Butterfly bush ( Buddleja davidii) is native to China but has become a popular garden ornamental in North America. However, it has escaped to invade roadsides, pastures and other disturbed areas.
While touted as beneficial for butterflies, it cannot be used as a butterfly host plant and it displaces native plants that butterflies need for reproduction.
Butterfly bush is a large deciduous shrub, growing up to 10 feet tall. The small, fragrant flowers are usually purple, but there are red, pink, blue, orange, yellow and white varieties.
Butterfly bush produces up to 3 million seeds per plant, which can remain dormant in soil for many years. When cut down, it resprouts readily from the rootstock. Butterfly bush can reach maturity in less than one year, which allows it to spread quickly, forming thickets that exclude native vegetation.
Since it reproduces by seed, clipping flower heads is an effective way to keep it from spreading, and persistent hand-digging is possible for small numbers of plants or seedlings. Contact the Whatcom County Noxious Weed Control Board for advice about chemical treatments.