Prune the Buddleia down to around 30cm (12 in) from ground level at this time. There may already be sprouting shoots on the Buddleia in a mild spring. No matter - Prune it back hard. This will make the Butterfly Bush grow many new arching branches, that will have larger flowers than if it had not been pruned.
The scribbles in left sketch, show the old bush at the top, with suggested pruning height. Yes, the pruned Butterfly Bush will look like a stag's antlers - but not for long. Within a few weeks it will start into grow the give you a nice arching shrub, that will soon
grow to around 6ft - 1.8 metres in a matter of a couple of months, and have fresh foliage, and large flowers drooping the branches over to give a pleasing arching habit.
The new main stems, will also send out some side shoots, which will also flower - but smaller flowers than the main tip end blooms.
Once you have finished pruning your Buddleia, you should end up with a framework of stems (stumps!) around 30-45cm from the ground..
For Buddleia bushes at the back of a large border, you can perhaps
just cut back to around 60cm each year. This will have the advantage of the flowering stems starting higher up and you will be able to plant other shrubs and perennials under or near to the base. The arching stems of the butterfly bush will then be seen higher up at the back of the border.
A pair of sharp loppers may be better for the larger branches on Buddleia.
It is nicer work on a sunny day! Time to prune when Forsythia and Daffodils are out in flower.
This is what the 'short back and sides' should look like when finished. (Be brave!)