Rhododendron

Rhododendron is a very large genus of about 1,024 species of woody plants in the health family (Ericaceae). They can be either evergreen or deciduous. Most species are native to eastern Asia and the Himalayan region, but smaller numbers occur elsewhere in Asia, and in North America, Europe and Australia. It is the national flower of Nepal, the state flower Washington and Wes Virginia in the United States, the state flower of Nagaland and Himachal Pradesh in India, the provincial flower of Jiangxi in China and the state tree of Sikkim and Uttarakhand in India. Most species have brightly colored flowers which bloom from late winter through to early summer. Azaleas make up two subgenera of Rhododendron. They are distinguished from “true” rhododendrons because having only five anthers per flower. They have frequently been divided based on presence or absence of scales on the abaxial (lower) leaf surface ( lepidote or elepidote). These scales, unique to subgenus Rhododendron, are modified hairs consisting of a polygonal scale attached by a stalk. Rhododendron are characterised by having inflorescence 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. Although Rhododendrons had been known since the description of Rhododendron hirsutum by Charles de l’Ecluse ( Clusius)  in the sixteenth century, and were known to classical writers (Magor 1990), and referred to as Chamaerhododendron (low-growing rose tree), the genus was first formally described by Linnaeus in his Species Plantarum in 1753. Linnaeus’ six species of Azalea were Azalea indica, A. pontica, A. lutea, A. viscosa, A. lapponica and A. procumbens (now Kalmia procumbens), as opposed to ten. As new species of what are now considered Rhododendron were discovered, they were assigned to separate genera if they seemed to differ significantly from the type species. For instance Rhodora ( Linnaeus 1763) for Rhododendron canadense, Vireya (Blume 1826). Most of the Balfouruan series are represented by Sleumer as subsection, though some appear as section or even subgenera. Sleumer based his system on the relationship of the flower buds to the leaf buds, habitat, flower structure, and whether the leaves were widely accepted, many in the United Kingdom continued to use the simpler Balfourian system of the Edinburgh group.

Sleumer’s system underwent many revision by other, predominantly the Edinburgh group in their continuing Royal Botanic Garden Edinburgh notes. Cullen of the Edinburgh group, placing more emphasis on the lepidote characteristics of the leaves, united all of the lepidote species into subgenus Rhododendron, including four of Sleumer’s subgenera (Rhododendron, Pseudoazalea, Pseudorhodorastrum, Rhodorastrum).moved to subgenus Azaleastrum, section Sciadorhodion. Similarly the single species in monotypic subgenus Mumeazalea (Rhododendron semibarbatum) was placed in the new section Tsutsusi, subgenus Azaleastrum. Genus Menziesa (9 species) was also added to section Sciadorhodion. The remaining small subgenus Therorhodion with its two species was left intact. Thus two subgenera, Hymenanthes and Azaleastrum were expanded at the expense of four subgenera that were eliminated, although Azaleastrum lost one section (Choniastrum) as a new subgenus, since it was a distinct subclade in a. In all, Hymenanthes increased from one to two sections, while Azaleastrum, by losing one section and gaining two increased from two to three sections.Terminology from the Sleumer (1949) system is frequently found in older literature, with five subgenera and is as follows;

Subgenus Lepidorrhodium Koenhne: Lepidotes. 3 section

Subgenus Eurhododendrom Maxim. : Elipidotes.

Subgenus PPseudanathodendron Sleumer: Deciduous azaleas. 3 section

Subgenus AAnthodendron Reader & Wilson: Evergreen azaleas. 3 section

Subgenus AAzaleastrum Planch. : 4 section

The largest of these is subgenus Rhododendron, containing nearly half of known species and all of the lepidote species.

Subgenus Rhododendron L: Small leaf or lepidotes ( scales on the underside of the leaves). 3 sections, 462 species, type species: Rhododendron ferrugineum.

Subgenus Hymenanthes ( Blume) K. Konch: Large leaf or elepidotes (without scales). 1 section, 224 species, type Rhododendron degronian.

Subgenus Pentanthera (G.Don) Pojarkova: Deciduous azaleas. 4 sections, 23 spacies, type Rhododendron luteum.

Subgenus TTsutsusi (Sweet) Pojarkova: Evergreen azaleas. 2 section, 80 spacies, type Rhododendron indicum.

Subgenus AAzaleastrum Planch. : 2 sections, 16 spacies, type Rhododendron ovatum.

Subgenus Candidastrum Franch.: 1 species, Rhododendron albiflorum.

Subgenus Mumeazalea (Sleumer) W.R. Philipson & M.N. Philipson: 1 species, Rhododendron semibarbatum.

Subgenus Therorhodion (Maxim.) A. Gray: 2 species (Rhododendron camtschaticum, Rhododendron redowskianun).This division was based on a number of what were thought to be key morphological characteristics. These included the position of the inflorescence buds (terminal or lateral), whether lepidote or elepidote, deciduousness of leaves, and whether new foliage was derived from axils from previous year’s shoots or the lowest scaly leaves .A number of insects either target rhododendrons or will opportunistically attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will preferentially devour them.

Rhododendron species are used as food plants by the larvae (caterpillars) of some butterflies and moths; see List of Lepidoptera that feed on rhododendrons.Rhododendron bud blast, a fungal condition that causes buds to turn brown and dry before they can open, is caused by the fungus Pycnostysanus azaleae, which may be brought to the plant by the rhododendron leafhopper, Graphocephala fennahi.They can self-propagate by sending up shoots from the roots. Sometimes an attached branch that has drooped to the ground will root in damp mulch, and the resulting rooted plant then can be cut off the parent rhododendron.Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States, most of them are on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might want, such as resistance to root rot when overwatered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.