How to Plant, Grow, and Care For Rhododendrons
Thinking of growing some rhododendrons in your garden this season but aren't sure where to start? These beautiful shrubs can really make just about any gardening space "pop" with color! In this article, gardening expert Jill Drago takes you through each step of planting, growing and caring for rhododendrons in your garden this season!
Rhododendrons have become a classic flowering shrub in landscapes all over the world, and it is easy to understand why. These large evergreen beauties come to life with their happy flowers in the spring, marking the unofficial beginning of the gardening season.
In many gardens, rhododendrons are the first flowering shrubs to bloom, sending a signal to gardeners everywhere to get ready! The large clusters of trumpet-shaped flowers have become a spring favorite over the years. These flowering shrubs bloom in shades of pink, white, red, purple and even yellow, and their deep green foliage lasts through the winter.
Gardeners adore these large, spring-flowering shrubs all over the world. Plant these flowering shrubs with your spring bulbs and other spring-blooming perennials such as hellebore, or bleeding heart and you will have an unbeatable spring display.
This genus is so large that there are species that grow in deep shade as well as species that will grow in full sun. Rhododendrons love acidic, well-draining soil and require minimal care once they are established. Let’s walk through all aspects of growing Rhododendrons in your garden.
Rhododendron Plant Overview
Plant type Flowering Shrub
Season Spring to Summer
Pests Black Vine Weevil, Lace Bugs, Mites
Exposure Partial Sun to Shade
Diseases Chlorisis, Fungal Diseases
Maturity Date 5 years
Species Rhododendron sp.
Growth Rate Slow
Soil Type Acidic, well draining
Native Area Asia, Europe, North America
Plant spacing 2-6 feet, variety dependent
Attracts Bees, Birds (Hummingbirds)
Hardiness Zones 4-8
Plant with Acid loving plants, spring bulbs
Planting depth Depth of root ball
Watering Requirement moderate to high
Don’t plant with N/A
Height 5-20 feet, species dependent
The history of Rhododendrons is a long one. There have been fossils that date Rhododendrons back to over 50 million years ago. Rhododendrons are native to many areas of the world, but the highest concentration of these plants is in Asia.
These plants were grown by Japanese gardeners and their details were recorded in a book “A Brocade Pillow” in 1692. However, it wasn’t until the mid 16th century that rhododendrons were discovered by a Flemish botanist named Charles l’Ecluse in the European Alps. His discovery was introduced to Britain, and Rhododendron hirsutum became the first rhododendron in cultivation.
This set off a wave of exploration and discovery in America between John Bartram, an American botanist, and Peter Collinson, an English Quaker. They searched America for new species of rhododendrons and were quite successful.
Their success set off another wave of discovery subsequently worldwide for the next 200 years or so. This exploration gave us the massive genus of rhododendron that we know today, with over 900 species worldwide. Today these gentle giants can be found gracing the front yards, and woodland gardens of many homes across America.
Rhododendrons belong to the Ericaceae family. They share their genus, Rhododendron, with Azaleas. Azaleas are similar plants, but have distinct differences.
- Rhododendrons are always evergreen.
- Some Azaleas are evergreen but many are deciduous.
- The flowers of rhododendrons are bell shaped.
- The flowers of azaleas are more of a funnel shape
- Rhododendron flowers have 10 stamens, Azalea flowers only have 5 stamens.
- Rhododendron leaves have scales, azalea leaves have small hairs on them.
There are over 900 species of Rhododendron in existence today, with countless varieties. The enormity of this genus makes it a bit difficult to wrap your head around. Most varieties are considered perennial shrubs, depending on your hardiness zone.
The species have been hybridized over and over again to create better-performing plants or more desirable flower colors. One thing is true: all of these species can be broken into two very broad groups: large-leaf rhododendrons (or elepidote), and scaled or small-leaf rhododendrons (lepidote).
The leaves of these shrubs are generally large, as is the size of the overall plant. The underside of these leaves will be smooth, without scales. An example of an elepidote rhododendron is Rhododendron catawbiense.
Lepidote rhododendrons are often referred to as small-leaved rhododendrons, which is true. However, the true mark of a lepidote rhododendron is the scales on the underside of the leaf. You may be able to see these scales with the naked eye. If not, you can use a magnifying glass to see them. A very common lepidote rhododendron is Rhododendron ‘PJM’.
When it comes to propagation, rhodies can be propagated in a variety of different methods. You can grow them from cuttings, seed, by grafting, and layering. Let’s take a look at each different method of propagation to help settle on which is best, should you decide to propagate your own plants or those from a friend.
