Rhododendrons are one of the biggest plant groups on the planet, and arguably, one of the most beautiful. Prized for their evergreen foliage and bright, showy blooms, these fabulous shrubs are a welcome and familiar feature of bouquets, gardens, woodlands, and jungles the world over.
Discover all about these fantastic flowering shrubs with our ultimate guide to the fifteen best types of rhododendron to grow, and how to care for them.
What are Rhododendrons?
Rhododendron is a vast and diverse genus of flowering shrubs and trees belonging to the Ericaceae family. There are over one thousand species of Rhododendron, and a great many more cultivars and hybrids, consisting of both evergreen and deciduous varieties.
The great majority of rhododendrons are native to Asia, although there are some varieties which are native to North America, Australia, and Europe. The habitat range for rhododendrons is vast and varied, covering cold mountainous regions to tropical jungles to temperate woodlands. Some species are able to withstand temperatures well below freezing, whilst others prefer warm, humid conditions.
Rhododendrons can generally be categorised into three different groups: Evergreen shrubs which have a small, dense growth habit and hold their foliage all year round; Deciduous shrubs which have the same dense and compact growth habit, but shed their leaves twice a year; and Evergreen trees, which grow much taller than shrubs, and hold their foliage all year round.
Because so many species are evergreen, tall and spreading rhododendrons are commonly used to create floral hedgerows with year-round foliage. Smaller varieties make excellent border plants and containerised ornamentals. The smallest dwarf varieties reach less than a meter tall at full maturity, whilst the largest tree varieties have been reported to reach up to thirty meters tall.
Most rhododendrons are prized for their truly abundant flowers which adorn, and sometimes completely cover, the foliage. Each leaf is arranged around woody stems in a spiral configuration. Flowers come in a range of colours, including whites, reds, yellows, oranges, pinks, and purples, many of which feature a contrasting spatter pattern of darker spots.
The flowers are usually bell-shaped or trumpet-shaped, often with frilled edges, and they envelope ten central stamens. Each individual flower forms part of a larger cluster, known as a truss. These trusses adorn the foliage like jewels from late winter to early spring depending on the variety, although the peak blooming time tends to be between the months of April to June. Many species of rhododendron have beautifully fragranced flowers, giving off a sweet but subtle scent whilst in bloom.
The History of Rhododendrons
From their name to their classification, rhododendrons have posed something of a puzzle throughout their history.
The name rhododendron is taken from the Greek rhodon, meaning ‘rose’, and dendron, meaning ‘tree’, so can be loosely translated as ‘rose tree’. This is somewhat misleading since rhododendrons are no relation to roses.
The classification of rhododendrons has proved a complex and problematic endeavour for botanists, a problem which would span 500 years. The first rhododendron to be officially discovered and recorded was the Alpenrose variety, Rhododendron hirsutum, in the 16th Century. It was brought back to Great Britain almost a century later and became the first cultivated rhododendron variety in Europe. Meanwhile, in Japan, similar classification and cultivation of local varieties had also begun.
Originally, rhododendrons and azaleas had been classified as belonging to separate genera. However, a dispute arose over whether the two plants were biologically distinct enough to justify this. In the 18th century, several azalea species were absorbed by the rhododendron genus, which had by now grown to many hundreds of discovered species.
In the 19th century, the genus was divided into eight subgenera. The classification was then revised again as recently as 2004 to incorporate just five subgenera under the Rhododendron genus.
Some species of azalea are still contained within the rhododendron genus, and this can often cause confusion when trying to tell the two plants apart. Generally speaking, rhododendrons have ten stamens, whilst azaleas only have five. Azaleas are deciduous, whilst rhododendrons can be both deciduous and evergreen. Azaleas also tend to be smaller than rhododendrons.
But before all the confusion with classification began, rhododendrons were known to humans for more sinister reasons. The pollen and nectar of rhododendrons contain a toxin called grayanotoxin which can cause serious illness and even death if consumed by both humans and animals. It can also cause mild hallucinations, leading to the rather unfortunate name of ‘mad honey’ being bestowed upon the honey made from rhododendron flowers.
In 401 BCE, the Greek warrior Xenophon wrote of the madness that consumed his soldiers after they feasted on honey made from rhododendrons. In 67 BCE, Roman general Pompey reported the deaths of many of his soldiers after they were lured into eating mad honey by the enemy.
Best types of Rhododendron to grow
There are thousands of rhododendron species, and even more hybridisation and cultivars. With so many beautiful options to choose from, it can be tricky to pick just one to grow. To help you decide, here are our top picks of the best varieties for your garden.
