Hi Megan,

I'm sorry to hear about your thistle problem!  Musk thistle (Carduus nutans) is a biennial, and relies on seed germination to persist as a population.  It's best to try to kill the one-year-old plants, which will be growing as rosettes of leaves close to the ground.  Spraying them with an herbicide (or severing/pulling them beneath the soil line) should work.  Be careful if pulling or digging, as the soil disturbance that this causes creates ideal habitat for new thistle seeds to germinate!

Flowering plants (two-year-old plants) should be removed before seed-set.  If you see flowers, remove the plants.  Spraying plants that are flowering may not prevent the maturation of seed.  

Recommended herbicides from the Colorado Department of Agriculture include Aminopyralid, Clopyralid, and Chlorsulfuron (which is the best choice if treatment is being done around woody vegetation, since aminopyralid and clopyralid are extremely dangerous to trees and are soil-active).

Always be sure to read and follow the label instructions for an herbicide.  Weeds need to be actively growing for herbicides to be effective.

I hope this is helpful!

Cordially,

John