Rhododendrons can easily be propagated from cuttings taken in the fall time from new growth. When choosing where to take your cutting look for new, green growth with at least one whorl of leaves on it. Cut these leaves in half to reduce the surface area of the leaf, and remove any flower buds. This will help the plant use its energy on rooting.
Dip this cutting into some rooting hormone, and stick it into a sterile planting medium. Cover this cutting with a plastic bag. Make sure the bag doesn’t touch the plant at all. Using planting stakes or even chopsticks can help with this.
It can take three to four months for large leaf rhododendrons to root. When they finally root you can transplant them into a larger container and begin to fertilize and treat as you would a typical nursery-grown plant.
You may want to wait until this cutting is larger before planting it in your garden, but that is entirely up to you. Keep in mind the full size of your plant and remember that you may need to transplant them again to accommodate for their large size.
Grafting takes the rootstock of one plant, and the shoots of another plant and combines them. This practice is used for plants that are not easily propagated using other methods or to improve upon an existing plant. For example, rhododendrons are shallow-rooted. One may wish to graft a rhododendron with a deeper rooting plant to improve upon the water needs of a rhododendron.
Layering is probably the most successful way to propagate Rhododendron and is very simple to do. Choose a low lying branch, and scrape away a few inches of the plant surface.
Cover the exposed section with garden soil, and weigh it down with a rock from your garden or a brick. The area that you have scraped will form roots and will become your new plant. It will take about two years for this plant to become independent from the mother plant. At this point you can transplant the new baby Rhododendron into a new shady spot in your garden.
When rhododendrons are done flowering seed capsules will form. As fall approaches the seed capsules will turn brown. Once they have dried on the plant, remove them and store them in a dry place until later in the winter.
Sew the seeds into sterile plant material by sprinkling the seeds on top of the potting mixture. The seeds require high humidity and using a plastic bag in the same way you would use one for cuttings is recommended.
The seeds will sprout in about two months. Once the seedling has its first two true leaves it can be transplanted into a larger flat. These plants will take about two years to be mature enough to be planted into your garden.
When you are choosing your Rhododendron it is so important to do your homework before purchasing a plant. Many species and varieties of Rhododendron are available at your local garden center. You can typically find them in large containers or as balled and burlapped larger specimens. There are also very reputable online sellers as well.
Ensure that the plant you want is hardy in your area, and that you have the correct conditions to grow this plant successfully. Rhododendrons are best planted in the early spring in cooler areas, and in the fall in warmer areas.
Get digging! You will want to dig a hole that is just a bit deeper than the root ball of your Rhododendron, and about twice as wide. Oftentimes some soil preparation will need to be done before planting a rhododendron. These plants prefer acidic soils, if you have alkaline soils you may need to add sulfur. Do not use aluminum sulfate, this is toxic to rhododendrons.
When you remove the Rhododendron from its container you will likely need to score the roots with a knife. This will help to encourage root growth. If you leave the roots as they are you may have a difficult time getting your rhododendron to establish in your garden.
Once you have the Rhododendron in the ground do your best to lay the roots out, this is where they will stay and continue to grow. These plants are very shallow rooted considering their size.
Growing & Maintenance
When growing rhododendrons, there are many different aspects to consider. You’ll need to make sure they have enough light, water, and fertilizer. You’ll also need to make sure the soil is hospitable for the variety you’ve chosen. Let’s take a deeper look at growing rhododendrons, step by step.
All rhododendrons require at least some direct sunlight each day to ensure the flowering is prolific. Some rhododendron species can tolerate full sun, where some do best in full shade. Rhododendrons need some sunlight to have strong and prolific blooms.
However, keeping your rhododendrons in some shade can help to prevent lace bug infestations. Be wary of planting in too deep of shade, this could lead to leggy growth and poor performing flowers. Be sure to read your plant tag and do some research to make sure that the plant you are choosing is right for the space in your garden.
Rhododendrons have shallow, fibrous root systems. Because these root systems are so shallow the plants tend to dry out in the hot summer months.
New plants should be watered twice a week. Mature plants may only require supplemental watering during a drought. Rhododendrons like nice moist soil that is not soggy making daily watering too much for their roots to handle. Slow soaks will suit these plants best.
Rhodys are acid loving plants that love some well draining soil that is high in organic matter. These conditions happen naturally when they are planted in wooded areas where leaf litter and pine needles are abundant on the forest floor.
If your soil is a bit too alkaline you can amend the soil with wettable sulfur. Aluminum sulfate is a popular acidifying product, however it is toxic to Rhododendrons and should be avoided.
Rhododendrons are native to many different parts of the world from the Himalayas to the Eastern United States. They can grow in differing climates as cold as Canada and as warm as Mexico.
