How to grow rhododendrons

By Andy McIndoe

Rhododendrons – jewels of the shrub world

Whether you grow them in your garden or not there is no denying that rhododendrons are amazing flowering shrubs. They are always at the forefront of my mind at this time of the year because they are such an important aspect of colour at RHS Chelsea Flower Show. Most flower naturally in late spring and their blooms are long-lasting; an important quality at a flower show as well, as in a real garden situation. They also respond well to manipulation of flowering time: flower show exhibitors use cold storage and gentle warmth to ensure that blooms are in the peak of condition at the right time.

For many the mention of rhododendron conjures an image of a big evergreen mound of a plant with shocking pink or purple flowers in large rounded heads in late spring. Shrubs suited to large gardens; a month of colour followed by eleven months of dark green. Rhododendrons need acid soil, so if your soil is alkaline you cannot grow them, but you probably yearn for them. Species, notably Rhododendron ponticum are widely naturalised in some areas, competing with native species and giving Genus Rhododendron a bad name in certain circles.

Rhododendron luteum

In reality rhododendron are a diverse group of plants that vary from small shrubs a few centimetres (inches) high to great trees reaching many metres (yards). There are both evergreen and deciduous species and perhaps this is the moment to clarify the difference between azaleas and rhododendrons. Botanically they are all Rhododendron. I often see puzzled visitors to gardens such as RHS Wisley, Surrey pondering over a label on a deciduous azalea such as Rhododendron luteum, the fragrant deciduous azalea. “I always thought this was an azalea?” I hear them say. In garden speak it is, in botanical Latin it is a rhododendron.

Rhododendron 'Diamant Group'

The same applies to the Japanese evergreen azaleas. These have small evergreen leaves and profuse single flowers; sometimes large and sometimes small. Their colours are often dazzling and given a few years some can reach up to 1.8 metres, 6ft in height with a broader spread. Usually they are much smaller than this and are ideal for small, shady gardens, either in the open ground or in pots and containers filled with a lime free growing medium. A group of evergreen azaleas in full flower has a jewel like quality. I remember staging a drift of evergreen azaleas at RHS Chelsea Flower show one year and the art student I was working with described it as arranging crushed velvet; an excellent description!

Rhododendrons and cornus at RHS Wisley

So how do you use rhododendrons (and azaleas) in the garden? Are these just large evergreen shrubs that are used to temporarily illuminate gloomy corners of large gardens? Far from it. Sadly they are often just gathered together with their own kind with little thought to colour combination and design. We’ve probably all been exposed to orange yellow and pink deciduous azaleas crammed together without a dilution factor. However when a little thought is put into a colour grouping of rhododendrons and azaleas the effect can be stunning. I loved this frothy pastel grouping of evegreen azaleas gathered around an early flowering cornus, the effect was just breathtaking.

Rhododendron loderi with bluebells

When I planted my garden, on acid sand, I wanted to incorporate some hardy hybrid rhododendrons (evergreen) and some deciduous azaleas. I’ve always loved the sight of the larger evergreen rhododendrons growing amidst a sea of bluebells under the dappled shade of trees. As we have oaks and bluebells I planted rhododendrons which sometimes flower at the same time as the bluebells. I have two young Rhododendron loderi hybrids which now reliably produce their blooms when the bluebells flower.

Rhododendron loderi

Rhododendron loderi can hardly be described as a popular garden plant, but if you have the space it is one to grow for the future. The massive trusses of funnel-shaped flowers are elegantly frilled and wonderfully fragrant. It is more open in habit than many rhododendrons and just gets better with age.

Rhododendron 'Horizon Monarch'

Many of the stronger coloured rhododendrons are more difficult to combine with other subjects. If I had to choose just one hardy rhododendron it would be ‘Horizon Monarch’, a wonderful plant raised in the US. It has good olive-green foliage and glorious blooms of clotted cream that open from orange-tinted buds. This colour is such a good mixer. I have it planted alongside the dark purple Rhododendron ‘Colonel Coen’. I often use it with purple or copper shades at RHS Chelsea Flower Show and its always one that is noticed.

Rhododendron 'Colonel Coen'

The deciduous azaleas are easier to mix with other plants; I like to think of them as more sociable. They are more open and looser in growth habit and change with the seasons; many display amazing autumn colour. Their delicate flowers are often fragrant and are carried in open clusters. I love the pale yellow and white flowering varieties such as Rhododendron daviesii, this has a delicious and powerful fragrance.

Rhododendron 'Daviesii'

Rhododendron ‘Irene Koster’ is a delicate shade of salmon pink with a gold flare in each bloom. I particularly like it under planted with the perennial honesty Lunaria rediviva. This has delicate silver-lilac flowers that sparkle amongst the lower blooms and apple-green emerging leaves of the deciduous azalea.

Rhododendron 'Irene Koster' with Lunaria rediviva

So if you’ve dismissed this amazing group of shrubs because you have the wrong soil or don’t think they will mix with other plants in your garden take another look. Many make excellent subjects for pots and containers and there is certainly a size, habit and colour to suit just about any planting combination.

Andy McIndoe

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