Q The butterfly bushes I planted about 10 years ago have matured nicely, and I have learned to fearlessly cut the stems back to 12 to 18 inches during the winter. Last summer, however, they had their longest flowering season, which I attribute to my gardener-friend's advice to "deadhead" the spent blooms. Could you elaborate specifically when to cut the dead flowers and at which point on the branch? Is deadheading good for other flowering perennials, such as black-eyed Susans and pink coneflowers? Will deadheading a crape myrtle bring out more blooming?
A Deadheading is the practice of removing fading flowers to prevent seed formation. This will encourage long-blooming plants to produce more flowers, and it is a useful technique for some summer blooming shrubs as well as perennials that flower on new growth and are not sensitive to the progressively shorter days of summer.
Tidying the dead flower heads on your butterfly bush will prolong bloom and spur the development of new flowers after the first wave has faded. An added benefit is that you are saving yourself some future weeding: butterfly bush has a reputation for sowing itself prodigiously throughout the garden.
I think that most gardeners rarely find the time to deadhead an entire shrub. The less laborious approach of shearing stems back after the first flowers have faded works quite well, and may result in larger panicles of flowers. Fertilize and water after shearing to boost growth.
The only shrubs that I would bother to shear or deadhead are those that are cut back severely every year, for example chaste tree, caryopteris and rose of Sharon. You may be able to encourage a longer bloom season in something like a crape myrtle, but all that deadheading and shearing will result in a shrub that is a tangle of small branches that produce undersized flower clusters.
Deadheading perennials is always a good idea, even if you are not rewarded with a longer bloom period. Energy otherwise spent in seed production can go to the plant's storage reserves or help to produce a healthy clump of foliage. Some, like purple coneflower, will continue to bloom if spent flowers are removed. Often, if the plant is vigorous, it will send up entirely new blooming stems from the crown.
I have a purple smoketree that was planted two years ago. In the summer, I made the mistake of fertilizing it. This produced branches that grew four to five feet but with none of the hazy flowers.
In late August, leaves on a couple of branches browned. I pruned them back but this was followed by the same demise on other branches. I was preparing to replace the smoketree with a small tree when I noticed new leaves near the main trunk. What happened? How do I prevent it from happening next season? And if it survives, how can I ensure blooming?
The fertilizer and a wet growing season predisposed your smoke tree to a bacterial disease caused by Pseudomonas syringae. It is also common on lilacs. Like many bacterial diseases of plants, it is most severe when the host plant has been severely pruned, fertilized heavily or watered frequently. The bacteria can proliferate rapidly on the rank, soft growth that results.
Smoketree can be cut back to the ground, and should be if bacterial blight is suspected. This can greatly reduce the chance for new infections as long as it is done in dry weather or in winter. But if you prune during wet weather, you may actually spread the disease on your pruning shears. It's a good idea to use bleach or alcohol to sanitize pruning implements between cuts if they have been used on a plant infected with a bacterial disease.
Smoketree blooms on new growth. It requires full sun for flower production and won't bloom much without it. You can still enjoy the striking foliage in partial shade. In fact, some gardeners treat it as a bold foliage element in perennial plantings by cutting it to the ground every winter. The dark-leaved cultivars (Royal Purple is widely available) are worth growing for their foliage alone.
Scott Aker is a horticulturist at the U.S. National Arboretum.
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