While they are among the most beautiful of the spring-blooming plants, the rhododendrons are also among the most sensitive to temperatures. They curl their leaves as a protection against the extreme cold, for example, and they also respond promptly when temperatures rise.
If you notice a brown or gray leaf tip this spring, it is a sign of winter damage. It points to the fact that this evergreen should be in a sheltered area. Rhododendrons thrive best in a location that is sunny for half the day, with protection from the hot afternoon sun. They do particularly well if planted against a backdrop of pine trees, which filter the sunlight and provide a constant mulch of needles. This layer of mulch is an important asset.
Moisture levels are important in both summer and winter. Rhododendrons, like their relatives, the azaleas, are surface feeders and should not be planted deep. Any of these plants that grow in light, sandy soil need to be watered thoroughly during dry spells.
When planting a rhododendron, dig the hole two or three times the size of the root ball. Put a good soil mixture at the bottom of the hole and set the plants no deeper than they grew in the woods or nursery. Do not mound up the soil, but make a basin around the base into which the feeder roots will spread.
A good planting mixture is one part decayed oak or other hardwood sawdust, leaf mold, or acid peat to two parts loam. Hardwood sawdust contains tannin, which is beneficial to rhododendrons. Also, all of these materials retain moisture in the soil, making a cool, moist environment which the roots enjoy.
There are two periods when these plants enjoy fertilizing - in the fall, when the plant is dormant and the ground not yet frozen; and in the early spring about a month before flowering. Overfertilization, however, can damage the shrub , which does best with an ammonium-nitrogen rather than a nitrate-nitrogen source.
After the plants have flowered, it is wise to remove the blooms before the seeds form again. Snap off only the bloom cluster. Removing too much will mean no flowers the next season. This is a good time for pruning, too. Take out the weak and dead branches and cut the old ones back lightly to make a well-shaped shrub.
Among insect pests to watch for are the taxus weevil, which makes quarter-inch, semicircular holes in the leaf edges. It can be controlled with an insecticide such as Orethene. Beware, too, the rhododendron borer. When the borer is present, the leaves will wilt and show a grayish, off-white cast. In determining the cause of a wilted branch, look first for physical injury, then for evidence of the borer or weevil, and last for wilt infection.
To protect your rhododendrons through the winter, make sure they have a good layer of mulch around the roots to help them retain moisture, as well as burlap wind shields to protect the leaves.