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Rhododendron are My stars of Nature in Springtime
What do you need to know about Rhododendron?
Rhododendron are a genus of 500 - 900 species of evergreen and deciduous trees and shrubs, sometimes epiphytic. You can find them in Europe, Asia, Australasia and North America, in particular in South West China, Tibet, Burma, Northern India and New Guinea.
They occur in diverse habitats. From dense forest to alpine tundra and from sea level to high altitudes.
They vary greatly and may reach a height of 25m or creep on ground level to form prostrate shrubs.
Rhododendron leaves are moslty lance-shaped and mid- to dark green. They are ranging in size from 4mm to 75cm long. Some have leaves and young stems covered with an indumentum (a dense woolly covering of hairs or scales). A few have leaves that are aromatic when crushed.
Rhododendron are grown mainly for their spectacular flowers, which sometimes are strongly scented. The individual flowers vary greatly in size and shape and are usually 5-lobed and often marked with flares or spots inside. There are thousands of hybrids, encompassing nearly every flower colour.
Some rhododendrons also have attractive young growth which ranges in colour from red to bronze-brown or betallic blue-green. A few have decorative, exfoliating bark, which may be any colour from brown-pink or deep maroon to silvery grey. A number of the deciduous rhododendrons are valued for their autumn colour.
Rhododendrons have a wide range of garden uses.
Dwarf alpine varieties are effective in a rock garden.
Larger Woodland rhododendrons are excellent for brightening shady areas.
The hardy hybrid rhododendrons are tolerant of more exposed sites and also suitable for hedges or informal screens.
Many of the modern compact hybrids are ideal for growers on shaded patios or in condainers or tubs.
In frost - prone areas, tender rhododendrons are best grown in a conservatory or cool greenhouse. The nectar of some rhododendron flowers may cause severe discomfort if ingested.
Fully hardy to frost tender
Under glass, grow tender rhododendrons in lime-free (ericaceous)potting compost in bright filtered light with moderate to high humidity. When they are in growth, water freely with soft water and apply a balanced liquid fertilizer monthly. Keep just moist in winter.
Outdoors, grow rhododendrons in moist, but well-drained, leafy, humus-rich, acid soil. Shallow planting is essential. All rhododendrons are surface-rooting and will not tolerate deep planting. Most large-leaved species and hybrids require dappled shade in sheltered woodland conditions. Avoid the deep shade immediately beneath a tree canopy. Most dwarf alpine species will tolerate full sun in cooler climates, provided the soil remains moist.
Avoid frost pockets to reduce the risks of waterlogging and bark split.
Mulch anually with leaf mould. After flowering, dead-head where practical to promote vegetative growth rather than seed production.
Surface-sow seed at 13° - 18°C in ericaceous (lime-free) propagating compost or fine moss peat. As soon as ripe or in early spring. Sow seed of hardy dwarf species and hybrids in containers in a cold frame as soon as ripe. Rhododendrons hybridize freely and garden-collected seed may not come true. However, seed collected from species in the wild or from hand-pollinated garden plants will generally produce plants that are true to type. Root semi-ripe cuttings in late summer.
Layer in autumn. Graft in late winter or late summer.
Pests and Diseases
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 and lime-induced chlorosis (if soil is not sufficiently acid).