I was under the impression that 2,4-d did didly against knapweed. Not so?
My understanding is that for chemical warfare, only tordon and clopyralid will work against knapweed. But they have such huge downsides, most folks have stopped using it and some places have even banned it.
Mowing at the right time (when it has its big growth spurt) kills off about 90% of it. Irrigation helps because knapweed exudes niacin to kill off any plants that might try to compete with it. Irrigation will wash away the niacin and help other plants outcompete.