Family Ericaceae > Subfamily Ericoideae > Tribe Rhodoreae.
A genus of 850 to over 1000 species plus tens of thousands of hybrids and cultivars.
There is a great variation in all features across the genus.

They are woody plants mostly evergreen but with some deciduous.
There are prostrate forms, shrubs and some tall trees.
Some are epiphytic or grow on rocks.

Leaves are spirally arranged.
Size varies from 1 cm to over 50 cms.
Leaf blades can be elliptic, lance-shaped or round, and thick or thin.

The leaf underside can be smooth, hairy or with scales (lepidote).
Scales may or may not have a short stalk attached the the edge or centre.
Scales can be simple, incised, star-shaped or lobed.
They are an important feature in identification and classification.
Scales are modified hairs confined to the subgenus Rhododendron.

Inflorescences can be a single flower or multiple flowers sometimes in trusses.
Trusses are where multiple flowers grow from a single bud.
In most species flowers are terminal but some have axillary inflorescences.
The funnel or bell-shaped flowers can be from about 1 to 15 cm across.
Most flowers have 5 petals but a few have 6 or 7.
A very wide range of colours except a true blue plus various markings.

There is a separate calyx and corolla and a superior ovary.
Some species have 5 stamens, most have 10 and a few have more than 10.
The fruit is a septicidal capsule.

Classification of genus Rhododendron.

In discussing the Rhododendron genus it is important to distinguish between the use of
    ‘Rhododendron’ as a scientific or common usage term.
In botanical terms ‘Rhododendron’ covers all the species that are commonly referred to
    as Rhododendrons, Azaleas and Vireyas.
‘Azalea’ is a popular usage term not recognised botanically.

The Rhododendron genus is divided into subgenera, sections, subsections and species
    based on features such as the presence or absence of leaf scales (modified hairs),
    evergreen or deciduous, branching patterns, flower position etc.

There have been frequent, and large changes and various authors use different classifications.

(After finding the third different classification labelled ‘current’ or ‘latest’ and
    contradictory descriptions in some areas I gave up trying to sort it out.
Following is a brief summary of one classification.

A. Subgenus Rhododendron. (Lepidote rhododendrons.)
There are 400 to 500 species.
They are characterised by having scales under their leaves.
Flowers are in terminal trusses.
Flowers have 5 (8) stamens.
It includes the 300 or so Vireya rhododendrons.

Rhododendron ferrugineum is the type species.
It is a small, rounded shrub up to 120 or 150 cm high.
The edge of the elliptic leaves is rolled under.
The undersides are covered in red-brown scales.
Inflorescences have 6 to 10 bell-shaped flowers 1.5 cm long.
They are a deep pink or reddish-purple with 5 stamens.
The fruit is a capsule.

B. Subgenus Hymenanthes. Leaves with no scales (Elepidote).
Shrubs to medium sized trees.
These have larger leaves that are usually deciduous.
Flowers are terminal and larger.

Rhododendron degronianum is the type species.
It is a shrub up to 2.5 m high.
The dark green, glossy leaves are up to 14 cm long and 3.5 cm wide.
They are elliptic to lanceolate with the edges rolled under.
The underside has pale brown hairs.
The funnel shaped flowers are in trusses of up to 15 bell-shaped flowers.
The 5 petals in white, pink or reddish.
There are 10 stamens.

C. Subgenus Azaleastrum. Leaves with no scales (Elepidote).
These are like the Hymenanthes but but with axillary flowers.
A group of about 120 species of evergreen plants.

Rhododendron ovatum is the type species.
An evergreen shrub to 4.5 (6) m high.
Leaves are ovate to elliptic and a glossy dark green.
They are about 6 cm long by 3 cm wide.
They always have a sharply pointed tip and sometimes a toothed edge.
The midrib may have a few hairs for a time.
The leaf stalks, calyx, ovary and fruit capsule are densely hairy.
Inflorescences are single axillary flowers near the ends of branches.
Flowers are white, pale purple or pink.
The 5 petals are round or oblong and the upper one has purple spots.
The 5 stamens are of unequal length.

Horticulturists and gardeners divide the Rhododendron genus on appearance only.

1. Rhododendrons.
Erect, usually evergreen plants up to 20 m or more high.
The ‘true’ rhododendrons have larger leaves.
The large flowers are bell- or funnel-shaped in terminal trusses.
Flowers mostly have 10 stamens.

2. Vireya rhododendrons.
Often put in a separate section as they do not look like other species.

3. Azaleas.
Mostly medium-sized shrubs to 2 m.
Small elliptic or ovate leaves.
Mostly deciduous (or partly) but a few are evergreen.
Inflorescences are axillary as well as terminal.
The small flowers are trumpet to tubular in shape.
They have 5 stamens.
Flowers are not in trusses (except the Homebush azaleas).