Rhododendrons & Azaleas
What are they?
The heath family is well known for its dislike of chalky soil, for carpeting great swathes of heathland with brilliant colour in late summer and for providing a number of our garden ornamental plants. Apart from their love of acid soils, the members of this family are rather diverse, but most are woody trees, shrubs or low subshrubs and most have flowers that have the five petals fused together to form a flask-like tube. This page covers the rhododendrons, which are rather different to other members of the family in their large and colourful flowers and in often growing to quite sizeable bushes (even trees in some cases). The group includes both evergreen and deciduous species and many are popular as garden ornamentals with a huge number of hybrids having been created in cultivation.
Where are they found?
The European Rhododendron has been widely introduced as an understory plant in wooded areas on acid soils, from where it has rapidly spread to be a highly successful, invasive alien. Other species and cultivated varieties of Rhododendron occur occasionally where planted in woodland on private estates or occasionally as relics of cultivation.
Identification
Almost all plants found are likely to be the European Rhododendron, but others do occasionally occur and should be taken into consideration. Flower shape and colour will be important to confirm identification, along with leaf detail, including leaf shape and size, plus the presence of any hairs and their colour.
NOTE: Plants in the UK included under Rhododendron ponticum have more recently been found to include a range of introgressed hybrids involving R. ponticum and a cluster of Asian and North American species currently in cultivation. It has been suggested that these hybrid events are at least in part responsible for the vigour of many plants in the wider countryside, though this concept is not universally accepted. So far, the presence of these hybrids - lumped under the name of Rhododendron x superponticum - does not seem to have been confirmed in East Anglia but this may simply be through a lack of looking. The hybrids are said to differ in the presence of hairs on the calyx lobes and ovary (found at the base of the flower, inside the petal tube) and sometimes on the underside of the leaves.
European Rhododendron Rhododendron ponticum
Introduced from southwest Europe and now widely found in wooded or heathy areas on acid soils. Highly invasive (but see note above) and vigorously controlled in a number of places. Flowers May to June. A many-stemmed, large bush or small tree, to 5m in height. Stout, leathery, evergreen leaves. Flowers in various shades of pinkish-purple, with orange, yellow or greenish spots. Ovary hairless or with just a few, scattered hairs.
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Hardy Hybrid Rhododendrons
Grown as garden ornamentals in a huge range of forms. Common in cultivation on acid soils and just occasionally found as garden throw-outs or more often as survivors from old plantings. Flowers April to June according to variety. A great range of forms is included here, from spreading shrubs to specimens that may attain 10m in height and with flower colours from white, through pinks and purples, to reds, with or without spots. Most of these cultivated varieties were artificially produced by initially crossing Rhododendron ponticum with three North American species (R. maximum, R. catawbiense and R. macrophyllum) and later with a range of Asian species. They make up almost all of the hybrids that were popularly planted in woodland on old estates and typically have large, evergreen leaves, an absence of scales on leaves and stems and flowers all in terminal clusters.
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Tree Rhododendron Rhododendron arboreum
Introduced from Asia as a garden ornamental and occasionally surviving where originally planted in woodland on old estates. Flowers late April to May. A tall species that may grow to 50m in the wild but is typically much shorter in cultivation, to 10m or so in height. A very variable set of cultivated forms exist as there are several subspecies of R. arboreum which differ in flower colour and leaf detail. Stout, leathery, evergreen leaves. The one illustrated here and found in woodland in West Norfolk is R. arboreum subsp. arboreum var. cinnamomeum, which has white flowers and richly felted undersides to the leaves.
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Cuneate Rhododendron Rhododendron cuneateum
Introduced from China as a garden ornamental and found long-naturalised in woodland in West Norfolk in 2021. Flowers May. A spreading species to 2m in height. Readily told from the commoner rohdodendrons in cultivation by the covering of shining scales on leaves and shoots (known as lepidote scales). Flowers relatively small (1.5-3cm across) and leaves broadly elliptic to 7cm in length.
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Yellow Azalea Rhododendron luteum
Introduced as a garden ornamental from SE Europe/Asia Minor and occasionally found persisting where originally planted in woodland. Flowers May to June. A deciduous shrub to 4m in height. Leaves 6-12cm long, rolled inwards when young and opening flat. Previously known in gardens as Azalea pontica. Some plants growing in the wider countryside may be cultivated varieties involving hybrids of several species. They come in a wide shades of various yellows, oranges and white.
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