Description

General aspect and origins - Rhododendron arboreum is a tree which can reach up to 90 ft (30 m) tall. It was discovered by Captain Hardwick in Kashmir in 1796, and was first introduced in UK in 1811, were the plant flowered for the first time in 1825. The red flowers were appreciated, but R. arboreum did not prove really frost hardy. The tree was thus crossed with hardier species, such as R. caucasicum and R. ponticum (Asia), R. Catawbiense and R. maximum (America) to produce hybrids such as 'Nobleanum', 'Altaclarense', 'Boddaertianum' and 'Broughtonii'.
Leaves - This tree has evergreen leaves, which are variable in texture, depending on the subspecies and on the selection.
Flowers - This species has round flower buds, and inflorescences which are about 4 to 8 in (10 to 20 cm) wide. The flowers can be white, red or pink.

Culture

Rhododendron arboreum is found in many parts of Asia, with subspecies such as :

R. arboreum ssp. arboreum : red or rose-red flowers. Hardy to about 14°F (-10°C). Selections such as 'Rubyiat' and 'Blood Red'. This subspecies is found in western Himalaya and at low elevations.

R. arboreum ssp. cinnamomeum : this subspecies can be white, pink or red, and can resist down to 5°F to 0°F (-15°C to -18°C). Selections such as 'Tony Schilling'. This subspecies is found in Central Himalaya.

R. arboreum ssp. Delavayii : has red flowers, and can be found in Eastern Himalaya.

R. arboreum ssp. nilagiricum : has red flowers and can be found in Nilgiri.

R. arboreum ssp. zeylanicum : has orange-red flowers, and can be found in Sri Lanka.

This species has fairly variable features, including height, leaves and flowers aspect, and frost hardiness.

This species can be grown in areas with moderate summer temperatures. It can resist down to 14°F for the most tender selections, and down to 0°F for the hardiest ones.
Soil - Rhododendron arboreum is native to high elevations, and needs a well-drained, acidic and fairly moist soil.

Propagation

This plant can be propagated by seeds, cuttings, air-layers or can be grafted.
As Rhododendron arboreum is highly variable, it is advised to prefer cuttings, air layers or graftings.