Henry Thoreau wrote, "The splendid rhodora now sets the swamp afire with its masses of rich color." It's no wonder rhododendrons have always been one of our most popular shrubs. Versatile, vibrant, with incredible flowerheads and often evergreen during the winter, it is a plant for all seasons. One can use these beautiful plants for energy-saving planting as well as the plainer ones.
The rhododendron family is large, with over 2000 listed varieties and hybrids that include rhododendrons and azaleas. They grow in every temperate country in the world, from Lapland to Australia, with twenty-seven species in North America alone. The ones we mostly grow in our gardens have come mainly from the Orient -- some were probably cultivated in Chinese gardens centuries ago.
In many parts of the world, rhododendrons are hard to grow and so, in that perverse human way, they are more prized. They can be made to grow in most of the United States, except for hot regions that have no cool winter. The East Coast and the Pacific Northwest are blessed with just the right soil and climate, which makes them easy to grow.
In other regions they often need protection, so special microenvironments have to be created for their survival. Some winters might be too cold for most varieties. Wind protection is usually necessary. The summers may be too hot, so open shade may have to be established. It is important that specific varieties be chosen for each region. Special soil may have to be created for the rhododendron as well. In areas with hard alkaline ground water, acid rainwater may have to be used.
If it gets too complicated, it might not be worth the effort to try to get them to grow outdoors, because they are easy to grow as potted plants, in special soil -- especially azaleas. In places with naturally acid soil, adequate rainfall and humidity and a temperate climate where they will happily thrive, however, rhododendrons are a most glorious shrub.
It is not necessary to prune a rhododendron, but proper pruning will help shape the shrub.
Rhododendron Requirements Rhododendrons must have acid soil, 4.5 to 6.5 pH. It must also be friable and soft, so the very fine roots, which grow quite near to the surface, can push through it. A heavy clay soil can be improved with large quantities of acid peat moss, bog soil, rotted oak leaves, pine needles, or acid woodland soil. Or a new bed about 2 feet deep may be excavated and proper soil brought in.
To keep it acid, don't add lime, gypsum, wood ashes, bone meal or calcium carbonate to the soil. Also beware of new concrete as rain can splash the calcium off of stucco or cement and into the soil, which will make it alkaline.
Most rhododendrons prefer a dappled shade, but they will grow in sun, as long as they have enough water. If the shade is too heavy, they will not flower well. Some varieties can stand more sun than others, but for all rhododendrons water is essential. The shallow roots easily dry out, which is why a mulch is always recommended.
It is essential to water new plants with 5 gallons of water once a week for the first year in the spring, summer, and early fall. Rainwater does not sufficiently moisten the soil. The second and third year they are still fragile and so must continue to be watched carefully and given supplemental water, because it takes at least that long for the roots to become established. You can tell when they are dry, for the leaves droop like limp hands.
Once established, rhododendrons need little care, except for the occasional soakings between June and September in dry years. Interestingly a good deep soaking in August makes them more winter hardy, but too much water after mid-September promotes winter damage. The plants need to get good water in late summer, to make as much carbohydrate as possible and to have time to harden off before winter.
Although rhododendrons need a moist root run, they will rot and die if they sit in standing water. For this reason, it is a good idea to plant the crown an inch or so above the ground level and slope the soil into a saucer to hold the 5 gallons of water for the first year.