Azaleas And Rhododendrons

Course CodeVHT106
Fee CodeS2
Duration (approx)100 hours
QualificationStatement of Attainment

Learn to Identify, Grow and Use Azaleas and Rhododendrons

Develop a broadened knowledge of Azaleas & Rhododendrons, their cultivars and cultivation; and in doing so, provide a foundation upon which you can better pursue your interest in this fascinating group of plants.

Considered by many to be the very best plants a garden can offer, azaleas and rhododendrons are some of the most popular plants grown anywhere. The course shows how to grow them to perfection, and builds your knowledge of varieties available, to give you a long lasting display of colour in the garden. Soils, weed control, feeding, pest and disease control, watering, landscape design and lots more are included over eight lessons

Lesson Structure

There are 8 lessons in this course:

  1. Introduction
    • Introducing Rhododendrons
    • Introducing Azaleas
    • Indica and kurume hybrids
    • Deciduous mollis hybrids
    • Review of the system of plant identification
    • Classification of Azaleas and Rhododendrons-sub genera
    • Information and networking
    • contacts (i.e.: nurseries, seed, clubs etc.)
  2. Culture
    • Soils for Azaleas and Rhododendrons
    • Most Common pest and disease problems with Azaleas and Rhododendrons
    • Other cultural considerations
  3. Propagation
    • Methods of propagating azaleas and rhododendrons
    • Using root stimulating auxins
    • Propagation of different types
    • Layering, cuttings, seed
  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.

What's the Difference between Rhododendrons, Vireyas, Azaleas and others? 
 
 
Azaleas and Rhododendrons are both in fact Rhododendrons. This was not always the case. Azaleas were considered to be a different species throughout the 18th and 19th centuries and were only grouped into the same genus with rhododendrons in the 1930’s
 
Today, there are around 800 acknowledged species of rhododendron; and many thousands of named hybrids and cultivars.
 
 
Rhododendrons belong to the plant family Ericaceae; as do many other common garden plants, including: Erica, Kalmia, Arbutus, Peris, Vaccinium (eg. Blueberry and Cranberry) and Gaultheria.  Most of these plants like acidic soils and high to medium levels of light (not heavy shade). Most prefer a temperate climate, though some are indigenous to arctic climates and others to tropical regions. Flowers are often conspicuous. Petals are commonly fused together. Corolla is often tubular to urn shaped. There are usually 10 stamens. Most plants in this family are shrubs or sub shrubs; occasionally epiphytic, and rarely trees.
 
 
Sub Groups
Botanically, rhododendrons are divided into eight sub genera.
 
Two of these sub genera (ie. Hymenanthes and Rhododendron) include all of those plants which we commonly know commonly as:
  • True Rhododendrons
  • Broad Leaved Rhododendrons
  • Asiatic Rhododendrons 
The remaining six sub genera include azaleas, and a few other species that have minimal significance in horticulture.
 
Species found in the sub genus “Rhododendron” are distinguished by the fact that the leaves have small scales on the lower surface. This characteristic is described as “Lepidote”.
 
Elepidote species are those that have no scales on the lower leaf surface.
True rhododendrons that are Elepidote (lack scales) are members of the sub genus “Hymenanthes”.
 
 
Other sub genera that are elepidote include:
 
  • Pentanthera –These are deciduous azaleas. Ecampoles include R. luteum. R. molle, R. nipponicum
  • Tsutsutsi – Evergreen azaleas
  • Azaleastrum 
  • Mumeazalea (includes R. semibarbatum)
  • Candidastrum (includes R. albiflorum)
  • Therorhodion (includes R. camtschaticum and R. redowskianun)
 
 

Rhododendron Hybrids
 
The earliest rhododendron hybrids date back to around 1825 with the introduction of R. arboreum. Since then a large number of hybrids have been produced using different species in different regions. Hybrids have been derived from: R. catawbiense and R. maximum: USA, R. arboreum: Himalaya, R. caucasicum: Caucasus, R. ponticum: Turkey, and to a smaller degree R. fortunei: China, and R. griffithianum: Himalaya. The majority of the hybrids derived from these plants are hardy, have dense foliage, and produce strong trusses laden with flowers. As such, they are referred to as the Hardy Hybrids.  Hybrids are particularly useful as informal hedging or screening plants. Hybrids more recently developed from R. yakushimanum tend to be smaller and so are ideal where space is limited.
 
