Land Plants - invasive

Meadow Knapweed (Centaurea × moncktonii) on Jul 26, 2019

Originally reported as Knapweed (species unknown) (Centaurea)

Submitter has sample
EDRR Status: Population treated

Description of specimen

Plants must have established a few yr ago but are spreading rapidly; I realized they were probably invasive last fall, and identified to knapweed using on-line plant ID guides. I've been trying to control them by pulling (last fall) and mowing (spring and early summer), but that's just made them mad. The stand is ~100'x100' on an open hillside, where they can be the dominant plant. They are starting to flower and I'd like to know how to control them before they go to seed.