Rhododendron

Hardiness and variety have ensured there is a species for every situation, says Miranda Kimberley.

Rhododendron ‘Cynthia’ – image: Flickr / Tim Waters
Rhododendron ‘Cynthia’ – image: Flickr / Tim Waters

Rhododendron is one of the most diverse genera in the plant kingdom. It is made up of around 1,000 species of evergreen or deciduous trees and shrubs. The majority are evergreen shrubs, with handsome foliage and showy, colourful flowers produced between mid-spring and early summer. The leaves vary greatly in form, colour, texture and size. Some have a dense layer of hairs on the underside known as indumentum, and these forms have become highly sought after.

Because it is such a huge genus it will be impossible to describe in full here all the types available. But the genus can be roughly divided into these categories - dwarf, small, medium and large rhododendrons and evergreen and deciduous azaleas.

For landscapers and professional gardeners of large gardens, it is medium to large rhododendrons, including the group referred to as the Hardy Hybrids, and the smaller forms, particularly the hardy hybrids of R. yakushimanum, which prove the most useful. But there are many other species and varieties on offer for the more adventurous gardener. Indeed the interest in rhododendrons is so high that backyard horticulturalists are constantly creating new hybrids for nurseries to propagate and bulk up.

The Hardy Hybrids were first created during the Victorian era, bred from several species including R. catawbiense, R. maximum, R. arboreum, R. caucasicum and R. ponticum. They were selected for their hardiness, free flowering nature and ability to withstand pests and diseases and pollution.

As a group they are indispensable for planting in cold and exposed locations and are unsurpassed for landscape effect. Many of them make excellent informal hedges or screens. More recently R. yakushimanum has played an important role in the development of many new hybrids suitable for the smaller garden.

In general rhododendrons need a humus-rich acid soil, with a pH between 4.5 and 5.5. They like to be planted shallowly, as they are surface-rooting. One of the most common failures with rhododendrons occurs because they have been planted too deep.

They require a location in dappled shade and moist soil, as they should never dry out, but nor do they cope with very wet soil.

While generally robust, rhododendrons can suffer from varied pests and diseases including vine weevil, leafhoppers, scale insects, bud blast, honey fungus, and lime-induced chlorosis if the soil is not acidic enough.

Phytophthora ramorum (Sudden Oak Death) and P. kernoviae have infected some rhododendron plantings in milder, wetter areas of Britain, particularly in the South West. To prevent the spread of the disease, the Food and Environment Research Agency, which is responsible for plant health, has issued guidelines for landscape managers on prevention measures. The fact sheet advises using reputable suppliers and explains quarantine methods, as well as offering help on recognising the symptoms of the diseases - see bit.ly/dHUowu (pdf).

What the specialists say

- James McCardle, sales manager, Osberton Nurseries, Nottinghamshire

There's a rhododendron for every garden. They range from very tiny types to 2m-high Hardy Hybrids, which can have a stately feel to them. Some of the popular varieties for landscaping include 'Cunningham White', and 'Albert Schweitzer' with its traditional pink flowers. I also like one called 'Jean Marie Montague' - a good red. There's also a lovely lilac one, 'Catawbiense Grandiflorum'.

The yakushimanum hybrids will grow to around 1m tall within five to 10 years, so more compact than the Hardy Hybrids. They are spectacular, very blowsy, providing a ball of colour from a distance.

As for Phytophthora, nurseries such as ours are checked regularly, and fortunately we have never had an outbreak. Our nursery is kept nice and clean, and therefore the stock is healthy.

- David Millais, owner, Millais Nurseries, Surrey

Hardy Hybrids are popular due to being evergreen, very floriferous and colourful over a long period in April-June.  Most are dense screening plants, which are good for noise suppression and privacy.  They are good on acid soils with a good humus content with up to pH6.0.  

They are ideal for large gardens and woodland gardens, and most varieties are good in sun also.  They may be relatively expensive to start with, but overall they are long lived plants requiring very low maintenance and offer exceptional value.  
 
Rhododendron fastuosum flore pleno is a good alternative to R.ponticum.  It is much more disease resistant, tidy and manageable, and is not invasive.  Double mauve flowers are an added feature. The other varieties which stand out for me are Gomer Waterer, Jean Marie Montague, Mrs Charles Pearson, Mrs TH Lowinsky and Nancy Evans.
 
With regards to Phytophthora, we have dropped from production any problem varieties.  In particular we do not grow R. ponticum and its close hybrids. We run a clean nursery and try to grow the plants as clean as possible with minimum stress so that they are strong enough to combat any disease. 

