I hadn't heard of plants in the bachelor's button family being alleopathic....I have cosmos growing right into my bachelors buttons, and the cosmos seems none the worse for wear.

The link below has some info on how Spotted Knapweed affects other plants:

Here is a link that might be useful: Spotted Knapweed

Yes some members of the Centaurea family are highly alleopathic. Those would be the knapweeds.

If it does contain the same chemical, it's not in the quatity that is of any consequence. At least not that anyone has studied. While bachelors buttons are certainly notorious for reseeding well, it's no thug like the knapweeds, lol!!

Here is a link that might be useful: knapweed article

Spotted Knapweed easily takes over an area. This summer I've noticed that people are not mowing areas infested when the plant is flowering as it is rather attractive. BUT BEWARE!!!!! It will reseed and take over. Minnesota is attempting to have it listed as a noxious weed or at least some counties are. But that is a long drawn out process.

It also spreads by its prolific underground root system

This is a hateful plant! Here on the shores of Lake Superior we are battling with it. It has crowded out all our native wildflowers on the roadsides and lines the edges of our lakes. I dig it out and also use round-up. I have had success on my property but more seed keeps blowing in. I am patrolling for it every day. If you see it...get rid of it!

keewenaw gal: Keep up the good work! I love the Lake and am disturbed to seed this infestation. Down here in Indiana, we are losing ground with Garlic Mustard and Amur Honeysuckle (introduced, btw by the Indiana DNR, some 20 years ago (always like to give them credit fo including it in their "Wildlife Packet!"); it's all over the place and in State parks here. Please people, DON'T spread invasive, non-native species!!!!