Rhododendron Bengal is a dwarf evergreen shrub variety, with a maximum height of around 50 centimetres. It has a spreading growth habit, which is great for filling borders, and delights with plenty of bright scarlet funnel-shaped flowers.
This small but mighty rhododendron likes a sheltered position in partial shade, and an acidic, well-draining substrate.
Rhododendron Black Satin is an evergreen shrub which can grow up to six feet in height. They take their name from the unusually dark foliage which they develop through winter, although they may behave deciduously and drop leaves in very cold northern climates. The foliage, which is almost black, creates a striking contrast against their bubblegum-pink and violet-coloured flowers, which usually appear from spring onwards.
Black Satin rhododendrons like a moist, but not waterlogged soil with a slightly acidic pH level and a sheltered position with partial sun.
Rhododendron Bloom-A-Thon is a stunning semi-evergreen hybrid variety which rewards gardeners with two blooms per year, once in April and again in July. The flowers range in colour from pastel pink to deep red.
Boom-a-thon is particularly sensitive to cold temperatures, which unfortunately means they are unlikely to survive in all but the very warmest gardens. They should be planted in a sheltered position where they will receive plenty of sunlight.
Rhododendron Blue Peter is an evergreen shrub which produces an abundance of frilly funnel-shaped violet flowers. Each petal is adorned with contrasting deep plum-coloured spatter markings in the centre. Easily reaching three meters in height, and with plenty of dense foliage, this variety is often used for hedges.
Blue Peter likes a fertile and acidic substrate and is particularly hardy, so is well suited to colder climates, although they also tolerate full sun and warm temperatures.
Rhododendron Bow Bells is a popular, slow-growing, evergreen shrub, with a maximum height of around two meters. They offer truly delightful displays of pastel pink bell-shaped blooms during late spring.
Bow Bells need a sheltered position in the garden with plenty of dappled sunlight. They like well-draining, acidic soil.
Rhododendron Boule de Neige is a dense evergreen shrub with a maximum height of around four feet. In late spring, they produce lots of pale pink buds which emerge into stunning off-white and cream-coloured bell-shaped flowers enveloping the central pink stamens. The name of this variety is French for ‘Snowballs’, because of its rounded white trusses.
Despite its elegant appearance, Boule De Neige is a robust plant, tolerant of warm, sunny conditions in the summer and cold temperatures in the winter.
Rhododendron Catawbiense is one of the largest varieties, often reaching heights of ten feet tall, making this evergreen shrub perfect for hedges. It offers lots of stunning trusses of bright violet bell-shaped flowers with delicate gold markings on some of the petals.
The Catawbiense prefers a sheltered position with partial shade and acidic, fertile soil with plenty of additional organic material mixed in.
Rhododendron Cornubia is an evergreen tree variety, which grows exceptionally slowly, taking ten years to reach two meters in height. It produces dense shocks of scarlet flowers which frame delicate black-tipped stamen. Some Cornubia varieties also display striking pale yellow and deep green variegation patterns on the foliage.
Tolerant of a range of light levels, from full sun to partial shade, this variety likes an acidic soil pH level. It may struggle to flower after a particularly cold winter.
Rhododendron Elviira is a bushy, evergreen shrub which is prized for its showy fuchsia blooms housing ten gold-coloured stamens. It is one of the hardiest rhododendron varieties, able to withstand winter temperatures of well below freezing, so is a great option for those of us who live in colder climates.
Elviira needs a position in partial sunlight and regular watering. Well-draining, acidic soil is preferred.
Rhododendron Gibraltar is a deciduous shrub which can grow up to five feet tall and six feet wide, making it perfect for hedgerows. It produces plenty of fiery orange flowers with delicately frilled petals.
Gibraltar likes a sheltered position with plenty of sun and protection from strong winds and low temperatures.
Rhododendron Golden Oriole is a deciduous hybridised azalea variety which produces a truly show-stopping display of butter-yellow and gold-coloured flowers. It has a faster growth rate than many other rhododendrons, with a full height of around six feet and a similarly large spread, so is a great choice if you want a quick-growing, dense hedgerow plant.
This golden variety likes plenty of sunshine and moist but well-draining acidic soil. It is tolerant of fairly cold conditions during winter.
Polar Night likes loose, well-drained, and mildly acidic soil. This hardy variety can survive even in an exposed position, so long as they receive partial shade.
Rhododendron White Angel is a semi-evergreen shrub variety with an upright growth habit, although they rarely exceed six feet in height. They produce an abundance of pale off-white flowers, often tinged with pink around the frilly edges, from mid-spring onwards.
