From what I understand, there is something in the seed shells also....if you look under the bird feeder where you put your sunflower seeds, there will be a bare spot.....
Hmmmm, those are in an old flower bed, but when they got established, the weeds did seem to be less. May be something to that.
I do have some on the edge of my garden, but still have weeds.
I do know the butterflies and bees like them. And my milkweed patches. I have seen a few monarchs at the milkweed, the real reason I let some grow.