Commentary: Rhododendrons Mess Up
David J. Goforth
Horticulturist, North Carolina Cooperative Extension
Concord, North Carolina
Reprinted from the Independent Tribune, Concord-Kannapolis, NC
Talk show host Dr. Laura has written several books including one on the ten worst things women do to mess up their lives and the ten things men do to mess up their lives. Eventually she will get around to the ten worst things a rhododendron does to mess up its life. Meanwhile, here is a preview.
Rhododendrons should remember the mantra, never get your roots wet and hot at the same time.
Hot and wet roots may not be preventable in Cabarrus County [North Carolina], but it helps to choose a cooler site. In our landscapes the north and east sides of the house are coolest. The north side of a hill stays cooler. Overhead shade helps keep them cooler. At the very least, rhododendrons should choose a site where there are trees to the west. A rhododendron will die by the numbers on a south-facing slope in full sun.
Rhododendrons can also help themselves by choosing well drained sites. Getting on top of a small mound of soil should be a priority. They don't need any peat moss in our clay soils. Peat moss helps hold water and clay soil already does that too well. I do recommend mulch even though it may keep the soil moist. Mulch also keeps the soil cooler and keeps the weeds down as well as helps with the third worst thing rhododendrons do, which is try to live in high pH soil.
Rhododendrons prefer a pH around 5 to 5.5. In our soil gardeners eventually learn to start throwing lime around like candy or Mardi Gras beads. Most plants respond but rhododendrons and other plants in the heath family won't.
The wimpy root system means rhododendrons should lay off the high salt fertilizers. Buy a special rhododendron type fertilizer or follow soil test recommendations with cottonseed meal or bone meal.
Lacebugs under the leaves won't kill the plant but they are unsightly. Rhododendrons should keep a check on them in April and again in August. Mites have similar symptoms to lacebugs on top of the leaf. You can tell the difference by looking for the varnish spots on the back of the leaf. Mites are far less of a problem.
Leaf gall falls into the same category of unsightly but not fatal. Judicial pruning to make the plants look better is the only solution that makes sense.
Rhododendrons occasionally get a dieback in the top limbs that can be pruned out (Botryosphaeria dothidea). It looks a lot like the dieback caused by diseased roots that can't be pruned back (Phytophthora spp.). The simplest way to figure out which is which is to prune back the affected limbs. If pruning solves the problem it is the first one. If pruning doesn't solve the problem, a rhododendron can be sure it has the second one.
The black vine weevil is severe as you get closer to the native mountain habitat of rhododendrons. The black vine weevil makes notches in the leaf margin. Locally rhododendrons have a worse problem with the cranberry rootworm. This weevil chews holes in the leaf, some of which are crescent shaped. You never see them during daylight (except possibly during major eclipses). Sevin [carbaryl] will take care of both these insects.
At least three scale insects are reported on rhododendrons. Scale insects are easy to control when the crawlers are present. Use Sevin [carbaryl], Malathion or insecticidal soap when crawlers are present. If you are not sure the crawlers are present, use one of the horticultural oils.
Rhododendrons occasionally lose some of their bark. The major culprit in our area is European hornets that gather the bark to make a nest. In some areas flying squirrels have been suspected. By the time the rhododendrons notice the problem, it is too late to do anything.
Rhododendron should keep a check on their smaller twigs. Two borers are commonly reported on rhododendrons. Both azalea stem borer and dogwood twig borer can be pruned out when found in the upper part of the plant.
Gardeners sometimes get excited when they notice lichens on the stems of dying rhododendrons. Lichens are a gray or green fungal/algae combination that grows on anything including plants, fences and rocks. Lichens won't kill the plants. It doesn't kill fences or rocks either. Lichens often thrive as the rhododendron canopy thins from another problem.
Rhododendrons make lovely plants. I planted three at the farmer's market in 1992 and they performed well with almost no care until this summer. One developed a root problem after blooming this year, but I expect the other two will be back next year. As your landscape matures, include a few rhododendrons. Just be sure to help them out with site selection, planting and occasional checkups.