By Matt Gibson and Erin Marissa Russell
Rhododendrons are often mentioned in the same breath as azaleas, as they are both flowering shrubs from the Rhododendron genus. Both shrubs are quite popular additions to spring flower gardens due to their stress-free care requirements as well as their gorgeous clusters of fragrant, showy blossoms that appear in late spring. Rhododendron’s flowers come in many different shades, including red, white, yellow, pink, and purple. The colorful clusters are framed by the shrub’s large, dark-green, leathery foliage that provides a green backdrop all throughout the winter.
Hardy to USDA hardiness zones three through nine (they grow best in zones five through eight), gardeners can enjoy rhododendrons all year long. Though most cultivars are spring bloomers, summer-blooming rhododendron varieties are also available. There are literally thousands of rhododendron varieties available, some are small ground cover plants that spread out to fill in empty spaces in your garden beds, stretching out while staying low to the ground. Some rhododendron cultivars are medium sized flowering plants, which send up flower stalks one to two feet tall. There are even large, tree-sized rhododendron cultivars that can grow up to 25 feet tall.
Rhododendrons enjoy areas with lots of rain and moist summer weather. Rhododendrons are more selective in their preferred growing environments than azaleas, requiring moderate temperatures and more stability in terms of climate.
When deciding which rhododendron varieties to grow in your flower garden, focus on flowering times to keep their color going in the garden all year long. Early blooming rhododendron cultivars bloom as early as March, while late bloomers flower in July and even well into autumn. When selecting plants from the nursery, look for rhododendrons that are deep green and well-provisioned (avoid yellowed leaves). Stick a finger in the soil to check the moisture level and take a pass on any plants that are sitting in a bone dry substrate.
If you live in a hot climate area, buy plants in 3-gallon pots instead of 1-gallon pots, as smaller flowers with less-developed root systems are more likely to struggle in warm springs and hot summers.
Varieties of Rhododendrons
With thousands of Rhododendron family plants to choose from, it’s nearly impossible to get to know the differences between the many plants in the genus. The American Rhododendron Society annually awards 8 different cultivars with the Rhododendron of the Year (RotY) honor, selecting one Rhododendron variety from every region of the U.S. that is worthy of the selection.
One way to sample new rhododendrons is to allow the society to choose them for you by growing the regional winner of the previous year’s award this year. So, each year, when the award winners are announced, jot down the species from your region and put in an order for seeds from that cultivar so that you can grow the regional winners each year, one year after they are awarded. Not only will growing the society’s top regional selections each year help you to become more familiar with the genus, you might find yourself becoming more familiar with the region’s growing tendencies as well.
Though there are way too many rhododendron species to list, there are quite a few known cultivars that have received high praise from flower enthusiasts. Here is a list of some of the most renowned varieties available, as well as a few standouts recommended by other gardeners around the globe. Start your own list of preferred cultivars and jot down the species that really stand out. Until your Rhododendron list gets fleshed out again by rhododendron varieties that you have first hand experience with, continue to pick out cultivars from the list below.
April Rose – A semi-dwarf shrub that reaches four feet tall at full maturity. The April Rose cultivar has green leaves that turn red-yellow during autumn and produces two-inch-wide deep-purple flowers in early spring. This cultivar is a previous winner of the RotY award.
Black Satin – The Black Satin rhododendron cultivar provides a stark contrast between dark foliage and bright flower colors, with eye-catching pink flowers rule the spring and dark purple-black leaves reign through the winter. .
Blue Diamond – Hardy to USDA zones seven through nine, the Blue Diamond cultivar is a dwarf variety that grows to five feet tall. Its flowers range from violet to blue.
Blue Peter – This sun-tolerant variety is a hybrid shrub that tops out at four feet tall. In mid-spring, it produces elaborate,showy, lavender blooms.
Boule de Neige – The Boule de Neige, or Ball of Snow in English, requires deep-shade, and is decorated with large, delicate, white flowers in the Spring.
Bow Bells – Growing only three to six feet tall, but with a six foot spread, the Bow Bells shrub produces pink blooms that start out deep pink and fade to pale pink as they mature.
