The ordinary rhododendron is popular with gardeners across the US for its showy clusters of trumpet shaped flowers that come in a huge array of colors from stark white to burgundies, yellows, and peach. Tough glossy leaves provide the perfect backdrop for the gaudy spring bouquets that bloom from early through mid spring.
Rhododendron is a vast and varied group, however, and includes not only the the popular evergreen, but multiple subgenera such as the azalea as well.
There are dozens of varieties of rhododendrons that grow in many portions of the US, excluding the desert Southwest, Southern California, Arizona, New Mexico, Texas, and a fair number of the Rocky Mountain states.
Rhododendrons like loamy, acidic soil and mild, humid climates. They do particularly well in temperate coastal regions like the Pacific Northwest and the South. Evenly moist, well-drained, rich soil promotes plant health and vigor. Established plants can get along quite nicely with minimal watering and feeding.
Acid soils should have a pH of about 5–5.5. Test your soil yourself with a kit from your local nursery, or contact your county extension service for information about soil testing. Once you know what the pH level is you can amend the soil accordingly.
For very clay-ey or sandy soils, add compost to break up soil structure and promote better drainage and water retention. (You want to hold enough water to make the plant happy, but not so much that it hates having its "feet wet".) Sulphur or iron sulfate is commonly used to change the pH. Dig it into the beds and work the soil well. To add acidity organically, use pine needles or other coniferous detritus. Check pH every year to make sure it's correct.
In more extreme climates like the Midwest and Northeast, choosing an appropriate variety and siting your rhodies is of great importance. In these less auspicious environments, plant to mitigate extreme temperature changes especially in spring and fall. Also, protect plants from drying summer or winter winds. Windbreaks of taller evergreen shrubs or trees can provide protection and frame your rhodies for spectacular spring color.
Early morning sun or filtered bright light provides optimum conditions for photosynthesis and bloom development. Rhodies are often under story plants in their native habitat. They tolerate some shade, especially from hot afternoon sun, but don't do well in deep shade. It takes a while, but this is one way you can successfully kill a rhodie in Western Oregon.
Plant rhododendrons where they have plenty of room to grow. Some varieties become quite tall—up to 15 feet. They have lovely round forms if allowed enough room, but overlarge types too closely planted to a house often become oddly shaped and need prodigious amounts of pruning.
Rhododendrons are shallow rooted so should be attentively watered their first year. Until they become established, especially if your area is subject to prolonged dry spells, this can mean the difference between success and the tragic demise of your plant. According to the American Rhododendron Society, maintaining moisture in the root ball is critical to the subsequent health of the plant. They advise setting a trickling hose once a week to keep the root ball damp.
Plant or transplant into a hole that is wide, but only as deep as the rootball. Back cover and tamp down well. Mulch and water regularly until the plant gets established. Planting a little shallowly is better than planting too deeply.
A rhododendron's root system is close to the surface of the soil. Keep as many roots as you can when transplanting. To minimize stress, you may need to cut back the plants to less than 12 inches to foster root development.
Select varieties that are appropriate to the location. If you want foundation plantings, choose a rhododendron that tops out at 4–5 feet. If the plant naturally tops out at 10–12 feet you'll do constant pruning to keep it compact. The American Rhododendron Society has a page on their website that can steer you toward the right sized varieties that are best suited to your locale.
With appropriate placement, your rhododendron needs only annual tidying up to keep it looking healthy and attractive. Pick off spent blooms after they are finished in the late spring by snapping off just the flower cluster. This is also the right time to prune if it's needed. Next year's blooms are set on the tips, so pruning later in the season can result in loss of next year's flowers.
Take out dead or diseased branches when ever you notice them. It won't affect the plant. If it's necessary to prune drastically, it's possible to take the entire plant to the ground and start over. In fact, in some climates like the marine Pacific Northwest, it's hard to kill them.
Rhododendrons and azaleas are dainty feeders. If the soil is right, additional fertilizer may not be needed. Where soil is more sandy or less rich, fertilizing may be required. If so, buy a good quality fertilizer formulated specifically for rhododendrons and azaleas, or use a little cotton seed meal. Go lightly with it in the spring; avoid fertilizing after June, especially if you live in an area with very cold winters. A late application creates rapid growth during the summer, which can be easily annihilated during a killing freeze. With rhodies, less is more.
Mulch every year with compost if possible. Many gardeners use decomposed leaf mold, pine needles, or bark chips to good effect. This enhances drainage but holds moisture. If using a heavy carbon mulch like wood chips, you may find it necessary to give a dose of fertilizer (aluminum sulfate) to prevent iron chlorosis, which is caused by lack of nitrogen in the soil.
Rhododendrons are pretty hardy and not typically subject to bugs or diseases, especially when growing in conducive climates. Plants taxed by poor soil, wind, or temperature extremes are susceptible to a number of complaints including scorching and iron chlorosis. The easiest way to diagnose the problem is to take leaf samples or a digital photo to your nursery or extension service for a solution. They can recommend an appropriate treatment that can succeed in your area.
A professional lawn service can help with either large annual cleanup or monthly chores. To find a qualified service in your area, ContractorNexus can help.