May 4, 1998
Choosing and Growing Rhododendrons in the Maritime Northwest
by Mary Robson, Area Extension Agent, WSU
Spring beauty in the maritime Northwest is defined by flowering rhododendrons and azaleas, the blooming highlight of many gardens and landscapes. We're now into the major bloom time of May, with many plants opening blossoms early after a warm March. How can gardeners get maximum beauty from the plant? And when the bloom is over, we see only the evergreen leaves. How can they be maintained so that the leaves 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-tan felting underneath the leaf, which shows up when the leaf is turned over. This under-leaf surface, called "indumentum" appears on many other rhododendrons, also. One called R. 'Sir Charles Lemon' has deep brown lower leaf color, giving the plant great beauty in fall landscapes. Check nurseries and garden centers for rhododendrons that enhance the year-round landscape. Select a likable leaf!
In selecting new rhododendrons, you may wish to choose plants that have some resistance to root weevil damage. Root weevil adults chomp on the edges of leaves, notching them with a pattern somewhat like that left by a dressmaker's pinking shears. Some which show good resistance to this company are 'P.J.J.' (pink) Dora Amateis, (white) and any of the Yakushimanum types (pinks and whites). Yellows 'Crest' and 'Odee Wright' also resist damage. 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. Check with your nursery for resistant cultivars.
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. But they do require light for best bloom and will not set buds well if the area is too dark. Morning sun in an east-facing situation is excellent for them if there is relatively little shade. Protect them from hot, exposed western or southwestern locations without tree canopy or other shade.
Both rhododendrons and azaleas 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. If construction has recently been completed, check for such conditions before planting. "Moist soils" mean those that have been amended with organic materials so that they will retain water, but won't leave water puddles or sogginess standing around the roots of the plants. If the native soils are clay, hardpan, or severely rocky consider planting in an amended raised bed. Add amendments such as compost to the entire planting bed, not just to the immediate hole where the new plant will go in. You don't want to create a planting hole that differs radically in texture from the surrounding soil.
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 to thrive. Occasionally, moving and resetting the depth of a rhododendron can improve its bloom if bloom has been scarce.
Place soaker hoses around the plants, or use an adequate irrigation system. Newly set plants of all types need water during their first two to three years while they are becoming established. To conserve water and reduce weeds, use a mulch. Cover the ground around and over the roots with two 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 truck of the plant. Use porous materials and keep the depth at two inches or less to avoid smothering the fine roots. 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. Grouping water-craving plants together in the garden can help with efficient watering. Older, established rhododendrons and azaleas require water during very dry spells, and shouldn't be allowed to wilt. However, big rhododendrons, if healthy, can sometimes survive and thrive with less summer watering than small ones. Many rhododendrons in municipal and park landscapes are watered sparingly during summer. Observe your plants carefully to assess their needs.
The buds for rhododendrons are set after they bloom, and a lack of water in summer can reduce the show the following year. Fertilize lightly with an acid-based fertilizer before bloom (in early spring) and after bloom. If the plants need pruning, during and after bloom is the ideal time to shape them.
Pruning a rhododendron can be daunting, but they respond well to spring pruning. Cut back to a leaf node, remove whole branches, or cut back to a "growth bump." Run your hand over a branch or the trunk, and you'll notice small, greenish lumps about the size of a pencil point, or larger. They will look slightly swollen and protrude from the bark. Each of them can potentially thrust out a new branch, so pruning back to one will allow the plant to fill out normally.
Do not prune rhododendrons in late summer, fall, or winter before bloom. The following spring's blooms will be pruned off. If this is no problem, prune away, anytime.