Butterfly bush

if it is a buddleja davidii they are usually pruned in spring( uk) but buddleja alternifolia is pruned after flowering as it flowers on last years growth,as does B.salvifolia and agothosma

I cut mine (B.davidii) back by a third in the autumn and then again more in the spring, to healthy shoots. This protects the shrub from wind-rock, as we get gales in this area.