You can't miss an outbreak of musk thistle. This towering weed can grow up to 6 feet tall and produce flowers larger than 3 inches in diameter. Musk thistle most commonly invades drought-prone, overgrazed land. Once it establishes itself, musk thistle spreads quickly. Each plant can potentially produce 120,000 seeds. But, especially if caught early, musk thistle is relatively easy to control.
Cut musk thistle back with lopping shears, as close to the ground as possible. Wait for it to grow back 4 to 6 inches. Herbicide is most effective when sprayed on actively growing plant tissue.
Spray the new growth with an herbicide prescribed for use on musk thistle (Tordon, Milestone, Transline and 2, 4-D are all acceptable choices). Wait for a windless day with no rain forecast for the next 48 hours. Spray the plant tissue until it is saturated, but just before the point where the herbicide drips off the plant.
Re-spray the thistle as necessary, as soon as new growth crops up. Spray before its flowers produce seed in summer or you will have a whole new outbreak to contend with. For the most effective control, switch to a different herbicide for the second treatment.
Plant the area once the musk thistle is gone to prevent it from being reinfested. Well-maintained grass or other rapidly growing ground cover suitable for your area will easily outcompete weeds.