Question: I planted two butterfly bushes. One reached 5 feet, the other 4. Should I cut them back this fall or wait until spring?
Answer: Fall pruning of buddleias or any other shrubs is inadvisable. Pruning inhibits the natural hardening off of tissues and can increase the likelihood of winter kill. In some cases, it stimulates late growth, which is similarly tender. It is better to wait until after the worst of the winter weather is past.
Buddleia davidii, the most commonly grown species, produces flowers on the current season's growth and blooms from mid-July to frost, so you can safely cut it back to a foot or so in the spring with no loss of show. There is one species that flowers on last year's wood. The fountain buddleia (B. alternifolia) has arching clusters of lilac flowers in mid-May. It is easy to distinguish by its alternate leaf arrangement; all the other buddleias are opposite-leaved. Prune this species by thinning out one-third of the oldest wood after flowering.
This post is excerpted from the September/October 1998 issue of Horticulture.
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