The Rhododendron family of flowering plants includes at least 1000 known species and cultivars. They are known as the "king of shrubs" and are native to Asia, India, Burma, Japan, and parts of Europe and North America. The genus Rhododendron belongs to the heath family, so they are related to blueberries, mountain laurel, heather and other ornamentals, including the azalea and Vireya, or tropical rhododendron.
Different Types for Different Climates
Wherever you live and garden, there is a rhododendron that will flourish in your yard. The Pacific Northwest is prime growing territory for rhododendrons, as is the entire Pacific coast as far south as San Francisco. A rhododendron variety called maddenii is adapted to warmer climates of Southern California, as are the tropical Vireyas, which are popular in Hawaii. A few very hardy varieties exist that are suited to growing in areas where winter temperatures drop to minus 20 degrees Fahrenheit. Your local chapter of the American Rhododendron Society can advise you about the best types for your area.
Rhododendrons prefer light soils with good drainage. But they also need moisture, especially during summer. If your soil is rich in organic materials such as rotted tree leaves of acidic plants such as pines or oaks, a rhododendron will fit in well. If your soil doesn't have these ingredients, dig in compost before you plant. Test your soil before you plant your rhododendron: the pH should be at 5.5. If you need to raise your soil's pH, add hydrated lime; to lower it, add sulfur. If your soil is clay, build a raised bed or mound and plant your rhododendron on top, above the poor soil that can cause the plant's roots to rot.
When you purchase a rhododendron, it will come in either a nursery pot or as a bareroot plant wrapped in burlap. Either way, include the soil that is in your bag or pot so you don't disturb the root system. If the root ball is dry, soak it thoroughly before planting. If the plant is rootbound, loosen the outer roots and cut some of them off if necessary to free the roots. In a cold climate, such as the Pacific Northwest, plant your rhododendron in early spring. In warmer climates, plant in the fall. Dig a planting hole that is slightly larger than your plant's root system, but be certain you don't plant it too deeply: leave the top of the root ball at the ground surface or above.
If the soil is right, your rhododendron does not require fertilizing. But if you use a mulch of wood chips or sawdust, the plant will need some nitrogen. You'll know if your rhododendron needs nitrogen because the leaves will begin to turn yellow and its growth will be poor. Ammonium sulfate is recommended in a situation such as this. In less fertile soil, apply a plant food made for acid-loving plants in the late winter or at the beginning of spring. Follow label instructions. If you live in a cold climate, do not give your rhododendron any nitrogen after the end of June.
To encourage flowering, fertilize with a plant food that is rich in phosphorus. Epsom salts can also help them to bloom: it provides magnesium. Only use Epsom salts if you see yellow areas between the leaf veins. You might need to add iron if you notice yellowing leaves---this is often the result of a soil with a pH that is too high for this plant. To correct this situation, you can add ferrous sulfate to the soil or spray your plant with chelated iron.
Pruning and Insect Control
Rhododendrons do not require heavy pruning, but you can cut your plant back to keep it a manageable size if needed. If you prune while it is blooming, you'll have some lovely bouquets. If you want to deadhead to remove spent flowers, it helps to keep the plant a nice shape and will encourage next year's flowers. You can cut back older rhododendrons quite a lot without harming them, but don't expect them to bloom again the following year.
The types of insects that attack rhododendrons vary in different geographic regions. In warm climates, the lace bug can be a problem: some growers report that they must use Malathion to rid their plants of this pest. The root weevil is also common on rhododendrons. Look for areas of bark that disappear, but many plants die before the damage is noted. Mites, aphids, the scale insect and thrips are insects that you can treat with insecticidal soap. Control caterpillars that eat leaves with Bacillus thuringiensis (Bt).