Q Last season, the leaves on my 10-year-old Stellar Pink dogwood were one-third their normal size. There were also some dead twigs on the tree. Could the cause be the perennials I grow at the base of the tree, though I have had them there for years? I put two to three inches of leaf mulch on the bed every March.
A My first thought was the leaf mulch: If it were of poor quality and contained things such things as vinegar or alcohol, it might have caused the stunting. But your perennials would have been affected too.
Dogwoods may also have small leaves if they are infected with powdery mildew. If this were the case, you would notice a white film on the leaves in addition to leaf size. Stellar Pink is more susceptible to dogwood powdery mildew than some of the other Rutgers hybrids, so this is worth investigating. Mildew is treated by spraying the foliage with a 1 percent solution of horticultural oil in spring when the leaves are fully expanded. Continue to treat the plant every 10 to 14 days throughout the growing season.
It's also a possibility that the tree has a girdling root, especially if the tree was first grown in a pot. Gently dig around the base of the tree, and if it appears that a root is strangling the base of the trunk, cut it away. Prune the dead twigs off in spring when the new growth begins. You can tell at that point what is living and what is dead. Keep the mulch away from the base of the tree.
I have an azalea bush, 12 years old, that has never bloomed. It is planted next to two older azaleas, identical in appearance to the non-bloomer, and they flower profusely each year. Any advice?
I know of no reason why an azalea bush would grow well for more than a decade and never bloom. The only conceivable way you could eliminate flowering is by pruning it every year after the flower buds are set. Azaleas should never be sheared for this reason, and any pruning should be done just as the flowers are fading.
You may also want to take a closer look. Some shrubs that do not have conspicuous flowers are close enough to azaleas in leaf texture and shape to fool the untrained eye. You may be dealing with a euonymus, a Japanese holly or some other shrub that is grown primarily for its foliage.
I have a rhododendron that in recent years has dropped its leaves. The only leaves that remain are the fresh ones. What's happening?
As long as the tip growth is normal and the buds are able to produce several inches of new growth every year, there is nothing to be concerned about. It may have lost more leaves than normal because of the harsh winter we experienced last year. Established rhododendrons need nothing more than an infrequent soaking during the worst droughts. Because they have very shallow roots that are sensitive to sudden changes in soil conditions, I don't recommend using lots of mulch or fertilizer on them. A thin layer of chopped leaves applied in the fall or spring is sufficient to conserve moisture and provide a slow release of nutrients as they decay.
Scott Aker is a horticulturist at the U.S. National Arboretum.
