Buddleja davidii is a vigorous plant and requires fairly drastic pruning to encourage flowering. The extent of the pruning depends, in part on where the bush is located – those at the back of a bed should be pruned back to a framework of about 3–4 ft height, while those further forward in a bed should be cut to no more than 2 ft height.
During the first spring after planting, the main stems need to be shortened to roughly two-thirds of their length, pruning just above a pair of buds. In subsequent springs, cut back the previous season’s growth and look to see if any of the older growth needs to be removed to reduce congestion. The timing of the prune will influence when the Buddleja will flower; pruning later into the spring will delay flowering, so it is possible to produce a succession of flowering across several plants by pruning one in January, one in February, one in March and one in early April.
Note that not all Buddleja should be treated the same. For example Buddleja globosa, which flowers on old wood, should be pruned immediately after the previous year’s flowering; if left until winter or spring this year’s flower buds will be removed.