Categories: Featured How to

How to Grow Rhododendrons

Rhododendron care may take some patience, given that there are so many varieties of the plant; thousands in fact. Some grow in the wild; others, the lush hybrids, are seen in people’s gardens and yards. Most rhododendrons are bushes between 3 and 10 feet tall. Some varieties, however, grow to tree-like heights of 70 feet or more.

Along with the many varieties of rhododendrons, the azalea is often thrown into the mix. The azalea is a close cousin to the rhododendron, although azalea care is, in some respects, rather different than rhododendron care.

History of Rhododendrons

From the middle of the eighteenth century to the early twentieth century, the British have dominated the gardening world. The English landscape garden, as it developed under Capability Brown, was composed of grass, water, and native trees.

Cedars of Lebanon were the only non-native trees that were commonly planted. This austere attitude frustrated people who wanted collections of plants, and by the late eighteenth century, it became increasingly fashionable to grow non-native plants in landscape gardens.

Huge numbers of plants were introduced to Great Britain during the nineteenth and twentieth centuries. It became fashionable for estates to contain a number of specialized gardens, such as rose gardens and herb gardens.

The earliest of these specialized gardens was the “American” garden. “American” gardens were collections of magnolias, kalmias, rhododendrons, and other peat loving plants. American rhododendrons and magnolias were introduced earlier than their Asian cousins.

The first American rhododendron to be grown in these “American” gardens was Rhododendron maximum, introduced in 1736 by Peter Collinson. Peter was a Quaker who had a partnership with an American plant collector named John Bartram. Peter financed John’s plant collecting expeditions by selling American plants to British gardeners.

Rhododendron catawbiense grows along the banks of the Catawba river in North Carolina, where it was discovered in 1809 by John Frasier and his son when they were collecting plants for the Emperor Paul of Russia. It was soon being grown in “American” gardens.

These were not the first rhododendrons to be grown in British gardens. The first introduction was Rhododendron hirsutum, which was introduced from the Alps of Central Europe in 1656. Rhododendron ferrugineum (the Alpine Rose) was introduced from Switzerland around 1740 and Rhododendron Ponticum was introduced from Spain in 1763. Rhododendron caucasicum was introduced from Turkey in 1803.

The first Asian Rhododendron to be introduced was Rhododendron arboreum from the Himalayas. Its seed was sent to England in 1811 and it first flowered in 1825. All of these species were grown in the “American” gardens. “American” gardens had never been purely American, they were collections of plants which were grouped together because they shared similar environmental and cultural needs.

As increasing numbers of Asian rhododendrons and magnolias were introduced, the “American” garden died a slow death. By the First World War very few of the British talked about “American” gardens; they talked about having a woodland garden or a rhododendron garden.

There are over 600 species of Rhododendron and there are rhododendron flowers in every color except true blue; all blue rhododendrons are at least slightly lavender. Out of these species, plant breeders have created thousands of hybrids. The main advantage of hybrids is that you are combining the genes of two plants that evolved in different environments, so hybrids tend to be more adaptable than species.

The first hybrid rhododendrons were created during the early 1830s by Anthony Waterer, at Knapp Hill in Surry, England. He crossed the rather tender Rhododendron arboreum with the much more hardy Rhododendron caucasicum. The result were plants which were hardier than Rhododendron arboreum and which had the advantage of repeat blooming.

These hybrids are still among the earliest blooming rhododendrons; they bloom when there is a strong possibility of frost. Repeat blooming means that unlike most rhododendrons, all of the flowers don’t open at once; instead, they open in succession.

If the first blooms get damaged by frost, there are still flower buds that aren’t damaged. Waterer named this hybid family the ‘Nobleanum’ grex. A grex is a family of hybrids which result from the crossing of two species rhododendrons, it even includes crosses of the same species done by different breeders. This term became very confusing and it is no longer used by rhododendron breeders, it is only used to describe antique hybrids.

The vast majority of rhododendrons are native to Asia, especially in the vicinity of the Himalayas. Many of these were introduced to Europe during the nineteenth century. Wealthy estate owners valued them as garden plants.

