Rhododendrons are grown in many North American and European gardens; however, the rhododendron species originated from the East, brought to Europe by plant explorers.
The rhododendron species belong to the Ericaceae plant family; rhododendrons are botanically related to azaleas, blueberries, heathers and mountain laurels. The name rhododendron is derived from the Greek word rhodos (meaning rose) and dendron (meaning tree).
There are over 1000, natural species of rhododendron; however, many rhododendron species found growing in North America and Europe today are hybrids.
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Rhododendrons are attractive evergreen flowering shrubs giving stunning displays of huge, brilliantly coloured blooms, but most species will only grow well in acid soil.
Rhododendrons, and the closely related azaleas, have almost everything; they’re fully hardy, and most have glossy evergreen foliage (although some are deciduous). The flowers are tubular or trumpet shaped, gathered in large trusses.
In some varieties the individual trumpets may be up to five inches across. They appear in a wonderful range of colours – white blue purple gold orange or red blooms, often bicoloured, or with splashes of contrasting shades. Some, too are fragrant.
Individual rhododendrons have a relatively short, albeit brilliant, flowering season. However, collectively they produce a display which can last from early spring to early summer. For the rest of the year their rich green glossy foliage, like that of holly, provides an attractive backdrop for other ornamental flowering shrubs.
Their only disadvantage is that almost all of them need a lime free soil. With this qualification, there’s a rhododendron for every garden, however large or small. The largest, such as the rich red Rhododendron barbatum may reach a towering 40ft (12 metres), while dwarf varieties may be as small as 1ft (30cm)tall.
Botanical Description of the Rhododendron Species
The Rhododendron Species is a diverse plant; rhododendrons grow as trees (some over 100 feet high), as bushes and as low ground cover.
The leaf of a rhododendron is just as diverse as the plant itself; rhododendron leaves can grow over three feet long or be less than a quarter of an inch. Rhododendron leaves grow in a variety of shapes too.
Rhododendrons flower from January through August, depending on the location and the climate.
Native Habit of Rhododendrons
The rhododendron species is native to Asia, particularly in the Himalaya mountains, northern India and western China; the rhododendron species was one of the plant specimens, together with the sweet box plant species, collected by Joseph Hooker, a director at Kew Gardens, on one of the many plant exploration trips to the East.
Today, there are many rhododendron hybrids which are native to North America and Europe, as a result of cross pollination of many rhododendron species.
Suitable Climates for Rhododendrons
Rhododendrons grow in a wide range of climates; native rhododendrons are found in both arctic and tropical places in the world. Rhododendrons usually require adequate rain fall and moderate temperatures to flourish most successfully.
Depending on the hybrid rhododendron species, rhododendrons grow well in the following areas in North America and Europe:
- Pacific northwest
- British Columbia (Canada)
- San Francisco area
- southern California
- west coast of the United Kingdom
Azalea or Rhododendron?
Azaleas actually belong to the Rhododendron genus and the botanical name for azaleas includes Rhododendron in the species name.
Azaleas may be evergreen or deciduous; evergreen species are native to southeast and central-east Asia whereas deciduous azaleas are native to Japan, China, parts of Eastern Europe and east and west Canada and the United States.
Many of the smaller rhododendron species resemble evergreen azaleas; however, most rhododendron flowers have 10 stamens and wider leaves whereas most azalea flowers have 5 or 6 stamens and narrow, pointy leaves.
As always, there are may be exceptions to this rule, so it is always best to check when identifying the difference between a rhododendron and an azalea.
Rhododendron Species and Hybrids
There are more than 900 different species of rhododendron, and, as it’s very easy to create hybrids, many hundred more different varieties in cultivation. Rhododendrons are a diverse and interesting plant species with beautiful flowers; the rhododendron species grows in various climates.
Choosing from such a vast selection isn’t easy, but theses popular and widely available varieties give an idea of the assortment available:
- Rhododendron ‘Albert Schweitzer’. Large Hybrid. Fine deep pink buds open to rosy-pink flowers with a dark reddish blotch in late May. A tough and hardy variety with an upright habit.
- Rhododendron ‘Blue Diamond’. Dwarf Hybrid. Mauve blue flowers in late spring.
- Rhododendron ‘Carmen’. Dwarf Hybrid. Deep red flowers in late spring. Not suitable for very cold areas.
- Rhododendron ‘Cunningham’s White’. Medium Hybrid. Pink buds open to white flowers with green or brown flecks. Flowers late spring/early summer.
- Rhododendron ‘Harvest Moon’. Medium Hybrid. Heavy trusses of cream/yellow flower with a pinkish flare in late spring.
