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Musk thistle

(Carduus nutans)

Photo of musk thistle
Photo credit: S. Kelly Kearns

Herbaceous biennial. Plants overwinter as rosettes in the first year and bloom in the second year. Flowering plants are one to seven feet tall. Musk thistle has a multi-branched stem that appears winged.

Overview

Other names for this plant 
  • Common names: nodding thistle
  • Scientific names: C. macrocephalus; C. nutans spp. macrocephalus; C. nutans ssp. leiophyllus
Ecological threat
  • Invades areas such as pastures, old fields, roadsides, waste areas, ditch banks and prairies.
  • When in meadows and pastures, grazing animals avoid musk thistle and focus on native plants giving the invasive the upper hand.
Overview map of musk thistle classification in WI
Restricted (orange) counties

Classification in Wisconsin: Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for musk thistle was based upon this literature review [PDF] developed by the department.

Identification

First-year plants-leaves: Basal leaves are up to 12-17” long, lance- to oblong-shaped and lobed with spiny margins.

Second-year plants-leaves: Alternate leaves clasp the stem giving it a winged appearance. They are coarsely lobed with prominent terminal spine and slightly wavy, yet smooth and hairless on both sides. Leaves are dark green with light green midribs and a white margin.

Flowers: Musk thistle has large red to purple, solitary, terminal flowerheads, 1.5-3” across, and usually bent over or “nodding”. Blooms May through August.

Fruits & seeds: A single flowerhead can produce up to 1,200 seeds that are viable for over 10 years. Wind-dispersed seeds can travel long distances.

Roots: Single taproot.

Similar species: Native marsh thistle (Cirsium muticum) has non-spiny stems and flower heads. Canada thistle (Cirsium arvense; invasive) does not have a winged stem or spines on its stem or flower heads. Bull thistle (Cirsium vulgare; non-native) has prickly winged stems and leaves have prickly hair above, with white wooly hair below. The single or clustered purple flowers, 1.5-2” across, are borne at the ends of branches and are enclosed by narrow, spine-tipped bracts.

Control

Mechanical: Sever root one to two inches below the soil surface. Repeated mowing (minimum two to three times per growing season) when flower buds are about to open will prevent seed production.

Chemical: Applications are most effective when plants are in the rosette stage. Foliar spray with glyphosate during bolting phase when plants are 6-10” tall, during the bud to flower phase, or rosettes in the fall. It can also apply a foliar spray with clopyralid or metasulfuron-methyl. For musk thistle on severely disturbed sites, apply of 2, 4-D ester or dicamba in early bolting phase. Or apply in combination – dicamba with 2, 4-D ester.

Biological: The flower head weevil (Rhinocyllus conicus) and the rosette weevil (Trichosirocalus horridus), have been released in some states but not in Wisconsin due to risks presented to rare native thistles.

Photos

View musk thistle pictures in our photo gallery.

Resources

Sources for content:
  • Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. University of Wisconsin Press. 2005. Pg. 101-102
  • Gassmann A. and L.T. Kok. Musk thistle In: Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p [exit DNR]. Last updated Nov. 5, 2003.
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