While this article applies to Rhododendrons, 99% of it also applies to planting and care for Azaleas, as they are in the same genus, Rhododendron. While all azaleas are rhododendrons, not all rhododendrons are azaleas. Leaves and flower characteristics differ, but site requirements, soil, and care are for the most part identical. Naturally, as with anything in gardening, there are exceptions and always read the plant tag of the cultivar you’re purchasing to understand its requirements.
There’s good reason that Rhododendrons are one of the most popular landscaping plants in the world. Their long life, beautiful flowers, unique, mostly evergreen foliage, beautiful shape, and ease of care make them a reliable standout in any perennial garden. If you take care to plant a rhododendron properly and follow some basic rules of care, it can conceivably live in your garden for as long as you do. No matter where you live in North America, there is a variety of Rhododendron for your climate and soil.
For those who are new to Rhododendrons, you might believe that most look the same because you’ve only seen rhodies at your local big box store plant nursery. Nothing could be further from the truth. This long-lasting shrub appears in an enormous number of shapes, sizes, and flower-type, from very small shrublets to majestic trees. Some have scented flowers, some have small flowers, and some have flower clusters larger than a softball. One thing rhodies have in common is that their flowers bloom for 2-3 weeks of the year, which makes the foliage an important focal point for the 95% of the year it’s not blooming. Fortunately, most Rhododendrons (but not azaleas) are evergreen and have beautiful deep green leaves.
Not every Rhododendron has the same requirements, so it’s wise to know what your plant requires before planting. Do some research online or if you’re old school, with a book. At the very least read the plant tag tied to the stem (so many people fail to do this one simple thing, that I’m stunned). Knowing your rhododendron’s sun/shade requirements and its mature height are critical to finding the best spot in your garden where it will thrive. Failure to know the mature height won’t cause problems this season or even the next, but in 5-10 years, you’ll be faced with a plant that obscures your picture window or trying to transplant and then rehab a mature Rhododendron, which does not always go well.
Where to plant your Rhododendron
Dappled shade is optimal
It’s a common misconception that Rhododendrons only like deep shade. While this is true of some varieties, it’s not true for all. In fact, if you deprive certain varieties of sun, they’ll only produce a few flowers each year. In the wild, Rhododendrons are found at the edges of woodlands where they thrive in dappled shade. Find a similar setting in your home garden – afternoon or dappled shade, soil that drains well, and shelter from cold, drying winds.
Rhododendrons love early morning sun
The flower buds on spring-blooming rhodies require light in late summer and fall to stimulate flower bud development. If light is too low, the flower show will be minimal. Make sure the plant receives enough direct light, but not so much that the leaves burn. A good rule of thumb is to site it where it will receive direct sunlight until midday and then afternoon shade.
Give it a little shelter from wind
Persistent wind, especially in winter, can damage or desiccate the foliage. If you must plant it in a particularly windy location, consider building a windbreak around the plant for the coldest months – wrap burlap around it, but do not let the burlap contact the plant.
Avoid sidewalks and foundations
Do not plant a rhododendron next to a sidewalk or other cement structure, as the lime that leaches from the concrete creates an alkaline soil. Rhododendrons need just the opposite – an acidic or at least slightly acidic soil. If you must plant near a concrete structure, monitor the soil’s pH annually, and add appropriate amendments to lower the pH.
“Rhododendrons and azaleas are healthiest in light shade, especially under oaks and pines with the lower branches trimmed. Do not choose a location near maples, elms, ashes, or other trees with shallow competitive root systems. Some varieties of azaleas and rhododendrons may survive in full sun, but avoid planting them in a south, southwest, or west exposure, especially if heat and light are reflected on the plants from a nearby building. In dense shade, the plants tend to grow spindly and do not bloom profusely. Protection from winter’s cold drying winds is also desirable. Avoid areas with old builder’s debris, particularly mortar which can raise the soil pH above the desirable range… Do not plant rhododendrons under downspouts or at the edges of sidewalks and driveways. Do not plant in places where other rhododendrons have wilted and died, because the site may still be contaminated with disease organisms.”
