Become an expert with Rhododendrons and Azaleas
- Learn to identify and grow different rhododendrons and azaleas
- Nurture, maintain, prune, water, fetrilise, propagate rhododendrons
- Explore your passion, improve your career, work in horticulture
A Brief History of Rhododendron Culture
Rhododendrons (R. hirsutum or R. ferrugineum) were originally cultivated around the mid 1600s; and called "Alpine Roses". One hundred years later Linnaeus officially designated Rhododendron as a genus and Azalea as a separate genus. In the 19th century 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.
Given that there are so many hybrids around which are able to boast: improved hardiness, more and longer lasting flowers, and a variety of sizes to suit all gardens, many of the natural species of rhododendrons are among those that have become less favourable. This is a shame because many of them are truly stunning plants. It seems that only the hardiest have survived the sways of fashion.
There are 8 lessons in this course:
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.)
Soils for Azaleas and Rhododendrons
Most Common pest and disease problems with Azaleas and Rhododendrons
Other cultural considerations
Methods of propagating azaleas and rhododendrons
Using root stimulating auxins
Propagation of different types
Layering, cuttings, seed
The most Commonly Grown Varieties.
R. arborescens -a very popular species
Other deciduous hybrids
Uses for Azaleas
Other important groups.
Other tropical Rhododendrons
Cultivated Rhododendron species
Lesser Grown Varieties.
Varieties that have become less popular
Alpine Roses R. hirstulum, or R. ferrugineum)
Lesser grown Azaleas
Making the best use of these plants. In containers, in the ground, as indoor plants, growing and showing, growing for profit.
A study of one selected plant or group.
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.
An Introduction to Rhododendrons and Azaleas
Origin: Europe, Asia, North America, extending south, though some are found in tropical South-East Asia with one species being native to Australia (R. lochae); between 500 and 900 species.
Appearance: Shrubs and trees, mostly evergreen but some deciduous; most flower in late spring to early summer with bright, showy, floral displays. The rhododendron genus includes azaleas.
Culture: All rhododendrons prefer fertile, freely-draining, moist, acidic organic soil; lime-haters; a protected position in part shade is best. Areas prone to hot, dry winds should be avoided; tolerant of air pollution. Hardy hybrids and alpine species can be planted in more open sites in full sun. Like other rhododendrons, azaleas prefer acidic, fertile and moist soil. Feed with well-rotted manure or compost at least twice annually, especially in autumn. Pruning is not necessary as both rhododendrons and azaleas tend to keep a compact shape, however, light pruning annually may be undertaken to maintain shape and improve vigour. Deadheading is best done by gently pinching out with the fingers so as not to damage shoot tips.
Propagation: Successful propagation can be achieved with seeds, cuttings, layering and grafting. Take cuttings of hardy hybrids, Kurume azaleas, and alpine species in summer or as late as early winter for small-leafed species and keep in a cold frame. Bottom heat and hormones are useful. Larger leafed species are often grown from seed in early spring and kept at 13-16 degrees C. Layering can also be undertaken at any time of year for large-leafed species as well as deciduous azaleas. Grafting is only used for hardy hybrids.
Health: Frost hardy to tender tropical plants; aside from the tropical species none are properly tender but they may be sensitive to cold winds and intense sunlight; frosts may damage buds of early flowering species. Pests may include the rhododendron leafhopper, whitefly, lacewings, weevils, leaf miners, rhododendron bugs, or mites. The leafhopper is most prevalent; they puncture buds with their mouthparts which causes bud blast disease. Mites can usually be controlled by releasing predatory mites. Diseases may include leaf spots, silver leaf, bud blast, petal blight, galls, and root rots. Honey fungus can kill plants. Chlorosis may appear if the soil is too alkaline.
Uses: Shrubberies, container plants, borders, massed displays of foliage or flowers, bonsai, rockery, alpine garden, woodland garden.
Rhododendrons include all Azaleas. Many different groups have been developed by hybridizing or breeding different cultivars within a species, for example:
Ghent Azaleas (R. gandavense): have been developed mostly by breeding R. viscosum, R. flavum, R. nudiflorum and R. calandulaceum.
Deciduous Azaleas: Have been created by breeding several species including R. flavum, R. calendulaceum, R. nudiflorum, R. viscosum, R. sinense, R. molle, R. occidentale, and others.
Indica Hybrids (syn. R. indica; Indian Azaleas): These are evergreen, sometimes tender, small shrubs derived mostly from R. indicum, R. mucronatum, and R. Simsii. Flower colours include white, pinks, reds, mauves, and purples.
Kurume Hybrids: These are derived mainly from R. Kaempferi and. R kiusianum. They are small, slow-growing, compact evergreen shrubs. Again, some may be tender in colder areas, but generally they are the most frost-hardy of the evergreen azaleas. They tend to be twiggier than Indicas with smaller leaves and denser foliage. Whilst normally smaller than the Indicas, the Kurumes also display great variation in height and spread.
Mollis Hybrids: These are deciduous small shrubs derived from R. molle and R. japonicum. Flowers often appear before foliage on the tips of stems. These flowers are trumpet-shaped, 6-7cm across, and appear in early spring. Colours include white, yellow, and orange. Growth is generally upright. Foliage is more open than with Indicas and Kurumes, and leaves tend to be larger.
Other Rhododendrons vary in size from dwarf shrubs from as little as 25cm to trees up to 20-25m tall (R. giganteum). Many smaller varieties have been developed specifically as rockery plants. Despite some rhododendrons being simply too large for a small garden there are hundreds of other varieties that are ideal where space is limited.
