Rhododendron

Rhododendrons are grown for their spectacular flowers, usually borne in spring. Some also have young leaves and stems covered in a striking dense woolly covering (indumentum) and some - the deciduous rhododendrons or azaleas - have good autumn colour.

Rhododendron 'Loderi King George'

Quick facts

Common name Rhododendron
Botanical name Rhododendron
Group Evergreen and deciduous trees and shrubs
Flowering time Mainly spring
Planting time Autumn or spring
Height and spread Variable
Aspect Generally light dappled shade
Hardiness Mostly fully hardy
Difficulty Moderate

Cultivation notes

Site selection

Choose a site with dappled shade in sheltered conditions. Avoid deep shade beneath other trees. Most rhododendrons will tolerate a more open site if sheltered from cold, dry winds. Dwarf alpine species will tolerate full sun provided the soil does not dry out. Avoid frost pockets and sites exposed to early morning sun.

  • Rhododendrons need moist but well-drained, acid soil between pH 5.0 and 6.0 that is rich in organic matter
  • If you have alkaline soil, grow rhododendrons as container plants or see our page on more tolerant species and cultivars. Reducing soil pH is not simple
  • Dwarf alpine rhododendrons are effective in a rock garden
  • Larger rhododendrons are excellent for woodland gardens
  • Compact hybrids are ideal for containers on shaded patios

Planting

  • Plant in October or March/April
  • Before planting, dig in plenty of neutral or acidic organic matter: composted tree bark, leafmould, decomposing pine or spruce needles, or composted chopped bracken. Avoid simply lining the planting hole; mix well into soil
  • Do not plant too deeply - all rhododendrons are surface-rooting and the roots should be just covered
  • Apply at least 8cm (3in) mulch of chipped conifer bark or other acid material. The mulch should be well-aerated, not packed or firmed down
  • Renew or replenish the mulch each spring when the soil is still moist

Watering

  • Rhododendrons grow best in areas of high rainfall. Even on suitably acid soils they are more difficult to grow successfully in the drier parts of the country
  • Tap water, especially in hard water districts, contains too much calcium for rhododendrons, reducing acidity around their roots
  • Use rain water for watering rhododendrons, but if rain water runs out, tap water is satisfactory for a month or two in summer

Feeding

In spring, an application of ericaceous fertiliser, controlled release fertiliser pellets, or ‘Vitax Q4’ at a rate of 70gms per m2 (2oz per sq yd) in cool, moist conditions is beneficial.

Container cultivation

  • John Innes ericaceous loam-based compost makes a good compost choice, generally providing easier management of watering and feeding
  • RHS research has found that soil-less potting media including ericaceous peat-based and peat-free potting composts are suitable for rhododendron growing. However, they can lose their structure over time leading to poor drainage and an airless root environment, causing leaves to brown and die back. Re-potting every other year into fresh potting compost in early spring as soon as signs of growth are seen is recommended. In the intervening year, replace the top 5cm (2in) of compost. You can re-pot back into the same pot if you trim off up to a third of the roots to make room for fresh potting compost

Pruning and training

Rhododendrons don’t require much pruning other than the removal of dead wood and deadheading of spent flowers if practical.

  • If pruning is necessary to restrict size, follow the guidelines in our profile on pruning evergreen shrubs
  • Many rhododendrons respond well to hard cutting back; response is best from deciduous azaleas and rough-barked rhododendrons such as R. ponticum, rather than from smooth-barked kinds. After cutting back, mulch, feed if soil conditions are poor, and keep well-watered to encourage new growth

Propagation

Only species rhododendrons come true from seed. Named cultivars must be propagated from semi-ripe cuttings, grafting or layering.

  • Rhododendron cuttings are taken from the current year’s growth during the late summer or autumn, once the shoots have completed their seasonal growth and the bud is fully developed. Take cuttings with a heel, and then wound the stem to encourage rooting
  • Cuttings of evergreen azaleas are taken in mid-to-late summer. Deciduous azaleas are harder to propagate from cuttings; commercial growers take cuttings in early spring, and grow plants on in frames for up to two seasons before planting out
  • Grafting is used where cuttings cannot be rooted. Rootstocks are taken from Rhododendron ‘Cunningham's White’. Saddle grafting (which involves cutting an inverted V in the scion stem) is done in January to March, and spliced grafting is done in spring
  • Layering can take 15 months to two years (some smooth-barked rhododendrons may take three years until rooted). Check layered shoots in the autumn of the second year.  If not well-rooted, partially sever and lift in the following spring as growth begins
  • Seed propagation: sow into pots in January and leave the seed uncovered on the surface of the compost. Alternatively, top off the pots with a 2cm (3/4in) layer of damp perlite, scatter the seed over the perlite and ‘plough’ in using a pencil. Place the pots in a heated propagator

