[7][8][9] Rhododendron is a genus of shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm (4–40 in) tall, and the largest, R. protistum var.

A recently discovered species in New Guinea has flowers up to six inches (fifteen centimeters) in width,[12] the largest in the whole genus.

[13][14] They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote).

[3] Rhododendron are characterised by having inflorescences with scarious (dry) perulae, a chromosome number of x=13, fruit that has a septicidal capsule, an ovary that is superior (or nearly so), stamens that have no appendages, and agglutinate (clumped) pollen.

This structure largely survived till recently (2004), following which the development of molecular phylogeny led to major re-examinations of traditional morphological classifications,[22][23] although other authors such as Candolle, who described six sections,[28] used slightly different numeration.

[33] The next major attempt at classification was by Sleumer who from 1934 began incorporating the Balfourian series into the older hierarchical structure of subgenera and sections, according to the International Code of Botanical Nomenclature, culminating in 1949 with his "Ein System der Gattung Rhododendron"[34] and subsequent refinements.

[3][15][23][44][45] Rhododendron Choniastrum Hymenanthes Azaleastrum Therorhodion The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who used matK sequencing.

They confirmed that the genus Rhododendron was monophyletic, with subgenus Therorhodion in the basal position, consistent with the matK studies.

Following publication of the studies of Goetsch et al. (2005) with RPB2,[3] there began an ongoing realignment of species and groups within the genus, based on evolutionary relationships.

Their work was more supportive of Sleumer's original system than the later modifications introduced by Chamberlain et al..[3][47] The major finding of Goetsch and colleagues was that all species examined (except R. camtschaticum, subgenus Therorhodion) formed three major clades which they labelled A, B, and C, with the subgenera Rhododendron and Hymenanthes as monophyletic groups nested within clades A and B, respectively.

In addition to the two separate genera included under Rhododendron by Chamberlain (Ledum, Tsusiophyllum), Goetsch et al.. added Menziesia (clade C).

Terminology from the Sleumer (1949) system is frequently found in older literature, with five subgenera and is as follows; In the later traditional classification, attributed to Chamberlain (1996), and as used by horticulturalists and the American Rhododendron Society,[50] Rhododendron has eight subgenera based on morphology, namely the presence of scales (lepidote), deciduousness of leaves, and the floral and vegetative branching patterns, after Sleumer (1980).

Older ranks such as Series (groups of species) are no longer used but may be found in the literature, but the American Rhododendron Society still uses a similar device, called Alliances[50] The system used by the World Flora Online as of December 2023[update] uses six subgenera, four of which are divided further:[53] Species of the genus Rhododendron are widely distributed between latitudes 80°N and 20°S and are native to areas from North America to Europe, Russia, and Asia, and from Greenland to Queensland, Australia and the Solomon Islands.

[15] The centres of diversity are in the Himalayas and Maritime Southeast Asia,[46] with the greatest species diversity in the Sino-Himalayan region, Southwest China and northern Burma, from India – Himachal Pradesh, Uttarakhand, Sikkim and Nagaland to Nepal, northwestern Yunnan and western Sichuan and southeastern Tibet.

The species in New Guinea are native to subalpine moist grasslands at around 3,000 metres above sea level in the Central Highlands.

[3][30] In the United States, native Rhododendron mostly occur in lowland and montane forests in the Pacific Northwest, California, the Northeast, and the Appalachian Mountains.

In areas with poorly drained or alkaline soils, rhododendrons are often grown in raised beds using media such as composted pine bark.

[80] Other examples include the PJM hybrids, formed from a cross between Rhododendron carolinianum and R. dauricum, and named after Peter J. Mezitt of Weston Nurseries, Massachusetts.

People have been known to become ill from eating mad honey made by bees feeding on rhododendron and azalea flowers.

Xenophon described the odd behaviour of Greek soldiers after having consumed honey in a village surrounded by Rhododendron ponticum during the march of the Ten Thousand in 401 BCE.

[82] Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BCE during the Third Mithridatic War.

Rhododendron, including its stems, leaves and flowers, contains toxins that, if ingested by a cat's stomach, can cause seizures and even coma and death.

[87][88] Animal studies and in vitro research have identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of flavonoids or other phenolic compounds and saponins the plant contains.

The flower is also utilized in making a healthful, antioxidant-rich juice that is popular during local festivities and summer months.

Additionally, Buransh flowers are incorporated into local arts and crafts, where they are used to make colorful necklaces and jewelry, symbolizing the spiritual and physical prosperity of the community.

Amongst the Zomi tribes in India and Myanmar, "Rhododendrons" called "Ngeisok" is used in a poetic manner to signify a lady.

[citation needed]The nineteenth-century American poet and essayist Ralph Waldo Emerson in 1834 wrote a poem titled "The Rhodora, On Being Asked, Whence Is the Flower".

The flowers are referenced throughout Daphne Du Maurier's novel Rebecca (1938)[citation needed] and in Sharon Creech's young adult novel Walk Two Moons (1994).

British author Jasper Fforde also uses rhododendron as a motif throughout many of his books, e.g. the Thursday Next series[95] and Shades of Grey (2009).

[citation needed] Records of the Rhododendron Society of America reside at the Albert and Shirley Small Special Collections Library at the University of Virginia.