There are more than 900 species of azaleas and rhododendrons
Q. How do I care for azaleas and rhododendrons?
A. Both azaleas and rhododendrons belong to the same genus: Rhododendron. Although there are more than 900 species and cultivars of azaleas and rhododendrons, only a small number can be successfully grown in Illinois.
To have success, you should choose cold hardy cultivars, select suitable planting sites, and pay attention to recommended planting procedures and plant care.
Azaleas like to be located where there is partial to full sun, while rhododendrons prefer partially shaded sites. Two winter enemies of these shrubs are cold winds and bright sun, which can cause leaf burn if they are exposed in unprotected sites. If your existing plants show winter damage, provide some protection.
Both azaleas and rhododendrons require well drained, acidic soils with lots of organic matter. Because the pH of most garden soils in the Chicago area fall in the neutral to alkaline range (7 to 7.5), you will need to lower the soil pH to between 4.0 to 5.5. Get a soil test to determine the acidity of your soil.
The results of the test will help you determine how much you need to lower the pH. If preparing a bed for planting azaleas or rhododendrons, you can lower pH by applying agricultural sulfur or iron sulfate to the soil surface. Generally, about 1¼ cups of iron sulfate or 2½ pounds of agricultural sulfate per 100 square feet will lower the pH one unit.
An alternate method if you are planting one or two shrubs is to use specific organic materials to assist in lowering pH. Prepare a hole twice the diameter of the shrub container, and use a backfill consisting of half moistened sphagnum peat moss and half soil. Sphagnum moss has a pH of 3.0 to 4.5. In addition to providing acidity, it will provide a good source of organic matter for the growing environment.
Maintaining soil acidity will help ensure good growth. After planting, use materials such as wood chips, pine needles or shredded oak leaves, which are mildly acidic, to help maintain the desired pH and provide organic matter.
A mulch cover that is 2 to 3 inches thick is helpful. Applying a dose of water soluble acidic fertilizers during the growing season can also help with your soil chemistry. Do not fertilize after the first of July to avoid forcing new growth that can be killed in winter.
Because these shrubs are shallow rooted, don't let them become waterlogged as this contributes to root rot, which is the No. 1 problem of azaleas and rhododendrons. Root rot produces stunted yellow leaves that turn brown and die or wilt and turn inward. Once symptoms of root rot appear, it may be too late to save the shrub.
If you have a poorly drained site, you can prevent their death by building beams or raised beds, which will promote good drainage. Remember the adage that an ounce of prevention is worth a pound of cure, which in this case would mean replacement of dead plants.
- Mary Moisand
• Provided by Master Gardeners through the Master Gardener Answer Desk, Friendship Park Conservatory, Des Plaines, and University of Illinois Extension, North Cook Branch Office, Arlington Heights. Call (847) 298-3502 on Wednesdays or email northcookmg@gmail.com. Visit web.extension.illinois.edu/mg.