Rhododendron Photos, Facts, and Care Tips
May is rhododendron month where I live. The flower buds on the rhododendron bushes open, producing beautiful, showy blossoms and a gorgeous display of color. This is a wonderful time of year for gardeners, photographers, and everyone else who loves the plants.
The word "rhododendron" comes from two Ancient Greek words—"rhodon", which means rose, and "dendron", which means tree. Despite the derivation of their name, most rhododendrons grow as shrubs, which are evergreen or deciduous. Some are wild plants, but there are many cultivated forms. These are popular with gardeners and landscapers because of their large and very attractive flowers.
I live in southwestern British Columbia, where the climate is perfect for cultivated rhododendrons. They grow well and abundantly here. Photographing the flowers is always enjoyable. Unless otherwise noted, all of the photos in this article were taken by me.
What Is a Rhododendron?
All rhododendrons belong to the genus Rhododendron. The genus is the first part of the scientific name of a living thing. There is a huge variety of plants within the Rhododendron genus. In addition, the different varieties easily hybridize, which can make classification difficult. Plant breeders frequently create new hybrids to produce flowers with interesting characteristics. Cultivated plants are generally identified by descriptive and attractive names such as Pink Pearl, Saffron Queen, and Paprika Spiced.
The cultivated rhododendrons in my area are evergreen plants with large leaves and spectacular flowers that open in late spring. Ornamental rhododendrons such as the ones shown in most of my photos have large, oval leaves with a glossy dark or medium green upper surface and a dull, pale green lower surface. The flowers are bell-shaped and are born in clusters known as trusses. They may be white, yellow, pink, red, or purple in color. Some types of cultivated rhododendrons bloom before or after May.
There are about a thousand species of rhododendrons. They belong to the family Ericaceae, which is often referred to as the heather family.
What Is an Azalea?
It may seem strange referring to azaleas in an article about rhododendrons, but azaleas are actually a type of rhododendron. The genus Rhododendron is divided into eight subgroups, or subgenera. Azaleas belong to two of these subgenera.
Azaleas are attractive flowering shrubs, but their flowers aren't as large or as showy as those of the rhododendrons that are so popular in my part of the world. Azaleas also have smaller leaves and are shorter in height. Ornamental rhododendrons can become very large plants and may reach the size of a tree. Most azaleas are deciduous while most ornamental rhododendrons are evergreen.
Another way of distinguishing the two plants is to look closely at the flowers. Azalea flowers tend to be more funnel-shaped than rhododendron flowers. In addition, an azalea flower has five stamens (the male reproductive structures) while an ornamental rhododendron has ten or more. The stamens produce the pollen grains. They surround the style, which is longer than the stamens and is part of the female reproductive structure.
All azaleas are rhododendrons, but not all rhododendrons are azaleas. Evergreen azaleas belonging to subgenus Tsutsusi, and deciduous azaleas are classified in the subgenus Pentanthera.— The American Rhododendron Society
Two Wild Rhododendrons
Rhododendron macrophyllum grows in the Pacific Northwest region of North America. This region is usually defined as the province of British Columbia in Canada and the states of Washington and Oregon in the United States. The common name of Rhododendron macrophyllum is the Pacific, Western, or Coast Rhododendron. The plant generally has pink flowers and is the state flower of Washington.
Rhododendron arboreum is the national flower of Nepal. It's known as laligurans or gurans in that country and forms magnificent mountain forests. At lower elevations the flowers are bright red. They change to pink or white as the elevation increases. The plant is also found in India, Sri Lanka, China, Thailand, Bhutan, and Myanmar.
Rhododendrons and Other Flowers in Nepal
Do not plant too deeply - all rhododendrons are surface-rooting and the roots should be just covered.— Royal Horticultural Society
Transferring the Plant from a Pot to the Soil
How to Grow Rhododendrons
Rhododendrons like a semi-shaded area with dappled sunlight. They will grow in an open area if the sun is not too hot, the soil is sufficiently moist, and there isn't too much exposure to wind. They don't do well in deep shade. The plant has shallow and fibrous roots that may be widespread. If it's planted in the partial shade of a tree, the tree's roots should be deep so that they don't interfere with the roots of the rhododendron.
Most varieties of cultivated rhododendrons require moist but well drained soil that is also slightly acidic. The soil should contain organic matter in the form of peat or compost. It should also be loose, crumbly, and well aerated. Rhododendrons require a good supply of water, but the soil mustn't be water-logged. Standing water can cause root rot. Mulch on the surface of the soil helps to retain moisture, discourage weed growth, and protect the roots from heat and cold. It's important that anything added to the soil—compost, fertilizer, or mulch—doesn't change its acidity.
Once a rhododendron is growing successfully it may need attention to keep it healthy and compact. Removing dead trusses, or dead heading, not only makes the plant look more attractive but stops it from producing large amounts of seed. This allows its energy to be used for other purposes and may increase its growth. Pruning can help the plant maintain a desirable shape.
Plant Care Tips
Because of their shallow roots, little or no cultivation should be done around rhododendrons.— The American Rhododenron Society
Grooming and Pruning Rhododendrons
The main toxin in rhododendrons is a chemical called grayanotoxin. This substance is quite similar in structure to pinene, a component of turpentine. Rhododendrons are pollinated by insects, including bees. The toxin is found in the nectar and pollen of the flowers and enters honey. Honey that contains a significant amount of grayanotoxin is sometimes known as "mad honey" because of its effects on people.
Honey from the Black Sea area in Turkey is especially prone to a high grayanotoxin level. An interesting story on the Kew Gardens website describes how this was historically useful for a particular ruler. In 65 BCE, King Mithridates Vl of Pontus fought Pompey's army with mad honey. Slices of honeycomb containing the toxin were distributed along the route of the army's approach. The soldiers stopped to eat the honeycomb and eventually fell into a stupor. The king's army then attacked and killed them.
Grayanotoxin is also present in the leaves of a rhododendron plant. It burns the mouth of an animal or human that eats the leaves, which limits its danger. Most animals avoid it. Our body can break the toxin down when it's ingested in small quantities, which also limits its effects. The toxin is harmful if a large amount is ingested, however. It causes stomach upset, nausea, and vomiting at first. Soon a low blood pressure develops, as well as muscle weakness and a lack of muscle coordination. Most people seem to recover from the effects of grayanotoxin, however.
A Variety of Flower ColorsClick thumbnail to view full-size
Some rhododendrons have delicately-colored blossoms while other have vibrant colors. The cultivar selection for gardeners and landscapers is lovely.
Finding the Flowers
In May it's hard to go for a walk in my neighborhood without seeing rhododendron flowers in someone else's garden. I find both rhododendrons and azaleas in multiple colors. Many landscaped areas, parks, and botanical gardens also have examples of the flowers for me to admire and photograph.
Plant stores and nurseries often have a wide selection of rhododendrons that can be purchased. They may also provide information about the plants and offer useful workshops. In addition, there are many rhododendron societies that provide information and growing instructions. Most of these societies have very helpful websites. It's easy for a flower enthusiast to find help.
References and Further Reading
- The American Rhododendron Society website contains a lot of helpful information. The society has chapters in the United States, Canada, and other countries.
- Useful rhododendron information can also be found at the website of the Royal Horticultural Society.
- Poisonous rhododendron nectar is described on a Kew Gardens webpage. The organization is also referred to as the Royal Botanic Gardens, Kew.
© 2013 Linda Crampton