Calophytum – Calophytum is a large flowering tree variety that can grow higher than 40 feet tall in the wild, but usually doesn’t grow much higher than 10 feet tall when cultivated. It produces trumpet-shaped, creamy pink flowers. This cold tolerant cultivar is a former winner of the Royal Horticultural Society’s Award for Garden Merit.
Cecile – Producing large, salmon-pink blooms in mid-season, the Cecile azalea variety can grow as tall as seven feet in zones five through eight.
Daviesii – Native to Mexico, this shrub can reach six feet tall and wide, with highly fragrant yellow blooms that turn to orange at the tips. A former RHS award winner.
Elvira – This variety thrives in cool-weather climates, hardy as low as zone 4. It’s showy bright-red flowers leave a lasting impression.
Freya – From May to June, the Freya cultivar produces gorgeous yellowish-pink flowers. This variety is easy to grow, but very difficult to propagate.
Gibraltar – Hardy to zones five through eight, the Gibraltar variety is known for its bright, trumpet shaped red-orange flowers and its wide, open-growth habit. Highly decorated, the Gibraltar is a former RHS and RotY winner.
Golden Lights – An especially cold hardy cultivar, the Golden Lights rhododendron has vibrant orange flowers and narrow, olive-green leaves.
Homebush – The showy pink flowers of the Homebush variety help the plant stand out in the spring. In the fall, its attractive foliage takes on the lead role. Attracts a wide range of pollinators in the spring. Hardy to zones six through eight.
Hydon Dawn – One of the only varieties to tolerate full sun locations, the low-growing Hydon Dawn cultivar is hardy to zones seven through nine. Forms pale pink midseason flowers
The Mighty Rhododendron – Produces frilly magenta blooms that look like pom-poms. Hardy to zones six through eight.
Nova Zembla – Hardy to zones five through eight, the Nova Zembla produces bright red flowers from mid to late season. An evergreen cultivar that grows to five to ten feet tall.
Rosy Lights – This cold hardy azalea variety grows to four foot tall and displays purple-pink blooms in zones three through eight.
PJM Elite – A cold hardy cultivar that grows well in zones four through eight, the PJM Elite is known for producing an abundance of red to purple flowers in mid-spring.
Purple Gem – This early-blooming dwarf rhododendron cultivar grows only two feet high and displays small purple flowers, making it perfect for front borders.
Rubiginosum – This variety is a small tree cultivar that can grow up to 20 feet high. Produces trumpet-shaped flowers that range from pale lavender to pale pink.
Saffron Queen – The pale-yellow trumpet-shaped flowers of this cultivar are known to attract bees, butterflies, and hummingbirds to the garden. This shrub can grow up to six feet tall.
White Angel – Growing as high as six feet and producing an abundance of showy long-lasting, white flowers in mid-April, the white angel variety thrives in partial shade environments.
Windbeam – This shrub’s flowers start out white and fade to pale pink. The abundance of blooms that are on display on this small shrub, can make this variety a show-stopper in the garden. A cold hardy species, Windbeam is hardy to zones four through eight.
Windsong – This evergreen species combines glossy-green foliage with strange yellow-green blooms.
Growing Conditions for Rhododendrons
Rhododendron requires an acidic soil with lots of organic material, a high-quality mulch, and consistent moisture levels. Some varieties prefer shady locations, but most enjoy full sun. The ideal soil pH for rhododendrons is between 4.5 and 6, which makes nutrients water-soluble, so that the roots can access them easily. If your soil pH is above six, use sulphur, never aluminum sulphate, to lower it. If it is too low, use ground limestone to raise it. Calcium, which is regularly leached out of the soil, is essential to rhododendron growth. To replace lost calcium, add a tablespoon of gypsum to your fertilizer prior to application. Boost calcium levels in your soil once per year.
How to Plant Rhododendrons
In temperate climates, such as zones three through six, rhododendrons should be planted in full sun to increase blooming and to avoid issues with mildew. Shrubs require at least six hours of full sunlight each day. Rhododendrons will suffer if exposed to cold, dry wind conditions. High winds can damage the plant, and cold, dry climates can cause the leaves and buds to dry out, wither, and die.