The tropical species found in South-East Asia and Australia plants are the vireya rhododendrons. These prefer warm, humid conditions and are generally not frost-hardy.
Some Lesser Grown Rhododendron Species
R. cinnarbarinum: a
2– 6m native to Northern Burma with fragrant, tubular flowers ranging
from red to orange. Flowers are borne in clusters of five to eight in
early summer and measure 2.5-4.5 cm wide and up to 6.5 cm long. Leaves
are ovate and a smoke blue colour all over when young, turning scaly
with grey green upper sides when mature.
R. decorum: a 1.5-6m
shrub from Western China and also Northern Burma. It has white to pink
fragrant, funnel shaped flowers sometimes speckled green or red. The
flowers which are 7.5-12.5 cm wide and up to 7.5 cm long appear in early
spring to early summer in loose clusters of eight to ten. The large
leaves are grey green above and smoke blue beneath measuring between 10
and 15cm. These plants are very hardy.
R. degronianum: a small
1-3m dome shaped shrubby plant from Japan producing speckled, bell
shaped pink to red flowers in late spring. The leaves are dark green
measuring up to 15cm long and have a red to fawn underside. The
subspecies ‘Wada’ has deep cinnamon coloured under sides to the leaves
and a more compact habit.
R. fastigiatum: This small evergreen, alpine species from Yunnan
Province in China has tiny leaves less than 1.25cm long and grows to a
height of 1m with a spread of 30-60cm. It produces terminal trusses of
three to five small, purple to lavender, bell shaped flowers measuring
2.5cm wide and 1.25cm long, in early to mid spring. It is relatively
hardy and free flowering.
R. irroratum: a medium shrub to small
tree measuring up to 9m from Vietnam, Indonesia and China. The flowers,
which are borne in early to late spring, are bell shaped, narrow, and
vary in colour from white, cream, or pink and are spotted red or purple.
Leaves are elliptic or oblanceolate, 6-13cm long, and green on both
sides. The subspecies ‘Polka Dot’ has striking white flowers which are
heavily marked with purple spots.
R. wardii: these evergreen
shrubs come from China and have a height of 3m with a spread of 1.5m.
They have oblong to elliptic rounded leaves up to 10cm long. Young
leaves a re a bright blue green and are smooth all over. Mature leaves
have a dark green upper side and blue grey under side. Saucer shaped
flowers emerge in late spring to early summer in clusters of 7-14,
measuring some 6.5cm wide. They are a rich bright yellow colour,
sometimes spattered with crimson. It is not the hardiest of species. The
Litiense group are thought to be a form of R. wardii var. wardii are
very attractive shrubs which produce wide saucer or bell shaped clear
yellow flowers and bluish, oblong waxy foliage.
R. macabeanum: a
large round shrub or small tree to 8m with a spread of 3-5m from
northern India. It makes a spectacular woodland tree and has leathery
leaves up to 30cm long which are dark green and veined on the upper side
and silver to white on the under side. Large trusses of bell shaped
flowers are borne in mid spring and are pale yellow flower with a purple
blotch at the base.
R. pemakoense: A small spreading evergreen
shrub from Tibet with a height of 60cm and spread of 90cm. One alpine
variety only grows to several centimetres tall producing many suckers.
Leaves are ovate to round up to 2cm long and have dark green upper sides
with scaly, blue green under sides. Flowers are large in comparison
measuring 3.75cm wide and 3cm long. Flowers are lilac pink to purple
funnel shaped and are borne singly, or in pairs, in early to mid spring.
They are highly floriferous but are susceptible to frost damage.
R. russatum: A low growing evergreen shrub with a compact habit from
China. These plants have a height and spread of 60-120cm. Leaves are
ovate, scaly and 2.5-3.5cm long with an orange yellow underside. Funnel
shaped flowers are produced in late spring to early summer in clusters
of 5-10. Flowers are 2.5cm wide and 1.75cm long and are a deep blue
purple or violet, often with a white throat. This species is hardy and
yakushimanum: is native to the Japanese island of Yakushima where it
grows on cool mountain sides however it is also able to tolerate some
sun. It is an important species because it is the parent of many modern
hybrids, often referred to as ‘Yak’ hybrids. These hybrids are
particularly suited to smaller gardens due to their compact habit. They
have leathery leaves and produce clear flowers in spring in a variety of
colours. Once they have finished flowering they produce new stems which
become covered in a white, downy layer. They are best planted in autumn
or spring and will tolerate slightly more sunny and exposed conditions
than many other rhododendron varieties. Examples include:
• R. 'Bashful': this plant has attractive leaves which are silver
grey when young. It is one of the more well known yak hybrids and is one
of a range of hybrids named after the ‘seven dwarfs’ of ‘Snow White'
which were cultivated in the 1970s. Flower colours vary from white
through to orange, yellow, pink and crimson
• R. 'Morning Magic': is one of the most striking of the white varieties and bears relatively large flowers.
• R. 'Fantastica': is recognised by its two tone flowers which begin as crimson and then turn to a spectacular shade of pink.
BENEFITS OF THIS COURSE
- Increase the number of Rhododendron species and cultivars you are familiar with.
- Develop a foundation of framework for understanding different groups of Rhododendrons. This will make it easier to learn, understand and retain information whenever you encounter new species or cultivars in the future.
- Discover how extensive the study of Rhododendrons can be - the more you learn, the more you will realize there is to learn.
- Improve your capacity to propagate and grow rhododendrons in different places and different ways.
- Begin to establish a reputation as an expert with Rhododendrons and Azaleas.
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