John Middleton, owner, Shelley Common Nurseries, Hampshire

Rhododendron is a large genus with a wide distribution around the world from the Equator to northern latitudes, found at sea level to high altitude. The wide number of species have adapted to cope with varying climatic conditions of temperature, precipitation and light levels. Hardy Hybrids are a good start and these are the plants that I tend to grow most of.
 
The old Hardy Hybrids many hybridized in Victorian times were selected for their ability to withstand low temperatures, pests and diseases, pollution as much as for their ability to flower freely. This gave the customer a relative trouble free low maintenance shrub, almost plant and forget with a guarantee of a good show of flower.
 
The good old favorites that have stood the test of time include Pink Pearl, Lord Roberts and Sappho. The yakushimanum hybrids have added a welcome addition to this range by giving a smaller more compact plant to suit many of the smaller gardens.
 
When planting Rhododendron it is important to remember that apart from the obvious of avoiding water logging and lime, that in the natural state their hair roots grow up into the rotted leaf litter rather than down into the soil, this means that once established they are self sufficient. In high rainfall areas they have found to grow on limestone rock. It is best to plant in a shallow hollow and mulch well, ideally with well composted leaf mould and dead head flower heads.
 
Hardy Hybrid Rhododendrons have few problems. Vine weevil can attack young plants if not treated in the nursery, but in general few problems.

In Practice

- Kevin Powell, head gardener, The Terraces, Regents Park, London

Rhododendrons typically like to be planted on the fringes of borders with a northerly aspect, in semi rather than dense shade. We water them as often as we can but they seem to cope with slight neglect.

We incorporate leaf mould and peat-based ericaceous compost when planting. We make shallow but broad holes and plant them slightly raised. We mulch them regularly and acidify the soil with sulphur chips, which are easier to spread and give a more even dose than ammonium sulphate. They last up to six years.

I am currently dealing with a site that is quite dry and has a pH of 6-6.5, which is not ideal for rhododendrons, but I am finding 'Sappho' and 'Cunningham's White' both cope with these conditions.

Species & varieties

- R. 'Cunningham's White' is a tough hybrid, good in exposure and useful as hedging. It bears white flowers with yellowy-green centre. Height: 1.5-1.75m after 10 years.

- R. 'Cynthia' Award of Garden Merit (AGM) (H4) bears trusses of rose-pink flowers in late spring. An old favourite, happy in full sun, reaching 6m tall and wide.

- R. 'Dopey' AGM (H4) is a deep-red yakushimanum hybrid with long-lasting flowers borne in upright firm trusses from May to June. It has a compact habit with dense light green foliage. One of the 'Seven Dwarfs' series. Height: 80-100cm in 10 years.

- R. 'Fastuosum Flore Pleno' AGM (H4) is a hardy hybrid that bears semi-double bluish-mauve flowers in late May-June. Ideal for screening. Height: 1.8m in 10 years.

- R. 'Gomer Waterer' is a striking, vigorous and well-proven variety. It flowers in early June, producing rosy-lilac buds which open to white flowers flushed mauve pink at the edges with a yellow-brown flare. Resistant to sun, heat and wind. Height: 1.5-1.8m in 10 years.

- R. 'Loderi King George' is a large hardy hybrid reaching 4m by 4m. It bears large clusters of highly scented, pink to white flowers in May to June.

- R. 'Markeeta's Prize' AGM (H4) produces large scarlet flowers with darker spots in May. It is a tall upright plant with thick dark green foliage. Best in partial shade to prevent fading. Height: 1.5-1.8m in 10 years.

- R. 'Mrs T H Lowinsky' has mauve buds which open to orchid pink, and fade to white with a conspicuous orange blotch in June. Very hardy and fast growing, it will perform well in all reasonable conditions. Height: 1.8-2m in 10 years.

- R. 'Sappho' is a hardy hybrid that is mauve in bud with flowers in pure white with striking purple markings. Height and spread up to 3m.

- R. 'September Song' produces large trusses of yellow-orange flowers with peachy-pink highlights. A floriferous and compact plant with olive green foliage. Good for a position in sun or light shade. Height: 1.5m in 10 years.

- R. yakushimanum 'Koichiro Wada' AGM (H4) Selected from seedlings raised at Exbury Gardens, this dense, domed form is considered among the best of yakushimanum varieties. The pink buds open to reveal white bell-shaped flowers in May. The young silvery leaves turn dark glossy green, with dense brown indumentum underneath. Height: 60cm. Spread: 1.3m in 10 years.


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