Most English estates contained groves of oaks, which provided ideal conditions for the latest arrivals. This is the most effective sort of rhododendron garden, creating a fantasy world unlike any place on earth. In nature the different species are usually separated from each other, that is why they became different species.

When different species of rhododendrons meet in nature, they often produce natural hybrids. Wealthy estate owners such as Lionel de Rothschild competed with each other in the crossing of Asian species with other Asian species to create exquisitely beautiful hybrid rhododendrons. These rhododendrons are often referred to as “species hybrids”. Lionel would raise a million seedlings on his estate at Exbury, employing a staff of over 100 gardeners. Out of these seedlings, only a few would be selected as worthy of growing, while most went on a burn pile.

The commercial rhododendron breeders focused on creating hardy rhododendrons with huge, brightly colored blooms. To achieve this goal they often crossed Rhododendron catawbiense with Asian rhododendrons. The most adaptable of these rhododendrons are called “hardy hybrids”. It became fashionable to line driveways with them.

By the 1920s there was a reaction against these “brightly colored cabbages” on bushes which were just mounds of foliage for much of the year. Collections of species rhododendrons were still admired, as much for the beauty of their foliage as their flowers, but the arbiters of good taste sneered at most hybrid rhododendrons, calling them “the stockbroker’s joy”.

How Many Species and Varieties of Rhododendron are There

There are over 300 species of the rhododendron and, as mentioned above, thousands of varieties. There are over 10,000 different named varieties of the plant. Rhododendrons can be grown in most climates, except for hot desert climates and in places where winters are very long and cold.

The azalea is a bit fussier, generally requiring a warmer climate than the rhododendron. Hybrid rhododendrons are generally showier than the natural growing types, but wild rhododendrons, such seen in Washington State, the state flower, can be quite attractive. Some varieties growing in the wild in Southeast Asia are even more beautiful.

How to Grow Rhododendrons

All rhododendrons are acid-loving plants. They will perform poorly, or not grow at all, in alkaline soil. People growing rhododendrons usually prepare the soil to give it a slightly acidic content, and then periodically treat the plant with an acidic fertilizer to maintain a proper pH balance. Once a plant is firmly established, rhododendron care becomes less of an issue. Given the right climatic conditions, these plants sometimes seem to thrive on neglect.

Just as important as the pH level of the soil, the way the soil is prepared is key when planting a tree or bush. Proper rhododendron care dictates that the soil drain quickly, yet retain moisture. The rhododendron is a thirsty plant that does not like to get its feet wet. To the beginning gardener, this may seem to be a bit of a challenge.

Mixing peat moss into well-prepared garden soil usually accomplishes the moisture retention and drainage objectives, while at the same time making the soil more acidic. Put another way, preparing the ground for a rhododendron plant does not require a degree in horticulture. Locate the plant in a sunny, well-drained area. Mix some peat moss and compost in with the dirt, and keep the plant moist until it becomes well established.

After that, caring for your rhododendrons consists mainly of picking off spent blossoms and occasionally pruning back a plant that may have grown larger than you intended. An annual feeding of commercial acid fertilizer certainly won’t hurt either.

A final planting tip: In areas where strong winds are the norm, plant rhododendrons in a sheltered location whenever possible. Rhododendrons can be damaged by windburn, especially when temperatures are very cold.

In places where the soil does not seem to lend itself to growing rhododendrons because it contains too much clay, or is too alkaline, or both, consider growing the plant in a raised bed. The rhododendron has a rather shallow root system, so a combination of a raised bed, with some of the original clay/alkaline soil beneath the bed removed, will usually work out just fine.

Smaller varieties of the plant can serve very well as container plants. Those who live in desert climates find that they can have great success growing smaller rhododendron varieties in containers by keeping the plants away from the hot late afternoon sun.

