September 18
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Plant Species of the genus Rhododendron

Information about this genus
Name: Rhododendron
Cultivation: CULTIVATION: Establishing an ideal growing environment before planting is the key to success with rhododendrons. Many of the problems likely to afflict them in the home garden can be minimized by maintaining soil quality and ensuring adequate ventilation. All prefer acidic soils between pH 4.5 and 6, high in organic matter and freely draining. A cool root run is essential and is best achieved by applying a deep mulch of organic material that also helps to reduce moisture loss and control weed growth, while minimizing disturbance of the delicate roots. Many rhododendrons, particularly those with larger leaves, prefer a shaded or semi-shaded aspect. They are ideally suited to planting under deciduous trees, allowing winter sun and summer shade. While most prefer some protection from wind, sun and frost, many others are tolerant of these conditions and some are well suited to exposed rock gardens. Rhododendron bugs can pose a problem in the Northern Hemisphere, and are most effectively controlled with a systemic insecticide for sucking insects, which is also the best defence against weevils, particularly vine weevils, thrips and mites. Disease problems are mostly fungal, such as petal blight and bud blight or blast and root rot, while rusts and mildews can pose a lesser problem in some areas. All can be controlled by the use of appropriate fungicides and minimized by improving growing conditions. Evergreen rhododendrons may be propagated by taking tip cuttings of new growth in spring, while deciduous azaleas are best grown from hardwood cuttings taken in winter. Plants may be grown from seed but germination and development is slow, and plants grown from the seed of hybrids are unlikely to be the same as their parents. Layering enables new plants to be created from low-hanging branches pinned to the ground and covered in a moist organic medium such as sphagnum moss. Plants which are difficult to propagate and establish by other means can be grafted onto the roots of stronger plants with more vigorous root systems. Regular pruning of rhododendrons is not necessary other than as required to control size, maintain shape and to remove damaged or diseased material, while some species and hybrids actually resent unnecessary pruning. Cultivated rhododendrons are normally more compact and attain only about half the height of similar plants growing in the wild. The growing habit of all species and hybrids varies widely according to the amount of shade the plant receives.
Description: This very diverse genus of 800 or more species of mostly evergreen and some deciduous shrubs is widely distributed across the Northern Hemisphere, with the majority growing in temperate to cool regions. Particular concentrations occur in western China, the Himalayas and northeastern Myanmar, while the so-called ?tropical? Vireya rhododendrons grow mostly at higher altitudes throughout tropical southeastern Asia, as far south as the northern tip of Australia, with more than 200 species occurring on the island of New Guinea alone. Deciduous azalea species are scattered across cooler Northern Hemisphere climates, notably in Europe, China, Japan and North America. Rhododendrons vary in form from tiny, ground-hugging, prostrate and miniature plants adapted to exposed conditions to small trees, often understory species in the forests of mountainous areas. Many species grow at high altitudes of 3,000 ft (900 m) or more and some can grow as epiphytes in the branches of trees or on rock faces. As members of the erica family, they are closely related to heathers (Erica and Calluna species), Pieris and strawberry trees (Arbutus species) and have similar growing requirements, particularly the need for a moist, organic and acid soil. Some rhododendrons have solitary flowers but most