Cultivar Selection

Early flowering (up to the end of April)

Rhododendron barbatum: spreading shrub or small tree with glossy, dark-green leaves. Tubular to bell-shaped, deep crimson to scarlet-red flowers in dense, rounded heads. Height 8m (26ft).
R. calophytum AGM: large, spreading evergreen shrub with long, narrow elegant leaves and large trusses of white or pale pink, bell-shaped flowers, spotted within with maroon. Height and spread 8m (26ft).
R. 'Christmas Cheer' (caucasicum hybrid): medium-sized, evergreen shrub, flowering in late winter and early spring with rounded trusses of funnel-shaped, very pale pink flowers from darker pink buds. Height and spread 2.5m (8ft).
R. (Cilpinense Group) 'Cilpinense': free-flowering small evergreen shrub of rounded habit with glossy, broad leaves and funnel-shaped, pale pink flowers. Height and spread 1m (3¼ft).
R. 'Countess of Haddington': compact spreading evergreen shrub, with hairy dark green leaves and slightly fragrant, funnel-shaped white flowers flushed with pink. Height and spread 1.5m (5ft).
R. dauricum ‘Midwinter’ AGM: Very hardy, deciduous or semi-evergreen variety with a height and spread of 1.5m (5ft) and rose-purple flowers in early spring.
R. fulvum AGM: large shrub or small tree with dark green leaves with striking cinnamon indumentum beneath. Pale rose-pink, bell-shaped flowers 4cm wide, in compact trusses. Height and spread 4m (13ft).
R. grande: small tree or large shrub with large, leathery leaves. Bell-shaped, cream to pale yellow flowers, each with a purple basal blotch, open from pink buds and are held in dense, rounded trusses. Height 12m (40ft).
R. Nobleanum Album Group: evergreen with narrow green leaves with silvery undersides. Flowers for several months in the late winter until early spring. Pale pink in bud, the flower trusses open to a pure white. Height 2.5m (8ft).
R. ‘Nobleanum Venustum’: evergreen leaves with silvery undersides. Flowering in late winter to early spring; trusses of mid pink flowers, spotted dark pink in the throat. Height 2.5m (8ft).
R. 'Praecox' AGM: More-or-less evergreen shrub with dark, glossy oval leaves and funnel-shaped, rosy-purple flowers 4cm in width, in small clusters at the branch tips from late winter. Height and spread 1.5m (5ft).
R. 'Ptarmigan' AGM: compact spreading dwarf shrub with small, scaly dark green leaves. Clusters of pure white flowers 2.5cm in width from early spring. Height and spread 1m (3¼ft).
R. sutchuenense: large shrub or small tree with drooping leaves up to 30cm long. Trusses of bell-shaped flowers ranging in colour from pale pink to a rosy lilac with red or purple spots. Height 12m (40ft).
R. (Tessa Group) 'Tessa Roza': small, erect evergreen shrub with dark, glossy ovate leaves and small clusters of deep rose-pink, funnel-shaped flowers 5cm in width in late winter and early spring. Height and spread 1.5m (5ft).

Mid-season flowering (May)

Rhododendron macabeanum AGM:  Evergreen tree rhododendron with huge trusses of creamy to deep yellow flowers in mid-spring and large leaves to 30cm (12in) long; eventual height of 15m (50ft) and spread of 6m (20ft).
R. 'Daviesii' AGM: Deciduous azalea with a spread of 1.5m (5ft), with bluish-green leaves and buff buds opening to fragrant white flowers with a yellow flare, at first tinged pink, in late spring and early summer.
R. 'Exquisitum' AGM: deciduous azalea with open habit; long-tubed, frilled, fragrant cream flowers suffused with pink and with an orange flare in the throat, opening in late spring and early summer.  Height and spread 1.5m (5ft).
R. 'Faggetter's Favourite' AGM:  large, hardy hybrid with shiny leaves. Pink buds open to funnel-shaped, fragrant white flowers 9cm in width, tinged pink and speckled green in throat, borne in elongated trusses. Height and spread 2.5m (8ft).
R. 'Fragrantissimum' AGM: medium-sized shrub with open habit, and veined, hairy, dark green leaves. Funnel-shaped, spicily fragrant white flowers with a rose-pink flush, yellow in the throat, and borne in small trusses. Height and spread 2.5m (8ft).
R. 'Irene Koster' AGM: deciduous azalea with an open habit, with fragrant, funnel-shaped deep pink and white flowers 5cm in width, with an orange flare and deep rose-pink tube. Height and spread 1.5m (5ft).
R. lindleyi: evergreen with dark green leaves. Fragrant, funnel-shaped flowers in clusters of 4-8 in creamy-white, often edged and tinged with pink and blotched orange or yellow at the base. Height and spread 1.5m (5ft).
R. 'Mrs A.T. de la Mare' AGM: large spreading shrub with glossy, dark foliage. Slightly fragrant white flowers 10cm wide, with green spots in the throat, open from light pink buds in late spring. Height and spread 2.5m (8ft).
R. (Loderi Group) 'Loderi King George' AGM:  large evergreen shrub with long, pale leaves, and scented, white, lily-like flowers to 15cm wide, opening in tall trusses from bright pink buds in late spring and early summer. Height and spread 4m (13ft).

