I think it is only some of the tagetes family that are supposed to deter couch grass by exuding something from their roots that disagrees with it...I tried growing tagetes 'minuta' last year....no germination from the whole packet sown three different times so I gave up!
I think poached egg plants are popular for attracring pollinating insects to lotties, that what I grow them for, but they are like forget me nots, once you have them....... :-\