NISAW: Musk Thistle

The second invasive species that we are going to meet during National Invasive Species Awareness Week is musk thistle (Carduus nutans) aka nodding thistle.   I worked closely with musk thistle two summers ago when working in the Mount Rose Wilderness.  We found a bunch of it in the Hunter Creek drainage at the north end of the wilderness area.  I led several volunteer trips into the wilderness area to hand pull the thistle. 

Musk Thistle nodding flower head

Musk thistle is a biennial, which means it lives for two growing seasons or two years.  Typically in the first year it forms a rosette and in the second year it bolts, flowers and goes to seed.  However, musk thistle can also be a winter annual, emerging in fall and going through all of its life stages within a single year.   Musk thistle reproduces by seed.  When the plant first emerges the leaves form a rosette, which is attached to a deep taproot.  The leaves are distinctive with a light-yellow/green midvein and silvery, purple edges.  Leaves can be between 4 and 15 inches in length and have spines that will  hurt you if you touch them too hard.  As the plant grows, it can get as tall as 2 to 7 feet in height.  The stem is sometimes covered with hairs and is branched.  It has spiny wings that extend down the length of the stem with leaves that  tend to alternate up the stem. 

Musk Thistle rosettes

The flowers of the musk thistle tend to be pink to purple in color and up to about 3 inches in diameter.  A distinctive trait of the musk thistle is the "nodding head" of the flower--it tends to droop at the end of the stem making it appear to be nodding.  The base of the flower has green and purple, large, spine-tipped bracts.  These large bracts are characteristic of musk thistle and can be used to help with identification.

Musk thistle tends to infest roadsides and waste areas where it forms dense stands crowding out native plants.  It can be found throughout most of Nevada.  According to the Nevada Noxious Weed Field Guide, mowing, tilling or hand removal after bolting, but prior to flowering is effective, you do need to dig down and remove the top 2 inches of the crown.  There are some biological controls that are available.  As for chemical control, apply 2,4-D, dicamba, chlorsulfuron, metsulfuron or picloram to actively growing rosettes; aminopyralid or copyralid between rosette and late-bolt stages.  Check with a certified herbicide applicator to get current information on pesticide use.

Sources:
Creech, Earl, Schultz, Brad and Blecker, Lisa. Nevada Noxious Weed Field Guide. University of Nevada Cooperative Extension, 2010. PDF available here.