Rhododendrons can grow in a number of different climates. They are hardy from USDA zones 4-8. These flowering shrubs prefer to live in climates that receive a moderate amount of rainfall. They also prefer areas that experience temperate climates. Rhododendrons need a period of chilling to produce flower buds. This cold requirement makes rhododendrons difficult to grow in hotter climates.
Rhododendrons do not need to be fertilized if they are growing in fertile soil. However, using a fertilizer for acid loving plants (such as Holly-Tone) can be applied in the early spring or the fall.
Rhododendrons do not require any deadheading or pruning at all unless you desire to do so. If you do not like the look of the spent flowers you can go ahead and snip them off with shears, or pinch them off with your fingers once the petals have fallen to the ground.
Pruning can be done if you wish to control the size and shape of your rhododendron. Prune your plants in the early spring. These are some tough plants and they will bounce back really well. Early spring is also a good time to remove any dead or damaged branches within the plant.
There are many different varieties of rhododendron to choose from! Depending on the look you are after, the colors, and the sun orientation, there’s a different type of rhodie for everyone! Let’s take a look at a few of the more popular types of rhododendron.
The white catawba rhododendron is considered to be an ‘ironclad’ rhododendron. An ironclad rhododendron has been proven to be particularly hardy and floriferous.
‘Album’ grows to six feet tall. Its flower buds are a very light purple, and the flowers open crisp white with a light green or yellow blotch. This particular rhododendron can tolerate partial sun, but performs best as an understory plant.
The catawba rhododendron is native to the southeastern United States, and this variety was bred in England at Knap Hill for cold hardiness.
‘Boule de Neige’ will grow to ten feet tall and wide with pure white flowers. The flowers are a bit more frilly than your standard rhododendron flower.
The leaves are pretty green with a hint of gray. This shrub is very nice used in a mass planting. ‘Boule de Neige’ does well in full to partial sun
‘Nova Zembla’ is a smaller rhododendron that will max out at five feet. The flowers are deep pink with maroon blotches. ‘Nova Zembla’ requires partial sun.
These flowers are particularly beautiful when they are backlit from the sun, they almost glow. This rhododendron would be really nice used in a mass planting, but the flowers are showy enough to make it a perfect specimen in a larger garden or as a foundation plant.
The Scintillation rhododendron is a personal favorite of mine. It is very large and can reach eight feet tall. Its flowers trusses of light pink flowers with gold freckles.
‘Scintillation’ will grow well in full sun to partial shade. This rhododendron will make a really nice privacy screen, but can be used just about anywhere in your garden that you are able to enjoy this exceptional blooming plant.
‘Yaku princess’ is a compact and dense rhododendron. It will grow to about five feet tall. Its flower buds are bright pink, and the flowers open to a mix of white and light pink.
The two toned effect of these flowers is beautiful. This rhododendron is best grown in partial shade amongst other acid loving plants.
Rhododendrons can be used in many ways. They make a really nice mass planting which can provide nice year round privacy due to the evergreen leaves. Rhododendrons are also really nice when used as specimen plants, or positioned in a foundation planting.
Like most ornamental plants, Rhododendrons are also susceptible to certain pests. While prevention is best, you can typically treat pests if you catch them early enough, and prevent further damage to your plants. Let’s look at the most common pests you’ll likely encounter.
Rhododendrons are a one stop shop for the black vine weevil. These insects will spend their entire life feasting on rhododendrons starting with the roots when the insects are in grub form, and ending with the leaves when they are adults.
If you are noticing holes in the leaves of your rhododendron but can’t seem to find any insects, it may be the black vine weevil that you are dealing with. These insects feed at night, so you won’t be seeing them out during the daytime.
The most recommended way to treat the black vine weevil is using beneficial nematodes. The nematodes will attack the grubs in the soil and will minimize the infestation. Keep your garden clear of hiding spaces around your plants such as older piles of mulch, and water only when needed because the grubs prefer moist soil.
Rhododendron lace bugs are small flying insects with lace like wings. These insects attack rhododendrons that are in full sun, and this usually happens in mid July when the weather heats up.
Lace bugs leave stippling, or small yellow spots, along the upper surface of the rhododendron leaves. The lace bugs will lay their eggs on the undersides of the leaves.
Some friendly garden pests such as spiders can help a bit with the control of these insects, although you can’t totally depend on them. Using a broad-spectrum pesticide can be difficult. You will need to spray multiple times during the insect’s life span.
Spider mites are very small insects that are distinguished by two small black dots on their back.
You will likely not see the insects themselves, but you will notice their webbing. Spider mites love to target stressed plants that are experiencing drought. They will feed on the leaves leaving a speckled appearance.