Recommended Rhododendrons

For early blooming:

  • ‘Christmas Cheer’: evergreen, compact shrub with pink flowers. Height and spread 3 feet.
  • ‘Rosa Mundi’: evergreen compact shrub with pale pink flowers. Height and spread 4 feet.
  • ‘Sierra Del Oro’: evergreen with glossy green leaves and light yellow flowers. Prefers a shady location. Height and spread 3 feet.
  • ‘PJM’: evergreen dwarf compact with bright, lavender-pink or pink-purple flowers. Height and spread 4 feet. Leaves turn mahogany purple color in the fall.
  • ‘Taurus’: evergreen with large, very dark green leaves and red flowers. Height and spread to 6 feet.

For a touch of the unusual:

  • ‘President Roosevelt’: evergreen shrub with variegated leaves (splashes of yellow on dark green). Funnel-shaped bright red flowers in early mid-season. Height and spread 6 feet.
  • Azalea Nol’s Variegata: evergreen with variegated leaves and bright pink flowers in early midseason. Height and spread 3 feet. Developed by Green Thumb nurseries in Nanaimo.

For exposed sites:

  • ‘The Honorable Jean Marie de Montague’: evergreen shrub with medium-sized leaves and funnel-shaped crimson-red flowers in mid-season. Tolerates heat, sun, and an exposed site. Height and spread 8 feet.
  • ‘Grace Seabrook’: evergreen shrub with medium-sized leaves and open, funnel-shaped crimson-red flowers in mid-season. Very tolerant, and makes an excellent hedge. Height and spread 8 feet (?).
  • ‘Anah Kruschke’: round evergreen shrub with medium-sized dark green leaves. Lavender flowers in mid-season. Height and spread 6 feet. Very tough; an ideal hedge or mass planting choice.

Tried and True:

  • ‘Unique’: evergreen shrub with rounded, medium-sized leaves. Produces funnel-shaped light cream-yellow flowers in early mid-season. Lovely mound-shaped form. Height and spread 4 feet.
  • ‘Hotei’: evergreen shrub with rounded medium-sized leaves. Funnel-shaped deep yellow flowers in midseason. Height and spread 5 to 8 feet. The best of the true yellows. Plant in full sun location for best flowering.
  • ‘Purple Splendor’: evergreen shrub with medium-sized leaves. Funnel-shaped deep blue-purple flowers with a black basal mark inside in late midseason. Height and spread 10 feet.
  • ‘Scintillation’: evergreen shrub with large, waxy curling leaves. Large funnel-shaped bright pink flowers in midseason. Height and spread 5 feet.
  • ‘Point Defiance’: evergreen shrub with large, leathery dark green leaves and huge white flowers with a picotee pink edge in midseason. Upright grower. Height and spread 6 feet in 10 years.

Dwarf Rhododendrons

When most people think of a rhododendron garden, they picture a woodland garden or a shrubbery planted with the larger rhododendrons. There is a third sort of rhododendron garden which evolved out of the tradition of creating rock gardens.

Rocks have always played a central role in the gardens of China and Japan, but the Western rock garden is a fairly recent development. Before the Romantic Movement of the late eighteenth century, there was very little interest in alpine plants.

Mountains were seen as barriers to cross. Shepherds and hunters explored them, but most people saw mountains as dangerous places, to be avoided whenever possible. The romantics of the late eighteenth century fell in love with the wild beauty of the Alps, and Switzerland developed thriving tourist industry. This was also the period when rockeries first became popular in European gardens.

There is a tendency for English speaking people to focus too much on British gardens. The British dominated the gardening world from the middle of the eighteenth century to the early nineteenth century. The British are still a major force in the gardening world; many of the top “gardening gurus” are British.

It is easy to ignore the fact that some of the most important romantic gardens were created in Germany. The Germans created some very dramatic gardens which featured water cascading among huge stones. The late eighteenth British rockery tended to be on a more modest scale and was often a display of various kinds of rocks rather than a garden.

Many alpine plants were introduced to cultivation during the nineteenth century and people began to plant them among rocks. The first rock gardens were rather unsophisticated, there was little attempt to arrange the rocks in a naturalistic manner.

By the late nineteenth century, designers were arranging rocks to look like natural outcroppings. The fashion for rock gardens reached its peak during the 1920s and they have remained popular, especially where large rocks are used to retain banks.

