But there's a dark side to this popular plant. Butterfly bush can be a very aggressive, or invasive non-native shrub that, in certain situations, can overtake native vegetation, according to horticulturists with the Oregon State University Extension Service and weed biologists with Oregon's Department of Agriculture (ODA).

http://extension.oregonstate.edu/news/story.php?S_No=770&storyType=garde

In Oregon, the ODA officially classified it as a class "B" noxious weed in 2004. Its strategic plan includes efforts to eradicate butterfly bush in the wild, but not from people's yards.

Impacts
This plant is a pioneering species that dominates open habitats. It poses an ecological threat to dry-land meadows, open slopes and dunes, dominating these sites as much as Scotch broom has historically. It also invades reforested sites, resulting in a loss of forest productivity.

They just outlawed the sale of this plant in Oregon. A pretty plant, but it is OBnoxious.

Not all butterfly bushes are invasive. I have 7 on my property and have never found even one seedling. I have seen pale lavender (and a few white) bf bushes in areas along I-5 in "generous" quantities, but they don't appear to be the hybrids commonly found in nurseries.