It is important to know a little bit about the life cycle of musk thistle so that you can spray it or cut it down before seed set. Musk thistle is an aggressive biennial weed. Its impressive spreading ability is made possible by the large amounts of seed that it produces. One musk thistle plant can produce 20,000 seeds every season, and some of that seed will remain viable in the soil for seven years. Therefore, the primary method of musk thistle control is to prevent seed production.
Monitor the landscape in late February and early March. This is when musk thistle begins its growth cycle for the season. It will begin to flower and set seed within the month. The best time to spray musk thistle is before it flowers. After that it will become more resistant to herbicides.
Inspect flowering musk thistle. Musk thistle flowers from late March through May, and its flowers will reach maturity at different times during that point. Purple flowers have not yet set seed, but they will in a matter of weeks.
Look for white thistle heads. These thistle heads have set seed. And it will be difficult to control their spread at this point. The tiny seeds have fluffy white tails that make them easily airborne. The best way to cull the seed heads at this point is to place a paper or plastic bag over the white head before pulling it off to prevent the seed from spreading.
Look for dead musk thistle. These thistles have already set seed.