Answer: The first thing I always say about butterfly bush (Buddleja davidii) is that it is invasive in Oregon. It's been added to the state's noxious weed list. There are some species and new hybrids that don't seed around so badly, but these are not the most popular ones you see commonly at nurseries. That said, I know a lot of people love it and resist taking it out. One way to grow butterfly bush and not feel guilty is to cut off the flowers before they go to seed. So you can still have the flowers, but once they dry out, get them out of there.
As far as pruning butterfly bush, they are very adaptable. Lots of people cut them to the ground in early spring (usually March). Doing this, called stooling, helps keep the size of the plant under control and does not affect blooming. But if you don't want to go to those lengths, you can cut on a butterfly bush pretty much anytime you want to and to whatever size you'd like.
Butterfly bush will be happy with one deep soaking per week during the spring and summer months if rainfall is sparse. Make sure your plant is getting lots of sunshine and you should see lots of flowers.
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