Snip spent flower heads, cut out dead branches. Give a good soaking in response to yellowing leaves, which probably indicate a need for it.

Cut back hard next March to renew. Or cut down and dig out completely, use the space for something else.

Here is a link that might be useful: ODA Plant Division, Noxious Weed Control - Butterfly bush

bboy gave you good advice up and until he said to dig it out. You didn't say what species it is. Many are border line hardy in the PNW. If it is B. davidii it is hardy except for during one of our extreme cold Artic blasts. So giving any of them a hard cut back would weaken them to the point of death if we had a cold winter. The National Weather Service is predicting a colder than normal winter for the PNW so be light with your trimming.

I have several species in my garden, including davidii. Bees, butterflies, and hummingbirds love them. Because of their fragrance, davidii attract more attention than the other species.

Thanks bboy and issafish! Yeah, bboy doesn't like Buddleias :-) I did hear about them being a problem, but I am with you issafish- we've really been enjoying the hummingbirds, butterflies and fragrance. One is a davidii, not sure about the other, as it was a transplant from my neighbor- different color.

They've just grown so huge they are overpowering the rest of my garden and I was hoping to be able to trim them down a bit. It is hard for me to be patient until March!

Most people are growing B. davidii or its hybrids when they talk about butterfly bush. I've grown them for years, still have a few specimens I have not removed. But we now have a new situation with the plants in this region - that is to be totally ignored only if you don't care about the world outside your fence.

This popular ornamental shrub has been establishing in not only disturbed areas, but natural areas as well, where it has negative ecological impacts and is difficult to control.

* It forms dense thickets, especially along river banks and river gravel bars, which crowd out native vegetation and may alter soil nutrient concentrations.
* The seeds, so easily spread by wind or water, can remain in the seed bank for three to five years.
* This shrub can begin producing seeds during its first year.
* Once established, this shrub is difficult to remove. It will re-sprout from the rootstock after its stems are cut, and the cut stems can also grow into new plants.
* Butterfly bush is a Class B Noxious Weed in Oregon, is one of the top twenty weeds in Great Britain, and is a serious problem in New Zealand

Here is a link that might be useful: Washington State Noxious Weed Control Board - Class B Noxious Weed - Butterfly Bush

ellen, Go ahead and trim the branches back. Where you might get problems is when you trim them back too far. I think I heard some where not to take more than a third out. Besides, it is still August, still time for any new growth to harden off.

I don't think that B. davidii is a big problem, not like English ivy. Like I have said before, the 1983 freeze took all the davidii out, dead, in my yard, along the road sides, and other people's lawns. At the time I was living in Montesano along the coast of Washington. And only now are people starting to grow it again in that area. And it is not as hardy as people like to say it is, claiming zone 5. A friend living in Walla Walla grew a hedge of them, a freeze that had temperatures a few degrees below zero killed them all, zone 6. The '83 freeze was at 4 F and stayed below freezing for the whole month of December. This last December I had four nights at 12 F with three weeks of below freezing weather. It didn't kill the davidii but it damaged them. bboy likes to tell people who grow tender exotics, one of these real cold spells will come along and turn them into compost. He is right, we will have another real cold spell, and I wouldn't be surprised to see most of them either damaged or killed outright.

If you planted your butterfly bush before it was listed (2004) you do not have remove it, but you are required to prevent it from propagating. You must deadhead the plant after blooming to prevent the seeds from spreading. Or better yet, replace your butterfly bush with some other native or non-invasive plants. Your local nurseries can help you chose a wide range of shrubs that can replace your butterfly bush

Here is a link that might be useful: ODA Plant Division, Noxious Weed Control Frequently Asked Questions

Of all the shrubbery I see around SE Portland, untended Buddleia is the most bedraggled and unsightly plant.

The Tiger Swallowtails are usually on the wane by the time butterfly bush is in bloom.

Which has nothing to do with the fact that the plant has become a public nuisance in this area. There is no fundamental difference between dreaming of a personal preference for planting only native plants becoming universal and dismissing the problems a particular exotic plant causes for others because one has a personal fondness for that plant.