Most butterfly bushes (Buddleja spp.) produce 4- to 10-inch fragrant flower spikes that look like lilac blooms, in shades of purple, pink, white and, sometimes, yellow or orange. Growing up to 15 feet tall and wide, butterfly bush needs either plenty of space for its arching branches, or it needs to be cut back. You can cut it back partially or severely without worrying about any damage to the plant.
Pruning After Blossoming
Some varieties of butterfly bush bloom on the previous year's wood and should be pruned immediately after they bloom. These varieties include fountain butterfly bush (Buddleja alternifolia) with lilac-purple flowers and 4-inch-long, blue-gray leaves, and "Honeycomb" (Buddleja x weyeriana "Honeycomb"), a hybrid butterfly bush with yellow flowers that can grow up to 7 feet in one season. Both varieties grow in U.S. Department of Agriculture plant hardiness zones 5 through 9.
Prune Before Spring Growth
Summer lilac butterfly bush (Buddleja davidii) blooms on new growth and should be pruned in early spring or late winter. This variety grows aggressively in USDA zones 6 through 9 and is considered an invasive species in some parts of the country, where only sterile varieties are sold. It typically reaches 15 feet tall and has 4- to 12-inch leaves that are dark green on top and white beneath. Dwarf cultivars grow 5 to 8 feet tall and 6 feet wide.
Types of Pruning
Prune to remove some branches, parts of branches or all the branches. To make your bush smaller overall, prune out three or four of the oldest shoots just above ground each year or cut back the entire bush to the ground in late winter and a new bush will regrow from the roots. Create globe shapes by pruning branches back partway or tree shapes by leaving only one main branch and pruning it multiple times to branch out midway.
Butterfly bushes do best when grown in full sun or light shade in rich, well-draining soil in either full sun or light shade. They need regular, weekly watering until they are established, when you can cut back to deep soakings every two to three weeks. Water less frequently in winter. Some varieties die back completely in areas with freezing winter temperatures, reemerging in spring.