Hi there!

Great question! Spotted knapweed can be quite a pain in the neck. Generally speaking, mowing can keep it from going to seed, but it won't significantly reduce the current population. Herbicides are widely considered to be the most effective and efficient way of controlling the population. You'll have to spot spray if there are other desirable plants in the field. If it's a heavy infestation across a large area, then using an application vehicle with a spray boom will be much faster. You may have to make additional applications over several years.

Montana State University publishes a great knapweed management guide that discusses the benefits and drawbacks of various methods starting on page 8. Download here: Biology, Ecology, and Management of Montana Knapweeds.

Let me know if you have additional questions.