- Rhododendron ‘Praecox’. Medium Hybrid, small leaved hybrid, and probably the earliest rhododendron to flower. Pink/purple clusters of flower in early spring.
- Rhododendron maddenii – large, fragrant flowers; native to Nepal, Thailand and southeastern China
- Rhododendron ciliatum – large, funnel flowers; native to Nepal, Tibet and Bhutan
- Rhododendron triflorum – small, funnel flowers in various shades of yellow: native to the Himalayas, northern India and south east Tibet
- Rhododendron edgeworthii – fragrant, white/pink flowers; native to the Himalayas.
Selecting a Rhododendron Plant
Rhododendrons bloom practically all year so the surest way to get the best plant possible for a certain area is to buy it when it is in flower.
Check the growing height details carefully. Rhododendrons can be dwarf, (Percy Wiseman, about 3ft.tall) right up to 20ft. tall trees. The colors available are pink, mauve, white, lavender, red, salmon and purple. The flowers are in clusters which cover the entire shrub.
Growing and Care Rhododendrons
Big or small, Rhododendrons can give punch to your landscape with their glorious flowers in spring.
Rhododendrons are the beautiful large bushes that flower with pom-pom like blooms during the spring. They have been popular for years and years and will continue to be because of their hardiness, ability to grow very large, and blooming capacity.
Rhododendrons and Azaleas are related but the one difference is that in most cases the Rhododendron is an evergreen and will not lose its leaves in the winter while the azalea does.
Rhododendrons are larger than Azaleas having the ability to grow to up to 80 feet in height and their blooms are much larger. Rhododendrons have a lance shaped leathery leaf while Azaleas have smaller, softer leaves.
Since it takes about 5 to 10 years to produce a bloom on a plant grown from seed it is much better to purchase a plant at your nursery. Taking a graft from an established plant is possible but home gardeners rarely have the equipment to do this.
Planting and/or transplanting should be done in early spring or early fall. Plant a nursery plant by digging a hole 2 to 3 times the size of the root ball. The top 1 or 2 inches of the root ball should be above the soil line. Be sure to mulch after planting.
Rhododendrons are acid lovers. They love a pH of about 5.5. Mulch them with pine needles (acidic) as their roots tend to lie close to the surface of the earth and dry out if not mulched. They do not appreciate bark or wood mulch.
They also enjoy shade and don’t particularly like to be planted where the sun would hit them directly. They do like moist, yet well drained soil and do very well in raised beds where the water will drain readily.
You may see many Rhododendrons planted near buildings. That is because wind tends to damage them and it is best to plant them near buildings to give them protection. Their roots are very shallow and wind will knock over a large plant very easily since there isn’t much to hold it in the ground.
Flowers will appear in the spring but if your plant is placed in a densely shaded area it may not bloom. Full sun will cause the plant to bloom but will also attract the pests to the plant and too much sun will cause other problems as well. This is a shade loving plant so remember to never plant a Rhododendron in full sun.
Never cultivate under a rhododendron as you will have the risk of disturbing the shallow roots.
Prune right after blooming. Rhododendrons set bloom by mid summer so if you prune past June you risk cutting off all blooms that would develop the next year. If you need to prune back an unruly bush it may not bloom for a year or two.
When rhododendrons are planted in a location that is not suited for them they can develop yellowing leaves that will fall off. This is called chlorosis and can be caused by an alkaline soil, potassium, calcium or iron deficiency, to much nitrogen caused by nitrate fertilizers, too much sun, and too much water and no drainage.
Rhododendrons are left alone by most pests but if you detect one call your local nursery to find the best antidote for them. There are other diseases that can harm rhododendrons and make their leaves turn a different color and fall off.
Check your nursery to find the best cure and what works best in your area. If you plant a tender variety of rhododendron in a cold area you may see damage from the cold. The leaves will be brown and brittle. Cut those leaves and branches off once the weather warms up.
Rhododendrons rarely need fertilization and if it is done it should be done carefully as their close to the surface roots can be harmed by too much fertilizer. Fertilizing should be done in May and not after the end of June as this would cause new growth that would be killed by the cold of winter. Do not fertilize at initial planting as this could harm the roots.
Interestingly enough established Rhododendrons that may not look as green as they should do well with a tea of 2 tablespoons Epsom Salts in 1 quart of water.
Spray this on the entire plant and let it drip onto the soil beneath. This will green up the leaves and make the plant look healthy. Some people say that putting coffee grounds around the bottom on the soil benefit the plants as well. It certainly would make the soil more acidic.