Rhododendron arboreum, the tree rhododendron, is an evergreen shrub or small tree with a showy display of bright red flowers. It is found in Bhutan, China, India, Myanmar, Nepal, Sri Lanka, Pakistan and Thailand. It is the national flower of Nepal. In India it is the state tree of Uttarakhand and state flower of Nagaland.
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), though more usually 12 m (39 ft) tall and broad. This plant holds the Guinness Record for 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).
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.
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.
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.