Late flowering (early June to mid-July or later)

R. arborescens AGM:  large deciduous azalea with glossy green leaves that produce good autumn colour. Strongly-scented, funnel-shaped white flowers in small clusters, with deep pink, protruding stamens. Height and spread 4m (12ft).
R. 'Corneille': bushy deciduous azalea with leaves colouring well in autumn. Flowers double, fragrant, cream, flushed deep pink on tube. Height and spread 2.5m (8ft).
R. ‘Cynthia’ AGM: Grows to a height and spread of 6m (20ft) with trusses of rose-pink flowers in late spring.
R. ‘Dopey’ AGM: Evergreen variety with a height and spread of 2m (6ft) and long-lasting red flowers in late spring.
R. decorum AGM: large evergreen shrub or small tree, with oblong leaves, pale beneath, and trusses of large, strongly fragrant, funnel-shaped white or pink-tinged flowers, greenish-yellow in the throat. Height 8m (24ft).
R. luteum AGM: Deciduous type for full sun with a height and spread of 4m (12ft) and strongly scented yellow flowers in late spring.
R. 'Norma': compact deciduous shrub with light green foliage that colours well in autumn, and double, orange-flushed, deep rose-pink flowers with a slight scent. Height and spread 1.5m (5ft).
R. occidentale: bushy medium-sized deciduous azalea with glossy foliage turning orange and yellow in autumn. Fragrant, funnel-shaped flowers to 10cm in width are white or tinged with pink, and blotched yellow within. Height and spread 2.5m (8ft).
R. (Polar Bear Group) 'Polar Bear': vigorous large shrub with mid-green leaves to 30cm in length. Lily-like, fragrant pure white flowers 11cm in width, with a small light green blotch at the base of the throat, are borne in late summer. Height and spread 4m (12ft).
R. viscosum AGM: bushy deciduous azalea with dark leaves that are glaucous beneath. Very fragrant, narrowly funnel-shaped white flowers up to 4cm wide, flushed deep pink on the tube, open in early summer. Height and spread 2.5m (8ft).
R. yakushimanum ‘Koichiro Wada’ AGM: Dome-shaped rhododendron to 2m (6ft) across and high, with rose-pink buds opening to pale pink flowers in late spring.

Links

RHS Find a Plant
AGM Rhododendrons

Rhododendrons can easily be grown in containers, as long as they are watered regularly.
Bold drifts, whether on a small or grand scale, can be impressive in spring.
Rhododendron 'Patricia's Day'
Rhododendron williamsianum
Rhododendron 'Rwain'
Rhododendron 'Caroline Allbrook'
Rhododendron leutum.
Rhododendron 'Martha Isaacson'
Rhododendron rex subsp. fictolacteum
Deadheading rhododendrons encourages better flowering.
    Rhododendrons can easily be grown in containers, as long as they are watered regularly. Bold drifts, whether on a small or grand scale, can be impressive in spring. Rhododendron 'Patricia's Day' Rhododendron williamsianum Rhododendron 'Rwain' Rhododendron 'Caroline Allbrook' Rhododendron leutum. Rhododendron 'Martha Isaacson' Rhododendron rex subsp. fictolacteum Deadheading rhododendrons encourages better flowering.

    Problems

    Non-flowering and bud drop

    • Rhododendron flower buds start forming in late summer. A short period of dry conditions at this time may cause flower buds to fail to form, or buds may only partially form, drying up and dropping unopened the following spring
    • Prevent this by mulching and watering thoroughly and regularly during dry periods from July onwards

    Leaf drop

    • Leaf drop can occur following a period of drought and is usually preceded by drooping and rolling of the leaves. The oldest leaves are the first to drop. Higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended periods of waterlogging
    • As with all evergreen shrubs, each year a proportion of the foliage (mainly older leaves) is shed in spring and summer. This is normal and not a cause for concern

    Leaf droop

    • Rhododendron leaves may droop in severe cold, but usually recover with a return to milder conditions

    Other problems

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