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JUNE 23, 1997

Home & Garden

Summer care for rhododendrons and azaleas

gardening Spring beauty in the maritime Northwest is defined by flowering rhododendrons and azaleas, the blooming highlight of many gardens and landscapes. Some do bloom in the summer months, but the greatest number have generally ceased bloom by mid-June. Now that most of the bloom is over, leaves offer most of the year-round interest on these plants. How can they be maintained so that the leaf looks healthy and attractive for the eleven months when the plants aren't blooming?
   Many rhododendrons, in particular, have stunning leaves. Those in the Yakushimanum group have attractive whitish felting underneath a green top surface. One rhododendron, called "Sir Charles Lemon," has deep brown lower leaf color, giving the plant great beauty in fall landscapes. The evergreen quality and structure of rhododendrons lends beauty and solidity to landscapes throughout the year.
   Azaleas and rhododendrons are in the same botanical family, and require similar growing conditions. Most of them do not thrive in constant bright sun, preferring filtered light such as that under open deciduous trees. They do require some light for best bloom and will not set buds well if the area is too dark. Morning sun is excellent for them.
   Both need moist, well-drained acid soil (pH 4.5 to 6.0). In general, the soil acidity isn't a problem, since our native soils tend to be within the desirable acidity range. Rhododendrons wouldn't do well in soils that have been heavily limed, or in areas where masonry rubble and slurry from concrete mixing might have been dumped into the ground.
   "Moist soils" mean those that have been amended with organic materials so that they will hold water well, but won't leave water puddles or sogginess standing around the roots of the plants. If the native soils are clay, lie over hardpan close to the surface, or are severely rocky, consider planting in an amended raised bed.
   Sometimes rhododendrons and azaleas fail to thrive because they have been planted too deep in the ground. When planting, after amending the soil, let the ground settle a bit and then place the plant with the root ball just slightly above the surface of the ground. ("Slightly" means less than 1/2 inch.) These plants have very fine, fibrous roots on the surface of the ground, and they need oxygen in the soil in order to thrive. Deep planting does not work.
   Place soaker hoses around the plants, or plant where summer irrigation will be feasible. Mulch with 2 inches, or less, of a coarse organic mulch (bark, pine needles, rough compost, or municipal compost mixtures such as Cedar Grove Compost, or Tagro). Don't let the mulch heap up against the trunk of the plant. And again, keep the mulch depth at 2 inches or less. Finally, don't cultivate over the roots of the plants (avoid tearing at the ground with weeding tools). The fine surface roots are easily damaged.
   Be sure to supply summer water in dry periods (not that this has been a problem yet this season). To have good flowering the following year, be sure to water during summer. The buds for rhododendrons are set after they bloom, and a lack of water in summer can reduce the show the following year. Rhododendrons appreciate a deep, thorough watering at least every two weeks in the driest parts of the summer.
   Fertilize lightly with an acid-based fertilizer before bloom (in early spring) and after bloom. If the plants need pruning, right now, in early June, is the ideal time to shape them.
   The most common problems in growing rhododendron-family plants come from not supplying enough water, having the plant in too much sun so the leaf scorches, planting in wet, soggy soils or failing to prune so that it becomes rangy. Other than these cultural problems, insects chewing on the leaves receives the most comments from gardeners. A group of root weevils of various types will chew on leaves, producing a characteristic notching that looks a little as if the plant has been attacked with pinking shears.
   If possible, choose plants that have some resistance to root weevil damage. Some which show good resistance are "P.J.M." (pink), Dora Amateis (white), and any of the Yakushimanum types (pinks and whites). Check with your nursery for others. The presence of chemicals in the leaves and the occurrence of a leaf edge that isn't receptive to being bitten keep these and others free of damage.
   A few techniques can also help to deal with the problem of root weevil chewing. The adult weevils (there are several species in this area) emerge from the ground after winter and feed on new leaves. Once the leaf has been damaged, it won't outgrow the notching. When they lay eggs, later in the summer, the larvae hatch and enter the soil to feed on roots. The larvae may damage roots or chew bark at the soil level, and can be particularly troublesome on plants in containers.
   Hand picking the adults off the plants is one way to reduce the population. They feed at night. Look after dusk with a flashlight, and drop the adults into soapy water. Any adults that are killed now will reduce later populations. Some people spread sheets under the plants and shake hard, then dispose of the fallen weevils.
   Wrapping the trunk of the plant with a non-toxic sticky substance such as Tanglefoot will sometimes help to catch the critters as they emerge from the soil and start up the plant for their leaf dinner. Spread the material on a strip of Visqueen, so it isn't directly on the bark. It should be firm enough so that beetles can't crawl under it. Other gardeners have also tried Teflon tape wraps, about 4 inches wide, which tend to be so slippery that the beetle pitches off down to the ground again. These two methods work only if the plants are not touching the ground or other bushes, which would provide alternative routes up.
   A biological control, the beneficial nematode, has been tried recently with some success in some locations. The nematodes are microscopic roundworms that infest and kill the weevil larvae in the ground. Make applications now that the soil has warmed; the product works only when the soil is both moist and above 55 degrees F., so this means applications in late spring and all the way up to mid-fall.
   The nematode moves on a film of water, so be sure that the soil does not dry out. Keep it evenly moist. These are living organisms, sold under several different trade names, ScanMask being one. Mix with water that is cool or lukewarm, under 95 degrees (they are killed by both cold and too much heat). Apply when they won't be exposed to direct sunlight or low humidity (early morning or late evening is best). They are not harmful to people or plants when used properly and they are effective against the root weevil larvae in the soil They don't affect the adults.
   Using these techniques will help to grow healthy, attractive rhododendrons without the use of broad-spectrum insecticides that can reduce beneficial organism populations in the garden. For more information about rhododendrons and azaleas, visit the display gardens at the Rhododendron Species Foundation, Federal Way, Washington. Their general information number is (253) 661-9377. Peak season of bloom is March through May, but garden classes and other events are held through summer. Gardens are open 11 a.m. to 4 p.m. daily but closed on Thursday and Friday.