R. arboreum is possibly the most widespread rhododendron in the world.  The plants grow in the wild in southeast Asia occupying a wide arc on the southern slopes of the Himalaya Mountains, from Kashmir through Nepal, Sikkim, Bhutan, and Manipur, a distance of about 1,500 miles.  In its native habitat some arboretum forms can grow up to 60 feet tall, but in cultivation, it rarely grows over 40 feet.  Most experts say it will grow to be about 6 ft. tall in 10 years. So don't let its mature height keep you from planting one in your garden.

The leaves of R. arboreum are large; 4 to 8 in. long and up to 2 in. or more wide. They are thick and leathery. Arboreum leaves are glossy, deep green on top, with the underside covered with a thin plastered or woolly indumentum that varies in color between different subspecies. The color of the flowers varies considerably, from white to shades of pink or red. Some of the white and pink forms sometimes will have deeper colored spots which add to their interest and beauty. All have nectar pouches at the base of the flower. Flowers are bell-shaped and are held in trusses of 15 to 20 flowers. The blood-red forms are generally considered to be the most tender.

Since the British Empire previously occupied much of the area where R. arboreum grows, and much of the early rhododendron hybridizing occurred in the British Isles, the species has been often used as a hybrid plant parent. Arboreum seed was first sent back to Britain from Nepal almost 200 years ago. Some examples of hybrids where R. arboreum has been used as a parent include:

R. 'Bibiani', ('Moser's Maroon' x R. arboreum ssp. arboreum)

R. 'Cornubia', (R. arboreum ssp. arboreum, red form x Shilsonii Group)

R. 'Doncaster', (R. arboreum ssp. arboreum x unknown)

R. 'Loders White', (R. arboreum ssp. cinnamomeum var. album x R. griffithianum)

R. Nobleanum Group, (R. caucasicum x R. arboreum ssp. arboreum)