In warmer locations, such as zones seven through 11, pick a location that gets afternoon shade to give your rhododendrons a break from the intensity of the sun. This is especially important in very hot areas. Azaleas, as well as rhododendrons, are known to bloom in full shade in tropical climates.
Plant your rhododendrons in the spring or in early autumn, spacing each plant two to six feet apart. Adjust spacing based on the approximate size of the cultivar at maturity. Dig a hole in the soil as deep as the root ball and twice as wide. New plants should be set in the ground so that their top roots stay at soil level or just beneath it. If the root ball is buried too deeply, the roots are likely to rot. After placing the root ball in the hole, back fill it halfway full with soil, then water it well to help the soil settle. After the first round of soil settles into place, fill in the rest of the soil.
It is better to plant rhododendron in the fall. The plants will tolerate spring or winter planting, as long as it doesn’t freeze, but fall planting will reap the best results. If you want to plant your rhododendrons in the summer, you will need to provide plenty of extra water, especially in the beginning until plants are established. Summer sown plants should be provided with a shady location that never gets overly hot. Avoid planting in areas that are flood prone, as these shrubs will not tolerate any standing water.
Care for Rhododendrons
Mulch plants in the spring with a two to five inch layer of pine bark or pine needles to shelter shallow roots and improve soil moisture. Insufficient moisture will reduce blooms. Leave a few inches around the trunk free of mulch to avoid rot issues.
Limit feeding to once a year, when flower buds begin swelling in the early spring. Heavy fertilization will burn your plants. Provide water in the summer when rainfall is below one inch per week. Deadhead spent blooms to promote more vegetative growth and limit seed production. Remove dead flowers carefully, as the next buds sit right below the old heads, and they will start to grow just after flowering occurs.
In areas with cold winter weather, protect evergreen rhododendrons by wrapping them with burlap in autumn and putting down extra mulch around the base of the plant. Transplant your rhododendrons and azaleas any time you wish during the growing season, however, fall or early spring transplants will have more success, when they are dormant and the weather is cool.
Spring flowering shrubs like rhododendron and azaleas do not require pruning. Lower height by pruning just after flowering in the spring if needed. Other than pruning to reduce height, the only pruning necessary is to remove dead, damaged, or diseased branches anytime they are noticed. Carefully break off faded flowers by bending them until they break loose from their stems. Take care not to damage the growing buds that rest at the base of each flower stalk.
How to Propagate Rhododendrons
The fastest and easiest propagation method for rhododendrons is cuttings, which can be prepared throughout the summer months. Collect six to eight inch cuttings taken from non-flowering stems and remove lower leaves. Keep only the topmost one or two sets of leaves. Dip the base of the cutting in a powdered rooting agent (rooting hormones are optional, as plants will develop roots without encouragement as well). Place cuttings into pots filled with a cutting soil mix. Keep the mix moist and put the pot near a light source, but do not expose it to direct sunlight. Roots will begin to develop in two to three weeks. Once roots develop, the plants can be transplanted outdoors.
Garden Pests and Diseases of Rhododendrons
Some of the trouble a gardener of rhododendrons may experience is not due to insect infestation or disease but instead to excess water in the soil. If the gardener is watering too much, the weather is especially rainy, or the soil where rhododendrons are growing does not provide sufficient drainage, the foliage and flower buds of rhododendron plants may start to turn brown. Rhododendrons growing in poorly draining soil that causes “wet feet” (water around the roots) can also show yellow discoloration instead of brown.
Rhododendrons become discolored like this to express their displeasure whenever the soil around their roots is oversaturated, so make sure not to overwater and to choose a spot for rhododendrons that drains well. You can improve the drainage in a location by mixing sand into the soil to improve its texture and loosen it up. When you’re planting a rhododendron, you can also line the bottom of the hole you dig for the plant with either pebbles or clay marbles. Both these items will create small spaces between them that water can flow through when it needs to drain.
