Buddleia alternifolia is not one of the more common Buddleia davidii - The Butterfly Bushes. It has an arching weeping habit, and needs different pruning to the common types of butterfly bush.
It flowers on wood (growth) made the previous year so its pruning needs are different.

Because of its weeping habit, it will make a spectacular flowering tree or even large hedge.
The flowers are sweetly scented - especially in the evening.
Buddleia alternifolia - so named because it bears its leaves alternately along the stems, (Butterfly Bushes) have their leaves in pair!
Early to mid summer flowering garden shrub, which flowers on stems that were grown in the previous year. It therefore needs pruning right after its flowering period, in order to allow new flowering stems to grow before the onset of winter.
Prune old flower stems on your Buddleja alternifolia back quite hard (about one third of the way) immediately after flowering. This will allow time for long arching weeping stems full of flower buds for next year to grow. Do NOT prune this Buddleia in the spring - which is the normal pruning regime for the Buddleia davidii types.