White Angel prefers a position in dappled or partial shade and is hardy at temperatures well below freezing, so ideal for northern climates. Like most rhododendrons, White Angel likes an acidic soil.
Rhododendron Vaseyi is a stunning azalea rhododendron variety. It is also known as the Pink-Shell Azalea because of the charming pink flowers which emerge atop long, leafless, upright stems during mid-spring. It offers another stunning display later in the year too when the glossy green foliage transforms to fiery reds and oranges. This statuesque variety can reach heights of around ten feet.
Pink Shell Azalias need plenty of moisture, but a well-draining, acidic soil. They do best in a partial sun position but are tolerant of fairly cold temperatures in the winter.
Rhododendron Windsong is a hybridised evergreen shrub variety with a slow-growing, spreading growth habit, often as wide as they are tall. They produce beautiful dense clusters of lemon-yellow trumpet-shaped flowers, dappled with tiny, deep-red spots. The foliage is a glossy, olive-green colour.
These rhododendrons grow best in a strongly acidic soil and can tolerate a range of light conditions, from partial shade to full sun. Windsong is a robust, cold-hardy option if you live in a cool climate.
Caring for your Rhododendrons
As such a diverse and versatile genera, rhododendrons offer a species or hybrid for almost every garden. Care requirements can vary greatly between species, so do your research before purchasing a specific type of rhododendron to make sure you can give it what it needs. Here are some general tips for rhododendron care.
Planting and Positioning
The best time to plant your rhododendrons is in the autumn, which gives them chance to get established and develop strong roots before the spring blooming season begins. You can also plant them in early spring, just make sure the last frost has passed.
Don’t plant your rhododendron too deeply, as this can make the roots more susceptible to root rot.
As a general rule, try to find a fairly sheltered position for your rhododendron which protects them from harsh winds and extremes of temperature.
If you hope to grow a rhododendron hedgerow, opt for a variety which has both a tall height and a wide spread, and leave around twenty per cent less distance between each plant than the maximum spread of the variety. This will create a denser hedge when fully matured.
Soil
Getting the soil right is the trickiest and most important factor in rhododendron care. Crucially, like most ericaceous plants, rhododendrons are acid-lovers, flourishing in soil with low pH levels of around 4.5–5.5. Ericaceous compost is ideal since it has been developed especially for acid-loving plants and you should incorporate plenty into the soil before planting.
You can amend highly alkaline soils by adding extra acidic matter, although this requires a significant amount of upkeep to maintain. If your soil is very alkaline, you’ll be better off planting your rhododendron in a container filled with ericaceous compost.
Many rhododendrons require consistently moist soil which mimics the conditions of their tropically rainy habitats. They hate waterlogged roots and oversaturated soil, however, so make sure you use a loose, well-draining substrate for your rhododendron plant, adding extra drainage material if needed.
Most varieties like moderately fertile and well-aerated soil, so incorporate plenty of nutrient-rich, acidic organic matter into the soil. Alpine species, on the other hand, can survive in a nutrient-poor substrate and will grow perfectly in rockeries or gravelly substrates.
Light
Light conditions can vary somewhat from species to species, but as a general rule, your rhododendron should receive at least two hours of partial or dappled sunlight per day for optimum growth. Some woodland species will cope in almost full shade, whilst tropical species prefer more sun, so always check that your garden can deliver the light requirements necessary for your chosen species before purchasing.
Water
Rhododendrons are mostly native to areas with high rainfall, so need regular watering, although the frequency can vary depending on the species. As a general rule, wait for the top two inches of soil to dry out between each watering, but amend this rule accordingly, depending on your chosen species and local weather conditions. Overwatering can lead to root rot, which is one of the biggest killers of rhododendron plants.
Try to avoid watering your rhododendrons with tap water, as this often contains calcium which can neutralise the acidity levels in the soil.
Food
Rhododendrons have sensitive roots, which can be easily damaged by excessive fertilisation. They don’t require much food, especially when planted in fertile soil. A sprinkle of specialist ericaceous food around the base of the plant during early spring should deliver enough nutrients to encourage plenty of blooms.
Pruning
You should deadhead your rhododendron regularly through the blooming season to encourage new flowers and prolong the flowering time. Hedgerow rhododendrons should be regularly pruned to help them retain the desired shape and height. The best time to do this is after flowering has finished in late spring or early summer.
From cute and colourful dwarf varieties to great sprawling shrubs, and tall, elegant trees, there’s space in every garden for a rhododendron or two! Sure to provide a welcome feast of colour each year as their beautiful blooms open to signal the arrival of spring, rhododendrons are a must-have plant for any gardener.
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