During a recent trip to the UK, I took this photo of a canal side wall full of Buddleia davidii seedlings. In the UK and here in New England, the butterfly bush is escaping our gardens and needs to be placed on the invasive species list before it becomes as invasive as Berberis thunbergii (Japanese barberry)
Here in Connecticut, there are alternatives. Clethra alnifolia is of similar size, fragrant
and comes in various colours
If you feel your garden can't survive without a butterfly bush, sterile varieties are now available.
Asian Moon, Blue Chip, Chip Jr, Ice Chip, Inspired Pink, Pink Micro Chip and Purple Haze are some.
Or, when your Buddleia shrub looks like the photo below, you can cut off all the spent flowers before they produce seeds, with pruners, or a hedge trimmer, which will in turn give you a second flush of flowers.