The popular and aptly named butterfly bush (Buddleia sp.) is very easy to prune. Here’s the nice thing about butterfly bushes: you can’t prune them wrong! Prune to any height you want, and at just about any time of year because buddleia are very tough and forgiving.
In habit, butterfly bushes are informal with a loose, open growth habit that does not hold a tightly manicured shape. Flowers are produced on new growth, so size can be controlled by cutting back quite hard in early spring, just before leaf buds begin to grow.
While pruning is not absolutely necessary, butterfly bushes tend to bloom better and keep a better shape if pruned hard each spring. They can also be pruned in summer to encourage new blooms, or just to tame untidy growth.
For non-dwarf butterfly bushes, cut back to 12 to 24 inches and remove any dead or broken branches. (Or cut back nearly to the ground. Butterfly bushes will still resprout and bloom!) For dwarf types such as ‘Blue Chip’ and ‘Buzz’, cut back by at least half the height in early spring. No special techniques are needed.