Ask the master gardeners
Q: When is the best time to prune a butterfly bush?
A: Although pruning may be done throughout the growing season, the best time to prune a butterfly bush (Buddleia davidii) is before new growth takes place, when daffodils and forsythias are in bloom.
This shrub should be pruned back hard to about 12 inches from the ground. Pruning will promote the growth of new branches and larger flowers. The plant should grow to at least 4 to 5 feet tall each year. During the growing season, cut off the faded and dead flower heads to prolong flowering into autumn.
If your butterfly bush is at the back of the border, you can cut back to about 2 ft each year. This will produce higher flowering stems, so smaller perennials can be planted under or near the base of the plant. To keep your butterfly bush shapely, give it a trim by cutting back the lateral branches, too.
Unlike Buddleia davidii, which has leaves in pairs, spring-blooming butterfly bush (Buddleia alternifolia) bears alternate leaves. This shrub gets pruned back in early to mid summer because it blooms in spring on stems from the previous year.
Pruning after it blooms allows to develop new buds on drooping stems well before the arrival of winter. Again, prune back hard to about one-third of the original size. Remember, if you prune this shrub at the same time as Buddleia davidii, you’re cutting off this season’s blooms.
When pruning, always use a sharp bypass tool, with a lopper for the larger stems.
Deborah M. Smith, Haverstraw, master gardener with Cornell Cooperative Extension of Rockland