Virginia Tech, Growing Rhododendrons and Azaleas in the Middle Atlantic States
Choosing a Rhododendron
Like roses, rhododendron varieties are vast and vary in flower color, shape, and mature size. It’s worth it to spend some time and search the American Rhododendron Society resource on rhododendrons before you make a decision. Your local weather conditions are also important, as are the amount of sun the plant will receive, wind at the plant site, soil type, and soil drainage.
Consider the foliage, not just the flower
The foliage of Rhododendrons varies greatly. Some have large ovate leaves, blue leaves, large leaves, small finger-like leaves, and most varieties are evergreen. Some varieties are covered with indumentum – a hairy covering of brown, tan, or silver on the underside of the leaves. This is strikingly beautiful as the leaves move in a breeze. On some varieties, new foliage is a different color than old foliage, which makes for an interesting display.
Rhododendron flower bud breaking (Photo courtesy of Todd Heft)
Optimizing garden soil for Rhododendrons
Soil pH is critical
Your garden soil’s drainage and pH (acidity level) are very important for the health of your Rhododendron. Alkalinity is a rhododendron’s worst enemy, turning the leaves yellow between green veins and giving it an overall unhealthy appearance. A rhodie prefers a pH between 4.5 and 6.0 (alkaline soil is in the 7.0+ range). A soil test should be your guide to determine your pH level. If you need to lower the pH, use wettable sulfur or ferrous sulfate. NEVER use ammonium sulfate (a common treatment for hydrangeas), as this chemical is toxic to the fine surface roots of Rhododendrons. Also, don’t try and lower the pH quickly or you may shock the plant – it should be lowered by .5 each season until the target pH is achieved. You should see the shrub improve significantly for every .5 the soil pH is lowered.
Soil needs to drain quickly
How do you know if your soil drains sufficiently?
Dig a small hole about 10-12″ deep. Fill it with water. If the water does not drain within 15 minutes, you have poorly draining soil and you’ll need to add organic material to aerate it. It’s important that you dig the test hole 12″ deep, as some soils have an upper layer of several inches that appears to drain, but beneath it is hardpan (tightly packed clay), which drains extremely slowly. Planting your rhodie in hardpan will lead to a struggle to maintain its health. An excellent addition to clay soil for Rhodies is coarse peat moss, as it breaks up the clay particles to introduce air into the soil and also lowers the pH over the long term. If your soil is impossibly alkaline, and a raised bed is out of the question, try growing your Rhodie in a container whose soil you can easily manage.
A Rhododendron requires sufficient soil aeration – the soil needs to drain quickly, as their roots are very fine and will “drown” in muddy soil. At planting time, make sure that roughly half of the surrounding soil contains fresh organic matter and layer compost over the root zone annually. The ARS recommends sphagnum peat moss, pine or fir bark fines, compost, and aged, chopped leaves should be worked into the soil to a depth of about 12“(with the exception of leaves from Walnut trees, which are toxic to rhododendrons and azaleas). The compost will find its way into the soil over the growing season, assuring a constant addition of organic material to the soil around the plant. If your landscape soil is heavy clay or if you’re planting in a low-lying area prone to waterlogging, consider planting your Rhodie in a raised bed so you can easily manage the soil.
“Raised beds should be built up with 12 to 18 inches of organic material such as oak leaf mold, other shredded acid type compost, pine bark, coarse peat moss, or decomposed pine needles. In heavy clay soil [rhododendrons] should at least be placed on top of the ground and the root balls covered with some of the above organic materials. If the garden topsoil is not heavy clay but is a loose loam or sandy loam containing much humus, the plant can be set in a 20-inch deep hole with at least the top inch or two of root ball above the soil surface. The bottom of the hole and area around the root ball should be filled with a mixture of equal parts loam and some of the above-mentioned organic materials. “
Virginia Tech, Growing Rhododendrons and Azaleas in the Middle Atlantic States
How much water does your rhododendron need?
Although too much water in the soil is harmful to Rhododendrons, not enough water and drought are harmful as well. But if you choose a proper site and plant correctly, you should have few or no problems after your plant is established.