To keep your rhododendron in fine fettle, nourish the plant with a fertilizer blend that includes heath at the end of each winter season. This treatment will both help the plant grow strong enough to avoid struggling with minor diseases and boost the beauty of its blooms.Once your rhododendron is established in its location, as long as the growing conditions match its preferences, it should develop strong roots and naturally resist most diseases or insect infestations.
On the other side of the coin, plants that do not have their care and maintenance needs met and those that need nutrition or water are vulnerable to health problems and may struggle with one minor illness after another. To learn how to diagnose when pests or insects cause trouble for your rhododendron and to find out what you should do if these problems rear their heads, reference the list below.
- Leaf Gall – Blisters that form on the foliage of rhododendron shrubs are caused by a fungus-based disease called leaf gall, caused by the Exobasidium vaccinii fungus. This disease is usually not critical, and only affects the plant’s ornamental value. In most cases, removing the blistered leaves is enough to control the disease’s spread.
- Rhododendron-Phytophthora Root Rot: There are a number of water mold pathogens behind rhododendron-phytophthora root rot, including P. cactorum, P. cambivora, P. cinnamomi, P. citricola, P. cryptogea, P. plurivora, and P. pini. These water molds are similar to fungi, and in addition to rhododendrons, they can also affect almond trees, andromedas, apple trees, azaleas, beech trees, blueberries, false cypresses, heathers, mugo pines, stone fruit trees, viburnums, and yews.
Phytophthora root rot is especially prevalent when soil is too wet, and it is exacerbated by plastic ground covers and poor drainage. Other risk factors include warm temperatures, high levels of salinity (salt), high pH level, and damage to plant roots. This disease can strike plants growing in containers as well as those planted in the ground. The pathogens behind phytophthora root rot can live in the soil or in plant debris for years as oospores or chlamydospores when the weather is not wet enough for them. They can also travel on gardening tools, plant containers, in potting soil, via water, or on infected plants.
This disease attacks the root system and begins causing it to rot and decay, then it spreads up through the tissues in the plant’s inner bark and stems. A canker may appear at the base of the stem as the root rot spreads up through the rest of the plant. Fibrous roots begin to show symptoms first, changing color to become brown or black, and then discoloration spreads through the plant’s entire root system. Brown and black lesions develop on large roots, and root tissue that is not discolored will appear pale, in shades of white, yellow or light green, in contrast, and it will feel moist. As the roots weaken, the plant will be less secure in the soil and may become wobbly or easy to dislodge.
After the rot has spread significantly, the plant may show signs of other stresses, such as drought stress or malnutrition, because it is not able to take in as much moisture and nutrients as it needs through its damaged roots. Sometimes the disease is not noticed until these symptoms appear above ground, which include discoloration of leaves to yellow or a reddish-green shade and leaf curling or wilting. This wilting will not be resolved regardless of how much water you provide. When the root rot is fatal, plants die with their foliage still attached to the branches.
Gardeners can avoid ever needing to battle phytophthora root rot by choosing resistant rhododendron hybrids from the outset. Resistant varieties include R. davidsonianum, R. delavayi, R. glomerulatum (R. yungningense), R. hyperythrum, R. keiskei, R. lapponicum, R. occidentale, R. quinquefolium, R. sanctum, R. pseudochrysanthum, R. simsii, and R. websterianum. Moderately resistant hybrids include Anna H Hall, Bali, Brittany, Crete, Ginny Gee, Hawaii, Ingrid Mehlquist, Martha Isaacson, Normandy, Peter Tigerstedt, Pink Trumpet, Professor Hugo de Vries, Red Head, Rocket, Serenade, and Vernus. Certain varieties, namely the Caroline and English Roseum cultivars, are tolerant of phytophthora root rot unless their soil is too wet.
Other control measures include establishing a one-year quarantine period when new plants are introduced to the garden, improving drainage in garden beds or containers using raised beds if the soil drainage cannot be sufficiently improved, watering plants at the base so moisture does not splash foliage, keeping the ground clear of plant debris and disposing of potentially infected debris or soil properly, decreasing salinity in soil and water sources and sterilizing garden tools. Fungicides may be used as a preventive treatment.
