Azaleas & Rhododendrons

Horticulture College -Learn to Identify and Grow Plants

Develop a broadened knowledge of Azaleas & Rhododendrons, their cultivars and cultivation.  

Azaleas and Rhododendrons are actually in the same botanical category (i.e. genus Rhododendron).  Azalea is a common name used to describe many smaller growing shrubs from the genus Rhododendron.  Broadly speaking, azaleas have both smaller leaves, and generally a lesser height.

Lesson Structure

There are 8 lessons in this course:

1. Introduction
   • Review of the system of plant identification, general characteristics of the group, information contacts (i.e.: nurseries, seed, clubs etc.)
2. Culture
   • Planting, staking, mulching, watering, pest and disease, feeding, pruning, protection from wind, salt, air etc.
3. Propagation
   • Methods of propagating this group of plants. Propagation of selected varieties.
4. The most Commonly Grown Varieties.
   • R. arborescens -a very popular species
   • Indica hybrids
   • Kurume hybrids
   • Mollis hybrids
   • Other deciduous hybrids
   • Azaleodendrons
   • Uses for Azaleas
5. Other important groups.
   • Scope
   • Rock Rhododendrons
   • Vireyas
   • Other tropical Rhododendrons
   • Hybrids
   • Cultivated Rhododendron species
6. Lesser Grown Varieties.
   • Obscure species
   • Varieties that have become less popular
   • Alpine Roses
   • Yak Hybrids
   • Lesser grown Azaleas
7. Making the best use of these plants. In containers, in the ground, as indoor plants, growing and showing, growing for profit.
8. Special Assignment
   • A study of one selected plant or group.

Each lesson culminates in an assignment which is submitted to the school, marked by the school's tutors and returned to you with any relevant suggestions, comments, and if necessary, extra reading.

Aims

• Discuss how Rhododendrons and Azaleas are classified.
• Describe the general cultural requirements that are common to all Rhododendron species.
• Select appropriate materials for propagation
• Propagate Azaleas and Rhododendrons.
• Describe species of azalea are most commonly grown.
• Describe a range of common varieties of Rhododendrons.
• Conduct valid research into lesser known varieties of Rhododendron and Azalea.
• Determine various uses and applications of rhododendrons in the home garden.
• Demonstrate the knowledge acquired for a specific group or individual plant in the Rhododendron group of plants.

Tips for Growing Rhododendrons

When you consider that rhododendrons are found over a wide geographical range it is not surprising that there are numerous species available that vary in habit, size, and flowering characteristics. Although the majority of species are naturally found in China and the Himalayas, species also occur naturally in North America, South East Asia, New Guinea, Australia, Korea, Japan and parts of Europe.

The tropical species from Northern Australia and New Guinea tend to flower throughout the year with a flush of flowers in autumn. These plants have only recently become popular and are now considered collectors items in many circles. In contrast, those species from temperate climates which only flower during late winter through to early summer, have long been sought after.

Flower colour and size changes considerably between species from single coloured to multi-coloured flowers some with speckles and variances in the throat to tiny single coloured flowers. However the overall appearance or form of both the leaves and flowers tends to be similar and are instantly recognisable as belonging to the rhododendron genus.

Some tropical species grow high in trees as epiphytes similar to orchids. Obviously, climate plays an important role in the choice and variety of species available to the horticulturalist and home gardener. Less hardy species will be difficult to grow in open land in cooler climates and may have to be kept as greenhouse specimens. Equally, some temperate climate varieties will struggle in the intense heat of tropical and sub tropical climates.

The fungus, phytophthora, which inhabits soil, has proved a bane to many rhododendron growers. It causes disease in the plant which is witnessed through wilt and root rot. The disease is more prevalent where soil moisture levels and heat are high. However, a number of rhododendron species and hybrids have moderate to high levels of resistance to the disease.

A Brief History
The “Alpine Roses” as they were commonly called (R. hirsutum or R. ferrugineum) were first cultivated around the mid 1600s. One hundred years later Linnaeus officially designated Rhododendron as a genus. Azaleas were classified as a separate genus until the 19th century when George Don ascertained that the two plants were far too similar to belong to separate genera. They were then brought together under the one genus, Rhododendron.

The Rhododendron genus is divided into three groups the first and second groups are what we commonly know as rhododendrons the third group comprises the azaleas.
• The first group is further divided into two sub groups: those with and without scales on their leaves. Lepidotes have scales. These plants are small and evergreen with scaly leaves and stems. Elepidotes do not have scales, the plants are large, some being trees, and have smooth leaves and stems.
• The second group are the evergreen Vireya also known as Malesian rhododendrons. They have scales and grow in tropical areas of South East Asia.
• The third group are the azaleas and are predominantly small evergreen or deciduous plants, with small leaves, depending upon the species.

It is generally not possible to cross breed between the above three groups. However, crosses occur readily between species from within each group.

Apart from the 800 or so species available there are a multitude of hybrids in cultivation and also many that are no longer popularly cultivated. These hybrids are often grouped into what is termed an ‘alliance’ to make it easier determine the parentage of the various plants. An alliance is a group of similar rhododendrons that also have offspring: seedlings, with similarities to the parent. For example: the R. kiusianum alliance. Hybrids usually have the same growth and leaf characteristics as the parent plant but may vary considerably in the flowers.

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