Shallow root systems makes Rhododendrons sensitive to heat and drought
Rhododendron roots are very shallow and grow like a spider web close to the soil surface. They have no taproot and as a result, are very prone to drought. Make sure your rhodie receives consistent moisture, especially the first year after planting when the root system is developing. The rule of thumb is to keep the soil damp but never soggy and make sure it receives 1″ of water each week in the absence of rainfall. Add a 2-3″ layer of organic mulch around the root zone to retain moisture, control weeds, and insulate the roots from heat extremes. If temps go above 95, Rhododendron leaves appreciate a misting of water to prevent desiccation.
In the wild, Rhododendrons are found on the edges of woodlands like this Catawba variety growing in the Blue Ridge Mountains of Virginia. (Photo courtesy of Todd Heft)
Planting Rhododendrons
Rhododendrons can be planted almost any time of year except winter. The best time to plant is early spring, with late fall being a close second choice. Fall planting is the preferred time if you live in a hot area because the warm soil allows the roots to establish before winter sets in.
Never plant the rootball below the level of the surrounding soil – it should be planted several inches above, to allow for adequate drainage around the roots. Planting a Rhododendron too deep can eventually kill the plant.
Loosen and expose the roots
After removing your new Rhododendron from its container, shake loose the potting medium and expose as much of the root system as possible, being careful not to damage the roots. With your fingers, gently loosen the roots and untangle any you see. If the plant is rootbound (roots wrapping around in the pot), thoroughly loosen them with your fingers and prune some of the outer, entangled roots. This will stimulate new root growth after planting and allow water and nutrients to penetrate the root mass. If the plant has been field grown and is delivered wrapped in burlap, completely remove the burlap before planting, and put it aside – do not plant the Rhodie with the burlap wrap in the hole and especially not still surrounding the rootball. Even though the burlap may slowly deteriorate – it is biodegradable – it can restrict root growth into the surrounding soil and take years to degrade.
Fertilizing a Rhododendron
Rhododendrons rarely suffer from fertilization problems, so check the pH first
In well-maintained soils, Rhododendrons need little to no fertilization after they’re established. But if your rhodie is exhibiting symptoms that lead you to believe it may need to be fertilized, chances are it’s a pH problem, so check that first (Rhododendrons prefer a pH of 4.5-6.0). Run a pH test of your soil and adjust it lower if necessary with ferrous sulfate or wettable sulfur.
Too much water may damage the roots
Rhodies may also struggle if their roots are perpetually wet. To help the soil drain easily, add 2-3″ of compost around the root zone each spring and fall. A layer of wood chip mulch is also helpful to insulate the roots.
Only use these fertilizers if necessary
If you’ve followed the above recommendations and your plant is exhibiting weak signs of growth, an organic nitrogen fertilizer may be useful for a slow feed. Use cottonseed meal, canola meal, fish meal, or blood meal. Pelletized high nitrogen fertilizers are not recommended as they may “burn” the fine roots.
Rhododendrons disease and insect problems
One of the most alarming signs to a new Rhododendron owner is a sudden leaf drop when the plant appears to shed many of its leaves. The good news is the cause of leaf drop is usually conditions and not disease. According to the Royal Horticultural Society, “Leaf drop can occur following a period of drought and is usually preceded by drooping and rolling of the leaves. The oldest leaves are the first to drop and higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended periods of waterlogging. As with all evergreen shrubs, each year a proportion of the foliage (mainly older leaves) are shed in spring and summer. This is normal and not a cause for concern.”
Rhododendrons are troubled by few insects, but are susceptible to fungal diseases, especially in poorly draining soils. For information on Rhododendron diseases see the diseases page from the American Rhododendron Society.
Todd Heft
Author
Todd Heft is a lifelong gardener and the publisher of Big Blog of Gardening. He lives in the Lehigh Valley, PA with his wife who cooks amazing things with the organic fruits, vegetables, and herbs he grows. When he isn't writing or reading about organic gardening, he's gardening. His book, Homegrown Tomatoes: The Step-By-Step Guide To Growing Delicious Organic Tomatoes In Your Garden is available on Amazon.
