David Robson Extension Educator, Springfield Extension Center, University of Illinois
Yard & Garden
Azaleas and Rhododendrons Done Right
Picking the right ornamental shrub for your garden
Growing certain plants in Illinois can be difficult, depending on your location. Our friends in the southern tip can do well with many things that our northern counterparts only dream about. On the other hand, Douglas fir, blue spruce and most birches do better in the northern part of the state.
Which only goes to prove that summer’s heat can be just as positive or negative as winter’s cold, depending on where you’re located.
And that brings us to azaleas and rhododendrons, the classic spring flowering shrubs noted for their masses of purple, white, orange or pink colored blossoms. You can find plants growing throughout the state, but maybe not the same plant. It’s sort of like the hydrangea issue.
Technically, all azaleas are rhododendrons, but not all rhododendrons are azaleas. That’s because, technically, rhododendron is the scientific name of the plant.
But if we still want to get technical, there is a difference between the common azalea and common rhododendron.
For the most part, but not always, rhododendrons tend to have shiny, glossy, leathery green leaves that are somewhat evergreen, at least staying on through most winters. Azaleas, on the other hand, tend to be more of an olive green, fuzzy and deciduous, turning somewhere between a yellow and burgundy wine color in the fall before dropping off the plant.
Again, for the most part. We have deciduous rhododendron and evergreen azaleas.
What really separates the two is the number of stamens in the flowers – rhododendrons have 10 and azaleas have five.
Stamens, the male parts of the flowers, are easy to identify as rhododendrons and azaleas only have one pistil, or female part.
By and large, azaleas tend to be hardier than most rhododendrons, probably because there’s less winter injury to azaleas because they are void of leaves.
There are literally pages and pages of rhododendron and azalea cultivars, with flower colors from blood red to red to pink to orange to yellow to cream and white.
There seems to be more emphasis on azaleas at garden centers and nurseries, probably due to their wider range of hardiness, portability and seemingly less transplant problems, though the latter is more of an urban myth.
Of all the azalea series, probably the Northern Lights series sees the most action. The Northern Lights group is noted for being cold-hardy to -30 degrees F., which means you can practically guarantee bloom year after year in Illinois. Most of the cultivars end in “Lights” such as Mandarin Lights, Orchid Lights, Golden Lights and White Lights.
Of the rhododendrons, the P.J.M. is the hardiest, with pink-purple flowers blooming even into Canada. There are all sorts of various crosses and cultivars related to the P.J.M. that are cold hardy.
Success with rhododendrons and azaleas comes down to two things – the correct hardiness and correct planting.
Rhododendrons and azaleas like an acid soil that is well drained. Give them those two criteria and they will go to town.
It’s easy to acidify a soil. Just add sulfur-related compounds.
Root rots occur with regularity on azaleas and rhododendrons and it’s related to a heavy soil that stays wet and doesn’t allow oxygen around the roots. Unfortunately, the symptoms cause homeowners to do the opposite of what’s needed.
Root rots will cause the leaves to fold under and look wilted. So what do gardeners do? They water more, which just makes the situation worse.
To avoid all this, consider digging the hole at least two to three times as wide as the root ball, but only one-half deep. Plant the specimen, bringing in soil from another part of the garden to gradually slope away from the stem, creating a little mound or berm. This way, it becomes more difficult, but not impossible, to over water.
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: 217-782-6515.
© 2008 Illinois Country Living Magazine.