Use your hose to spray the bugs off of your plant. Keeping your rhododendrons appropriately watered will keep spider mites from attacking as well. Typically their damage will not be detrimental if your plants are well cared for. Even if your rhododendrons do sustain some damage they will likely bounce back next year.
Diseases also tend to strike rhododendrons for a variety of reasons. The good news is that some are treatable and can result in minimal plant damage if you’ve caught them early, and put a care plan in place. Let’s look at the most common diseases you may encounter.
Chlorosis is a very common occurrence on rhododendrons. It presents itself to us as the yellowing of the leaves. This typically happens due to the lack of iron in the soil.
The type of yellowing we are looking for occurs between the veins of the leaves. If your entire leaf is yellow it is likely leaf senescence which is the natural dropping of older leaves that will happen in extreme heat or just before winter.
Once you’ve seen the yellowing of chlorosis it is a good idea to do a soil test before amending your soil at all so that you don’t worsen the situation. If you find that the soil is alkaline it is in fact a lack of iron that is causing the yellowing. You can try to amend the soil with wettable sulfur, or you may opt for transplanting the rhododendron to a better location.
This is a potentially devastating fungal disease that attacks rhododendrons. You may notice an entire branch or two that are completely dead on a plant that you would assume is healthy.
An easy way to determine if your rhododendron has fungal dieback is by scraping the bark of the affected branch. If you see green underneath you are in the clear. However, if it is red you have fungal dieback.
This is a tough disease to control. Start by pruning the infected branches out of the shrub and moving them far away from your plant. You can apply a copper fungicide to help control this fungus. Use the labeled instructions for application timing and rates.
This is a fungal disease that affects the leaves of the rhododendron. Native rhododendrons are more susceptible to this fungus than hybridized rhododendrons. The leaves will begin to look distorted. They will thicken, curl and begin to lose color and turn white. The leaves will then harden.
This is usually not a large enough problem to require chemical control. Typically you can remove the infected leaves by hand. However, if it does happen that this leaf gall becomes a widespread problem you can use a chlorothalonil fungicide according to labeled instructions.
This fungus attacks only the flower petals on your rhododendrons. It will start as small brown spots on your petals that will rapidly spread.
This fungus will ruin your bloom for the year. The infected petals will fall and attach themselves to leaves of the rhododendron, or will just fall to the ground.
The best thing you can do to prevent this fungus, or at least stop it from coming back the following year is keeping your garden free from debris. Raking the fallen petals and taking them far away from your plants will be a great help.
Winter burn isn’t so much a disease, but it is a common issue that rhododendron growers may experience. This will occur if the plant loses too much water over the winter. Rhododendrons are broadleaved evergreen shrubs and water can easily be wicked from them. Symptoms of winter burn are large brown almost rust colored splotches on the leaves.
Keeping the soil moist around your rhododendron will be important if you are trying to prevent this. WInter burn commonly occurs on rhododendrons that are receiving too much sunlight in the winter, or are getting hit with too much wind. Both of these are drying to any plant.
Applying an anti-desiccant can help to prevent the loss of water through the leaves. This would need to be applied in the fall, and again as the label of the product specifies. Anti-desiccants provide a waxy coating to your rhododendron leaves which helps keep the leaves hydrated.
Are Rhododendrons Invasive?
Some rhododendrons can be invasive in some areas. They are prolific at self seeding, and oftentimes their roots can produce suckers which will cause the rhododendron to spread.
Rhododendron ponticum is a species that is native to the mediterranean region of Europe. This shrub has been labeled as being invasive in many areas, spreading through seeds and suckers and creating a thicket making it impossible for other nearby plants to survive.
Can I Plant Rhododendrons in Containers?
Yes! These plants are shallow rooted making containers a great place for them to live. Whiskey barrels and larger pots will work best to accommodate the size of the large pot.
You will need to water your potted rhododendrons more frequently than any you may have planted in your garden. This is because the roots are so close to the surface and will dry out more quickly with the small amount of surface area they have in the containers.
What Should I Plant With My Rhododendrons?
Rhododendrons are wonderful when planted on their own, in a mass planting. However, if you wish to add them into an existing garden, or create a garden around them here are a few ideas:
- Add spring bulbs for an extra pop of springtime color.
- Surround your rhododendron with other shade loving perennials such as showy types of hostas, or ferns.
- If you are wanting to create a woodland garden, consider adding Mountain Laurel, or Japanese Andromeda.
There is so much to love when it comes to rhododendrons. The variety available to gardeners is so vast that there is a rhododendron for every garden and every situation in every garden. Doing a small amount of research before purchasing and planting is the best thing you can do for your rhododendron because you will be setting yourself up for easy success.