In the early nineteenth century many colourful and ornamental hybrids were developed from species found in China and the Eastern Himalayan region. These hybrids exhibit a wider variation in foliage, habit, and flowers but are less tolerant of extreme conditions than the popular Hardy Hybrids. 

 
Hybrid rhododendrons range from prostrate alpines to small trees and their average height is 1.8-3m tall.     
 
 
 
Examples of Hybrid Rhododendrons
 
R. ‘Cornubia’ - This is a sparsely leaved large shrub or tree to 4m tall. It is a strongly growing plant with bright scarlet red bell shaped flowers, lasting for 2 months from early spring. It needs a sheltered spot. 
 
R. ‘Fragrantissimum’ - This is a beautiful mid-sized shrub to 2m tall producing large fragrant wide funnel shaped white flowers with a rose blush up to 7.5cm long. The stamens have brown anthers. Leaves are dark green and corrugated with a paler underside. They are not dense but have an open habit. These plants are heat tolerant in warmer climates, but in temperate climates need to be in a conservatory with the exception of the mildest areas.
 
R. ‘Beauty of Littleworth’ - This was one of the first R. griffithianum hybrids and remains one of the best Hardy Hybrids. It is a spectacular large shrub with huge conical trusses of white flowers with crimson speckles.   
 
R. ‘Isabella’ - This is a spectacular Leonardslee hybrid. It is a large shrub or small tree with oblong-lanceolate leaves up to 20cm long.  It produces large trusses of huge white, scented, trumpet shaped flowers with a frilled edge, and rose coloured speckling near the base. Flowering is in early summer.
 
R. ‘Princess Anne’ - This is a very attractive hybrid dwarf shrub having a dense habit. The leaves are a light matt green and are elliptic, slightly pointed around 6cm long. The 3cm wide funnel shaped flowers appear in early to mid-spring and are pale yellow with faint green speckles. This plant is also known as ‘Golden Fleece’.
 
R. ‘Purple Splendour’ - This is a strong, dense, erect bush to 1.7m tall. The deep purple violet flowers have raised black blotches, and are borne prolifically in late spring to mid-summer. 
 
R. ‘Trewithen Orange’ - A spectacular hybrid with loose trusses of dark orange to brown flowers with a faint rose blush appearing in late spring to early summer.  
 
R. ‘Venetian Chimes’ - A vigorous growing small shrub with a compact spreading habit. The flowers which appear in late spring to early summer are bell shaped, 5cm wide, and form trusses up to 13cm across. Flowers are earth red, scarlet towards the base and speckled with dark black red.
 
R.’ Windlesham Scarlet’ - A fast growing medium sized shrub with a compact habit. Flowers appear on domed shaped trusses and are widely bell shaped, frilled, and deep crimson with black speckles inside.  
 
 
 
Rhododendron Species
 
Aside from the hybrids there are many species of rhododendron which are commonly grown. One of the first species known to be grown outside of its native country was R. hirsutum which was introduced into the UK from Europe in 1656. Other species were taken to the UK by Sir Joseph Hooker in 1849-51. These included R. campanulatum, R. campylocarpum, R. falconeri, and R. thomsonii. As with hybrid rhododendrons there is much variety amongst the natural species. They vary from tiny prostrate alpines to trees with huge leaves. Dwarf species make excellent rock garden plants. Some of the more popular species are outlined below. Most rhododendron species do not flower until well established. 
 
 
 
 
 

More from ACS