Rhododendron arboreum

Its specific epithet means "tending to be woody or growing in a tree-like form". It has been recorded as reaching heights of 20 m (66 ft),[2] though it is more usually 12 m (39 ft) tall and broad. This plant holds the Guinness Record for the World's Largest Rhododendron. The tree discovered in 1993 at Mount Japfü in the Kohima District of Nagaland, India, holds the Guinness Record for the tallest Rhododendron at 20 m (65 ft).[3] In sharp contrast to this, the New Guinean epiphytic species Rhododendron caespitosum never exceeds four inches (ten centimeters) in height.[4]

In early- and mid-spring, trusses of 15–20 bell-shaped flowers, 5 cm (2 in) wide and 3–5 cm (1–2 in) long are produced in red, pink or white. They have black nectar pouches and black spots inside.

Rhododendron arboreum prefers moist but well-drained, leafy, humus-rich, acid pH soil, in dappled shade. It has broad, dark green leaves, 7–19 cm (3–7.5 in) long, with a silvery, fawn or brown hairy coating beneath.

This plant is suitable for woodland gardens.

Shelter is imperative to prevent wind damage to leaves.

Rhododendron arboreum from Fyson[5]

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

  Flower of Rhododendron arboreum from Yumthang Valley in North Sikkim, India
  Rhododendron arboreum subsp. zeylanicum rare ssp from the highlands of Sri Lanka, named after Zeilan, the name used by Arabian traders to refer to Sri Lanka
• 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
• Rhododendron arboreum subsp. nilagiricum (Zenker) Tagg is found in Tamil Nadu, India.[6]

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.[7]

The plant is known as Maha ratmal, Maha Rath Mal, Asela mal in Sinhala. The Sinhalese name "rathmal" is mostly used for the Ixoras, which are not in this family. However, the Rhododendrons are not a common plant in Sri Lanka, and the unsystematic local name seems to be"Maha-rathmal", i.e., "Big-Ixora", applied to smaller varieties which are more like ornamental Azelias. Maha Rath Mala is commonly available in Horton Plain, Knuckles mountain range and many of the parts towards the center of Sri Lanka.

• Catawbiense hybrid – hybrid with R. arboreum