bear terminal racemes, known as ?trusses?, of up to 24 or more spectacular blooms, in a wide palette of colors including whites, pinks, reds, yellows and mauves, excluding only shades of pure blue. Flowers may be a single color but are often multi-colored, with spots, stripes, edging or a single blotch of a different color or shade in the throat of the flower. With the exception of some Vireya species and hybrids, fragrant rhododendrons are always white or very pale pink. Blooms vary in size and shape but are generally campanulate (bell-shaped), with a broad tube ending in flared lobes, and usually single. Flowers with double petals do occur, particularly among the evergreen azaleas, which may also be ?hose-in-hose?, when the calyx is enlarged and the same color as the petals. Most rhododendrons flower from early spring (early season) to early summer (late season), although some bear spot flowers briefly in autumn, and Vireya rhododendrons can flower at various times during the year, often in winter. Deciduous azaleas flower in spring on bare branches just before or at the same time as new leaf growth starts to emerge. The fruit is a many-seeded capsule, normally woody but sometimes soft, and sometimes bearing wings or tail-like appendages to aid transportation. Rhododendron species and hybrids are cultivated as ornamental plants, valued for their masses of colorful flowers and year-round foliage in great diversity of form; some are also sought for their attractive textured bark and rich fragrance. The new leaf shoots of evergreen rhododendrons often form attractive perpendicular ?candle-sticks?, while mature leaves vary enormously in size, from less than 1?2 in (12 mm) long to as much as 3 ft (1 m) or more. The foliage of deciduous azaleas progresses through the growing season from bright green shoots in spring to bronze in summer, followed by rich reds to yellows in autumn before falling. The genus is divided into 2 botanically distinct groups known as lepidotes and elepidotes, and these groups are subdivided further into the various rhododendron types. Plants from one group may not breed with plants from the other, thus limiting the options for hybridizers. The leaves, and sometimes the flowers and other parts, of lepidote rhododendrons are covered with scales, which is thought to aid transpiration. This group includes many of the cool-climate evergreen plants, including the Vireya rhododendrons. The rest of the genus, the elepidote rhododendrons, with no scales on leaf or flower parts, includes the remaining cool-climate evergreen plants and the evergreen and deciduous azaleas, which are normally rather more compact plants with 5 stamens rather than the more usual ten. Azaleas were originally classified as a separate genus but are now regarded as botanically part of the rhododendron genus. Vireya rhododendrons can be grown in just about any climate as long as protection from frost is provided. Many are well suited to growing in hanging baskets and containers. The nectar of some species and some flower parts are poisonous and care should be taken when handling the flowers.

Specie Vernacular Zone
arborescens 4?8
atlanticum 6?9
calendulaceum 5?8
canadense 3?8
canescens mountain azalea
lapponicum Lapland rosebay
maximum 3?8
periclymenoides 3?8
prinophyllum 4?9
Rhododendron 3 mucronatum 5?8
Rhododendron 3 obtusum 6?9
Rhododendron aberconwayii 7?9
Rhododendron aganniphum 7?9
Rhododendron albiflorum 4?8
Rhododendron arboreum 7?9
Rhododendron argyrophyllum 6?9
Rhododendron arizelum 8?9
Rhododendron augustinii 6?9
Rhododendron aurigeranum 10?11
Rhododendron austrinum 6?9
Rhododendron brachycarpum 6?9
Rhododendron brookeanum 10?11
Rhododendron bureavii 6?9
Rhododendron burmanicum 9?10
Rhododendron callimorphum 7?9
Rhododendron calophytum 6?9
Rhododendron calostrotum 6?9
Rhododendron campanulatum 5?8
Rhododendron campylocarpum 6?9
Rhododendron campylogynum 7?9
Rhododendron carneum 8?10
Rhododendron catawbiense 4?8