Most, but not all dwarf rhododendrons are true alpines that grow in large colonies above the treeline. The word “dwarf” is a bit deceptive. Some dwarf rhododendrons form mats of foliage 2 inches (5cm) tall; most grow from one to four feet (30cm – 1.20 m) tall.

It isn’t unusual for dwarf rhododendrons to reach 6 feet (2m) in height. Rhododendron minus can reach 30 feet (10m) in the wild, but in cultivation, it rarely tops 6 feet (2m). This is also one of the dwarfs which are not a true alpine; it is native to forests on the mountains and plains of the Southeastern United States, from Carolina to Alabama.

Alpine rhododendrons can be tricky to grow in lowland gardens. They prefer full sun, but the sun which is usually filtered through cool mists. I live on the southern Oregon coast; we have very cool summers and the sunlight is usually hazy, so I can grow alpine rhododendrons in full sun.

In most of the United States, alpine rhododendrons prefer a bit of shade. Shade protects them from heat and strong sunlight, but it also makes them less compact and they won’t bloom as heavily as they do in full sun. The trick with all rhododendrons is to give them as much sunlight as the leaves will take.

Too much sun scorches the leaves, too little reduces the number of blooms. The design of all rhododendron gardens is dictated by the needs of the plants, so it is best to plant a few in different amounts of sunlight. Find the best locations before you plant huge masses of them.

Dwarf rhododendrons are often used as foundation plantings. They are the right size for this, but most foundations are concrete. Lime from the concrete leaches into the soil and most rhododendrons hate lime. You can grow them next to foundations but you will need to be careful about maintaining a pH between 4.5 and 5.5, if it goes above that level most rhododendrons will have difficulties absorbing nutrients.

If the garden soil is acidic this need not be a problem. Dwarf cultivars of Rhododendron are perfect for pots and containers. Use ericaceous compost, put stones or crocks in the bottom of the pot to aid drainage and water regularly until the plants are established.

There are hundreds of varieties encompassing nearly every flower color:

  • Rhododendron ‘Blue Tit’ makes a compact shrub with small mid-green leaves and trusses of 2 or 3 funnel-shaped, grey-blue flowers at the top of the shoots.
  • Another attractive choice is ‘Baden-Baden’ with its cherry-red flowers.
  • ‘Scarlet Wonder’ is exceptionally free-flowering and carries loose trusses of funnel-shaped, slightly frilled, bright cardinal-red flowers.
  • ‘Shamrock’ is dwarf and spreading. The pale green buds open to trusses of 5-9 funnel-shaped pale yellow blooms.
  • Variegated foliage is unusual in Rhododendrons. ‘Molten Gold’ which reaches 1 to 1.2 meters tall has golden splashed centers to its leaves. Light purple flowers are displayed in May/June.
  • Hybrids of rhododendron yakushimanum such as ‘Aloha’ are great for pot culture. They have a tight domed shape with dark green leaves. An added attraction on some cultivars is the rusty, felty surface on the underside of the leaves.

Pests and Diseases

Rhododendrons tend to be relatively pest and disease-free. If yellow leaves appear, it is often a sign of an iron deficiency. This deficiency is known as chlorosis and is easily cured by spraying the plant with an iron solution.

As far as pests are concerned, root weevils can at times be a problem. An application of diazinon or malathion quickly takes care of those annoying critters. If root weevils are allowed to hatch, the adult weevils may feed on the leaves of the plant, but there are usually many more leaves than the weevils can deal with. You may notice a few leaves with notches and nothing more.

If you feel there are too many notched leaves on your plant, spray it with Orthene.

Pruning and removing spent blossoms was mentioned earlier as on-going rhododendron care for established plants. Usually, the blossoms break off easily.

Just be careful not to break off the buds, which are next year’s growth, that lie just under the blossoms. Light pruning can be done at any time, but heavy pruning is best done in the spring. Light pruning and pinching off new growth will usually result in a more compact and bushy plant.

Rhododendrons are, for the most part, a pleasure to work with. Once you find the right choice of plants for your garden, the plants will provide a lifetime, or at least a good many years, of enjoyment.

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