Answer to the October's Horticulture Challenge

This is the Butterfly bush also known as Buddleja davidii and also known as the summer lilac. Butterfly bush is a non-regulated Class B noxious weed. This means due to how widespread it is in King County, property owners are not required to control this species on their property, though it is always encouraged. Butterfly bush is also on the Washington quarantine list and it is illegal to buy, sell or offer it for sale in the state.

Source: The King County Noxious Weed Control Board