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The Homeowners Column
Amazing Azaleas and Riotous Rhododendrons
Extension Educator, Horticulture
Landscapes are now dotted with vibrant splashes of pink, salmon and orange; the characteristic colors of azaleas and rhododendrons. With so many obviously happy plants, do they deserve their reputation as difficult divas?
First, let's discuss their family tree. Although we often think of them as different plant groups, azaleas and rhododendrons belong to the same genus - Rhododendron. Azaleas are mostly deciduous and rhododendrons are mostly evergreen. Azalea flowers are usually shaped like a funnel where rhododendrons are bell-shaped.
Are azaleas and rhododendrons difficult divas or just misunderstood darlings? They can be grown successfully in Illinois if we pay attention to a few details: soil preparation; plant care; and cultivar selection.
Since rhodies grow naturally in wooded areas in well drained acidic soils with lots of organic matter, we should mimic these conditions for best plant growth.
Ideally beds intended for rhodies should be prepared the season before planting. They are best grown where they will receive afternoon shade. Get a soil test to determine the acidity and how much sulfur is needed to lower the pH to 4.0 to 5.5. If soil pH is not corrected, the leaves may appear yellow, especially between veins. A regular dose of water soluble acidic fertilizers during the growing season will also help. Before planting add organic matter such as leaf compost or peat moss to the top twelve inches of soil.
Azaleas and rhododendrons are shallow rooted. Mulch with wood chips and water regularly. Root rot is their number one problem. They like to be moist, but not waterlogged. Symptoms of root rot include stunted yellow leaves that eventually brown and die. Leaves may also wilt and roll inward. Although fungicide soil drenches may help, root rot is systemic and travels through the plant. By the time we see symptoms, it may be too late for the distressed plant. Prevention is a better solution.
Azaleas can also be reluctant to grow out into the surrounding soil once they are planted. If possible find field grown azaleas or be sure plants are not potbound with many circling roots. To help pot grown azaleas grow out into the surrounding soil, score the root ball with a sharp knife before planting by running the knife down the root ball in several places to encourage new root growth.
To encourage good drainage plant azaleas and rhododendrons on a mound of soil so the top surface is a few inches above the surrounding soil level. Any pruning should be done immediately after bloom.
Worthy azaleas and rhododendrons include:
- Catawba Rhododendron: large flower clusters; several color selections available; 6-12 feet tall shrubs with leathery, evergreen leaves.
- Korean Rhododendron: 3-6 feet tall; cultivars include 'Cornell Pink'- pink flowers and 'Crater's Edge' - dark red buds opening to lavender pink flowers.
- 'Girard' Hybrids: large group of azaleas includes both deciduous and evergreen cultivars; vary in flower color and winter hardiness.
- 'Knapp Hill' Hybrids: deciduous azaleas; result of crossing several species of Rhododendron; includes 'Knapp Hill', 'Exbury', 'Slocock' and 'Ilam' Hybrids; flower color depends on cultivar; mildew can be a problem in planting areas with poor air circulation.
- 'Northern Lights' Hybrids: deciduous azaleas; developed at University of Minnesota; very cold hardy; 8-10 feet tall; wide variety of flower colors; cultivars include 'Lemon Lights', 'Northern Highlights', 'Pink Lights', 'Rosy Lights' and 'Orchid Lights'; magnificent fragrance; maroon fall color.
- 'P.J.M.' Hybrids: evergreen rhododendrons; compact size of 3-6 feet tall; shades of lavender pink to mauve flowers; leaves purplish in winter; reportedly not as finicky about soil pH.
For more information about these and other shrubs, check out new U of I Extension shrub selector website .
Or contact the American Rhododendron Society .