www.invasivespeciesinfo.gov/plants/muskthistle.shtml

http://www.uaex.edu/Other_Areas/publications/HTML/FSA-3054.asp

http://www.agdepartment.com/NoxiousWeeds/pdf/MUSKTHISTLE.pdf

http://www.agry.purdue.edu/Ext/forages/publications/WS-19.htm

http://www.natureserve.org/explorer/servlet/NatureServe?searchName
=Carduus%20nutans

http://www.na.fs.fed.us/fhp/invasive_plants/weeds/musk-thistle.pdf