You know its spring when the azaleas bloom. Once the snow melts, these acid-loving shrubs are covered with blossoms ranging from pink to purple to red to orange to yellow or white. Azaleas are in the genus Rhododendron, with evergreen azaleas in the subgenus Tsutsusi and deciduous azaleas in the subgenus Pentanthera, according to the Azalea Society of America. Most azalea varieties grow best in U.S. hardiness zones 7 to 9, but Illinois ranges from zones 4b to 6b. Despite that, you can grow azaleas in Illinois with the proper care and the right choice of cultivars.
Choose a cold-hardy azalea cultivar. The Northern Lights series is very popular with northern gardeners because it is hardy to -30 degrees F. Most of the cultivars end in “Lights,” such as Mandarin Lights, Orchid Lights, Golden Lights and White Lights, according to David Robson, University of Illinois Extension Service educator.
Other cold-hardy cultivars include "Knapp Hill" hybrids. These deciduous azaleas are hardy in zones 5 to 7. "P.J.M." hybrids are evergreen and hardy in zones 5 to 7.
Choose a site that will protect azaleas from Illinois' cold winter wind, using buildings or slopes as barriers. You also can plant evergreen shrubs or trees to the south or west of your azaleas for a wind block.
While azaleas will grow in the shade, they prefer filtered sunlight or full sun only in the morning. Strong afternoon sun will bleach the azalea's flowers.
Azalea roots cannot penetrate water-logged, rocky or heavy soils, and they need soil with a pH between 5.0 and 5.5. If your soil is too alkaline, amend it with peat or sphagnum moss. When choosing your location, avoid planting near shallow-rooted trees like elm, maple or ash that will compete with your shrubs. Plant your shrubs 3 to 4 feet apart.
Mulch your azaleas heavily. They are shallow rooted and need the protection to conserve moisture and minimize injury from snow and frost. The best mulches are acidic, such as partly decomposed oak leaves or pine needles. You also can use hardwood chips or aged sawdust. Keep the mulch lower on the stems during the spring and summer. If you use sawdust or hardwood chips, make the mulch about 2 inches deep. Pine needles or oak leaves should be 4 to 6 inches deep.
Use fertilizer specifically for azaleas. General-purpose fertilizer will not give your plants the proper nutrients. Fertilize after the last frost, ideally in May, but never after July 1, as it may spur tender growth that would be killed during the winter.
Ease up on watering your azaleas in September to harden them off for the winter. If you are in drought conditions, or the plants are beginning to wilt, water lightly and again after the first frost.