At the other house I lived in the ones planted in front of the storm drain were eight feet tall and had little baby nandinas at the base. Very easy to pluck out. Here I have never had a seedling. Less moisture and lots of mulch. Also Butterfly bushes are supposed to be invasive but in an area where there is frost they always dye down to the ground each year and have never been a problem. I have noticed, btw, that they have some new cultivars that are sterile.
I bought one from Santa Rosa called 'Asian Moon' it is beautiful. Our friends in Georgia and Florida will be happy. Also Lantana, I have no problem but if you talk to people in Georgia it's another story, they are just cussing.
Again, there are new ones out that are sterile, as a matter of fact I think mine are sterile, because they are fairly new cultivars, just bought them last year. I have never seen any seeds at all.