RHS Garden Hyde Hall

Plant of the month: April

Rhododendron arboreum @ Hyde Hall

During spring the Woodland Garden at Hyde Hall always comes into its own with an array of dazzling spring colour. This begins early in the New Year as the first camellias burst into flower, hinting at what is on the way. As winter ends and spring begins, so the colour increases as the rhododendrons produce a vivid and varied display. Rhododendron arboreum is a tall and stately plant with flowers that are slightly variable in colour, but are generally shades of pink. These clothe the plant to near ground level and make a wonderful site as you walk through the Woodland Garden.

Rhododendron syn. Azalea

This genus contains between 500 and 900 species of evergreen and deciduous trees and shrubs, found widely distributed across the northern hemisphere, in temperate to cool regions. The name rhododendron was the name for the rose-flowered oleander Nerium oleander, derived from the Greek words rhodon, a rose and děndron, a tree. The name was transferred to the genus Rhododendron.

The so-called ‘tropical’ Vireya rhododendron grows at higher altitudes throughout tropical Southeast Asia and as far south as the northern tip of Australia. Many species grow at high altitudes above 900m (300ft), and some grow as epiphytes in the branches of trees or on rock faces.

Most rhododendrons are grown for their spectacular and sometimes scented flowers that come in a range of colours excluding blue, sometimes with spots or blotches of different colours. Most bloom in spring, but some flower between late autumn and late summer.

The leaves are simple and some have a coloured covering of hairs on the lower side called an indumentum.

The rhododendron genus is often divided into four groups: evergreen rhododendrons, Vireya rhododendrons, azaleas and azaleodendrons.

Rhododendron arboreum

This is an evergreen tree found in China, Thailand, northern India, Bhutan and Sri Lanka. Its name means ‘tending to be woody or growing in a tree-like form’.

It has broad, dark green leaves, 7-19cm (3-7in) long, with a silvery, fawn or brown hairy coating beneath.

In early- and mid- spring trusses of 15-20 bell-shaped flowers, 5cm (2in) wide and 3-5cm (1.25-2in) long are produced in red, pink or white. They have black nectar pouches and black spots inside.

This plant is suitable for woodland gardens.

Rhododendron arboreum subsp. cinnamomeum has leaves with cinnamon-brown hairs beneath.

Rhododendron arboreum subsp. cinnamomeum var. album has white flowers with small blood red spots on the inner surface of the petals.

Rhododendron arboreum subsp. delavayi has red flowers.

Cultivation

Grow in moist but well-drained, leafy, humus-rich, acid soil (pH 4.5-5.5), in dappled shade in a sheltered position. Shallow planting is essential, as rhododendrons are surface-rooting and will not tolerate deep planting.

Rhododendrons are susceptible to vine weevil, rhododendron and azalea whiteflies, leafhoppers, lacebugs, scale insects, caterpillars, aphids, powdery mildew, bud blast, honey fungus, rust, leafy gall, petal blight, silver leaf, phytophthora root rot and lime-induced chlorosis.

Pruning is not usually needed except for the removal of unsightly shoots or diseased growth, in which case it should be done after flowering.

Propagation

Surface-sow seed at 13-18°C (55-64°F) in ericaceous compost, as soon as ripe.

Root semi-ripe cuttings in late summer.

Layer in autumn.

Graft in late winter or late summer.