Rhododendron cephalanthum 7?9
Rhododendron cerasinum 7?9
Rhododendron ciliatum 7?9
Rhododendron cinnabarinum 6?9
Rhododendron concinnum 7?9
Rhododendron cultivars
Rhododendron dauricum 5?8
Rhododendron davidsonianum 7?10
Rhododendron decorum 7?9
Rhododendron degronianum 7?9
Rhododendron edgeworthii 9?10
Rhododendron elliottii 9?10
Rhododendron facetum 8?9
Rhododendron falconeri 9?10
Rhododendron fastigiatum 6?9
Rhododendron ferrugineum 4?8
Rhododendron floribundum 8?9
Rhododendron formosum 8?9
Rhododendron fortunei 6?9
Rhododendron fulvum 7?9
Rhododendron genestierianum 8?9
Rhododendron glaucophyllum 8?9
Rhododendron glischrum 7?9
Rhododendron griersonianum 8?9
Rhododendron griffithianum 8?9
Rhododendron hanceanum 7?10
Rhododendron hippophaeoides 6?9
Rhododendron hirsutum 4?9
Rhododendron hyperythrum 8?9
Rhododendron impeditum 4?8
Rhododendron indicum 6?9
Rhododendron irroratum 7?9
Rhododendron japonicum 5?8
Rhododendron jasminiflorum 10?11
Rhododendron javanicum 10?11
Rhododendron johnstoneanum 7?9
Rhododendron kaempferi 5?8
Rhododendron kawakamii 9?11
Rhododendron keiskei 5?8
Rhododendron kiusianum 6?9
Rhododendron konori 10?11
Rhododendron lacteum 7?9
Rhododendron laetum 10?11
Rhododendron lanigerum 7?9
Rhododendron lepidostylum 6?9
Rhododendron leucaspis 7?9
Rhododendron lindleyi 9?10
Rhododendron lochiae 10?11
Rhododendron loranthiflorum 10?11
Rhododendron lutescens 7?9
Rhododendron luteum 5?9
Rhododendron macabeanum 8?9
Rhododendron macgregoriae 10?11
Rhododendron maddenii 9?10
Rhododendron magnificum 9?10
Rhododendron makinoi 8?9
Rhododendron mallotum 7?9
Rhododendron megeratum 9?10
Rhododendron mekongense 9?10
Rhododendron minus 4?9
Rhododendron molle 7?9
Rhododendron morii 7-9
Rhododendron moupinense 7?9
Rhododendron mucronulatum 4?8
Rhododendron nakaharai 6?9
Rhododendron nipponicum 7?9
Rhododendron niveum 7?9
Rhododendron nuttallii 9?10
Rhododendron occidentale 6?9
Rhododendron orbiculare 6?9
Rhododendron orbiculatum 10?11
Rhododendron oreodoxa 6?10
Rhododendron orthocladum 6?9
Rhododendron pachysanthum 7?9
Rhododendron pemakoense 6?9
Rhododendron phaeochrysum 8?10
Rhododendron pleianthum 10?11
Rhododendron polycladum 7?10
Rhododendron ponticum 6?9
Rhododendron protistum 9?10
Rhododendron pruniflorum 7?9
Rhododendron prunifolium 6?9
Rhododendron pseudochrysanthum 8?9
Rhododendron quinquefolium 6?8
Rhododendron racemosum 5?8
Rhododendron reticulatum 6?9
Rhododendron rex 7?9
Rhododendron rubiginosum 7?9
Rhododendron russatum 5?9
Rhododendron saluenense 6?9
Rhododendron scabrifolium 8?9
Rhododendron schlippenbachii 4?8
Rhododendron scopulorum 9?10
Rhododendron simsii 7?9
Rhododendron sinogrande 8?9
Rhododendron smirnowii 4?8
Rhododendron strigillosum 8?9
Rhododendron sutchuenense 6?9
Rhododendron thomsonii 6?9
Rhododendron trichostomum 7?9
Rhododendron tuba 9?11
Rhododendron valentinianum 9?10
Rhododendron vaseyi 4?8
Rhododendron veitchianum 9?10
Rhododendron wardii 7?9
Rhododendron weyrichii 5?9
Rhododendron williamsianum 7?9
Rhododendron williamsii 10?11
Rhododendron yedoense var. poukhanense 5?8
Rhododendron yunnanense 7?9
Rhododendron zoelleri 10?11
Rhododendron: Azaleodendron Hybrids 6?9
Rhododendron: Gable Hybrids 6?10
Rhododendron: Ghent Hybrids 5?9
Rhododendron: Hardy Medium Hybrids 3?9
Rhododendron: Hardy Small Hybrids 3?9
Rhododendron: Hardy Tall Hybrids 4-9
Rhododendron: Ilam and Melford Hybrids 5?9
Rhododendron: Indica or Indian Hybrids 7?10
Rhododendron: Inter-group Hybrids 6?10
Rhododendron: Kaempferi or Malvatica and Vuykiana 6?10
Rhododendron: Knap Hill and Exbury Hybrids 5?9
Rhododendron: Kurume Hybrids 7?10
Rhododendron: Mollis Hybrids 6?9
Rhododendron: Occidentale Hybrids 6?9
Rhododendron: Other Deciduous Azalea Hybrids 5?9
Rhododendron: Rustica Flore Pleno Hybrids 5?9
Rhododendron: Satsuki Hybrids 7?11
Rhododendron: Tender Hybrids 5-10
Rhododendron: Vireya Hybrids 10-12
Rhododendron: Yak Hybrids 4-9
viscosum 4?9

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