there are a bunch of plants under the common name butterfly bush ..

any chance you can be more specific.. or post a pic.. or browse google images and find the one you are talking about????

ken

If you are talking about a buddleia, most people prune them every year - only in the Spring after they have started to show signs of new growth. Cut them down to between 6 and 12 inches. They grow really fast and bloom on new wood. If you don't prune them hard every Spring the blooms will always be over your head. Never prune them in the Fall.

Deb

Ditto what Deb said too - with the caveat that you can choose to not cut it back that hard if you want a taller plant. I keep some of my butterfly bushes tall on purpose. The important thing I find is to force development of flowers on the sideshoots to provide maximum bloom and extend the blooming period. When you deadhead, the two dormant buds behind where you cut off the flower develop and put out flowers - so you cut off one flower and get two to replace it. You can do the same with the developing branches (i.e. cut off their ends) to force it to branch more densely, which gives you more flowering branches. You can also shape it by selectively cutting off branches if, for example, you actually want it to look more tree-like so other things have room to grow underneath it.

This isn't the best of pictures... I only cut this one down to about 3' because I want it to be a substantial presence in the bed by late summer. It gets to be about 8' tall or so. We got lazy about making it branch out this year so it's not as densely flowering as it should be.
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That's a pretty Buddleja Woody. I did what you describe one year to my 'Pink Delight' - cut it back to about 2-3 feet so it would grow a bit taller. It was quite compatible with the form and the new growth that year. Usually I cut them down to about 8-16 inches high sometime in April, after they show new growth. I cut the stalks back to new green shoots between 8-16 inches high.

Here is what the 'Black Knight' looked like after I cut it back in late April last year. Later I also thinned out some of the overcrowded shoots near the base of the plant. By June or July this one is 6-7 feet tall again.
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I posted a follow up last night but it looks like it didn't "take". My plant is the same one that Woodyoak shows above. It looks bad right now, it's so tall and not really very "bushy" at all. It doesn't bloom very much either. It always looks dried out and there aren't a lot of flowers. I will cut it way back, and give it some fertilizer. Thanks everyone!

Depending on where you are, you can cut them in the fall. I have about a dozen and prune each autumn.

I prefer a two step pruning. In the fall I cut them to about 4 feet and then in the spring will cut again to about one foot. One year I cut to a foot in the fall and the cut developed slime flux disease that all but killed the plant. Al