- Yellowing leaves: If the foliage of your rhododendron plant grows pale and then becomes discolored to yellow, your rhododendron may have developed rhododendron chlorosis in a reaction to insufficient minerals or other problems with the soil. Chlorosis is an especially likely cause for yellowing leaves when the discoloration occurs mostly between the veins in the leaves. Discoloration tends to affect the newest growth first, and when it is severe, foliage may eventually turn brown, and the plant’s growth can be stunted. Rhododendron chlorosis is also sometimes referred to as interveinal chlorosis.
Rhododendrons love acidic soil—a pH level between 4.5 and 6.0 is acidic enough for them to really thrive. If you aren’t sure of the pH level of your garden soil, you can read about how to find out in our article How to Test pH in Your Soil. If you’ve planted a rhododendron in alkaline soil (that’s when the pH level is above 7.0), the plant is likely taking in too much calcium while simultaneously lacking iron.
You can amend the soil with acidifying treatments such as aluminum sulfate, elemental sulfur, ferrous sulfate, iron chelate, or iron sulphate. Top the soil with these treatments, and they will become incorporated with the soil as you water the plants or rain falls. For quicker results, you can mix these amendments with the top few inches of soil, but take care to stay shallow so you do not damage the root systems of your plants. You can also opt for water soluble versions of these acidifying treatments, which can be administered using a root feeder.
If your garden has any alkaline areas, transplanting your rhododendron there may be the quickest, easiest solution. Nourishing your soil and acidifying it by applying a layer of two to four inches of acidic organic material, like well rotted compost, peat moss, or leaf mulch, can also help correct discoloration of foliage. You can also grow rhododendron in a container, which gives the gardener complete control over the soil.
Yellow foliage is also a symptom when the rhododendron has its root ball planted too deep. This is an easy mistake to make, because burying the root ball at a depth that would be the standard for most plants will be too deep for a rhododendron. For best results, situate your rhododendron so the root ball is level with the surface of the soil where you can feel it if you run your hands over the ground around the plant. If your rhododendron is planted too deeply, simply dig it up so you can make adjustments.
Your rhododendron’s leaves may also turn yellow if the plant is hungry for fertilizer or thirsty due to insufficient water. Give rhododendrons an annual dose of fertilizer between late May and June. (Or, if you miss the ideal window for fertilizer application, you can do the treatment at any time of year.) If you suspect a lack of water to be the cause for discolored leaves, give your plant a deep watering and watch to see if the problem improves.
While you wait for soil amendments or other gradual solutions to kick in, you can use foliar feeding as a quick fix to correct the color of your rhododendron’s leaves. Spray the foliage with an iron compound, such as iron chelate, iron sulfate, or a soluble organic iron complex. Give the plant a dose of several squirts every two to four weeks. While this is a cosmetic remedy that does not resolve any issues with the soil that may contribute to discolored foliage, it’s a quick fix that can keep your rhododendron vibrant and green for a season as the more long-term treatments do their magic.
If an iron treatment has the opposite of its intended effect and your plant’s foliage turns even more yellow, then you are probably dealing with a manganese deficiency instead of a lack of iron. (Don’t forget that when foliage turns yellow, poor drainage is another possible cause, for which the best prescription is transplantation to a location that offers better drainage.) Leaves may also turn yellow as a response to chemical treatments, so if you’ve recently sprayed an insecticide or other chemical near the plant, you may have found the culprit.
Getting the most out of your rhododendrons means amending and treating the soil prior to planting and picking out the best possible location. If you amend properly and pick a location that provides your rhododendrons with an ideal growing environment, caring for the flowers becomes a very simple process. All rhododendrons need, aside from rich soil and a good location, is consistent watering, deadheading, and a very occasional pruning. When plant care involves very little effort and little to no stress, gardeners can really enjoy their hobby to the fullest extent. Rhododendrons offer months of color and ornamental beauty and they are one of the easiest flowers to grow, which makes them a very popular choice for flower gardens around the world.