Also known as Creeping Thistle, this plant is toxic to livestock. They often outcompete native vegetation. Differentiated from native thistles by relatively small flower heads, long & narrow leaves, and the large number of flowers per branch. 

For small patches, dig up and remove plants. For larger patches, cut/mow down to base annually before flowers bloom. Gloves highly recommended due to spikes and toxins. 

(Image credit: Family Handyman)