Michael Thornley,
Anyone who reads Harold Fletcher’s ‘A Quest of Flowers’ (Edinburgh University Press 1975) which describes the plant hunting expeditions of Frank Ludlow and George Sherriff, and unfortunately now out of print, will realise why the west coast of Scotland is a particularly favoured area for growing rhododendrons. On the Himalayan passes, between India, China and Tibet (the source of large numbers of rhododendrons introductions) the rain appears to fall almost continuously. High rainfall, and acid soil, are the key factors to growing rhododendrons successfully. But the terrain on which rhododendrons grow in the wild provides other clues: the land is often mountainous and steep and, consequently, is also well drained so that the ground water is carried away. At lower levels the larger leafed rhododendrons co-habit in stands of the same or similar species, often within a forest. In other cases medium sized rhododendrons grow at forest margins and at higher level, above the tree line, the small leafed dwarf rhododendrons grow on moorland.
It should be noted that, in nature, rhododendrons are not molly coddled, but fend for themselves. They have shallow, spreading root systems which are easily damaged if they are forked over or hoed in a garden bed, but make for the easy moving of plants if they have outgrown their allotted spaces. Much can be gained from studying the photographs in books of rhododendrons growing in their native habitats or reading accounts by plant collectors. Kingdon Ward, for instance, in his book ‘Assam Adventure’ (London 1941) describes descending the Poshing La:
“ In some places the water runs along the narrow crest of the ridges before tumbling over the side. The result has been to cut deep grooves along the ridge, with banks twelve or fifteen feet high, overhung with trees.”
Reading this I doubted his description but following the same route in 2004 I found that it was exactly as Kingdon Ward had reported. Moreover the cutting action of the water had revealed that the ground in cross section consisted of a thin but rich layer of humus and where everything below was grit and sand. The plants had no incentive to develop deep root systems and instead these were confined to the thin humus layer.
Therefore most rhododendrons will not survive in claggy or permanently wet ground. The problem is exacerbated if the usual advice is followed to dig a reasonable sized hole and fill it up with leaf mould, as this will often only result in a water filled pit. If the soil is self draining there should not be a problem but when planting on flat ground it will be worth taking Peter Cox’s advice and create a slightly raised mound, placing the rhododendron on top of this and spreading the roots over the soil and temporarily stabilising the plant with a couple of large stones. In this way good drainage is established with the slight risk of drying out, which can be dealt with by mulching the area around the rhododendron using leaves raked up from the grass.
Planting on sloping or steep ground often ensures a well drained site. But often we find that the natural leaf mould and top soil can be quite a thin layer over clay, or barren sub soil, necessitating the need to create shelves or pockets, to provide an anchor point on the slope and into which compost can be dug, prior to planting. One method of working on a steep slope is to cut out a divot on threes side and hinge it over and outwards to create a platform. Again it is worth placing the plant in a slightly raised location or on an slight angle, or alternatively creating a drainage slot out of a pocket on steep ground, to avoid the build up of water around the roots. Also take some time to carefully tease out the roots, especially if they have been in a pot for a while and an application of ericaceous fertiliser always gives the gardener a psychological boost, and probably helps the plants too.
In the wild some rhododendrons grow epiphytically, their root systems anchored to the well drained but damp deep moss and to the bark of the host trees. Archie and Sandy Gibson, who created the garden at Glenarn, sometimes placed small rhododendrons, such as R. valentinianum, on tree stumps, which provide very free draining sites, but our efforts to emulate this approach have only met with limited success. You need a tree stump with just the right amount of moss and, ideally, it should be a Scots Pine, which seems to harden in the ground like a fossil rather than softwood stumps, which soon rot down and can become a source of honey fungus.
Almost all rhododendrons, except some azaleas, are evergreen. Large leaved rhododendrons are like a boat under full sail, and therefore susceptible to damage from high winds. Likewise, these rhododendrons will find it difficult to survive in exposed and cold gardens. However, they will thrive in sheltered places and glades, even in the most unlikely locations, as Sir Osgood MacKenzie demonstrated so successfully at Inverewe. Some gardeners, such as the late Geoff Dutton who created what he called a marginal garden in the southern Grampians (‘Some branch against the sky: the practice and principles of marginal gardening’ Newton Abbot 1997), plant quick growing spruce in a large area of their gardens into which they will later cut openings, which can be gradually expanded once the shelter has been established. However, many gardens benefit from the shelter that is provided by established planting or from the historic woodland canopy.
Only the most tender rhododendrons require head cover as protection against frost but if the overshadowing is too dense it will also inhibit flowering. Better to plant tender rhododendrons in particularly favourable locations: those warmer, less frosty sites that can only be identified by a careful study of the garden’s microclimate. (It is said that tender rhododendrons benefit from the close proximity of other plants and by being placed on steeper or raised ground, to gain protection and to divert frost
away, as well as by avoiding an easterly aspect and morning sun that can encourage too early flowering and make them prone to frost damage).
Ideally, rhododendrons planted in sheltered parts of the garden should still have clear sky above them, to get them away from the major root systems of large trees but, as important, to allow them to grow straight up towards those clear voids in the air into which they can fully expand. A site that appears open in winter can become as dark as a curtained room in summer, all the light excluded by the heavy canopy. In direct contrast to rhododendrons that benefit from some protection, dwarf rhododendrons prefer a completely open site, with high light levels and no overhead cover whatsoever. The classic location for dwarf rhododendrons is the rock garden.
Rhododendrons are gregarious and grow well together, each affording the others protection and also creating wonderful effects of texture and colours. However, getting the spacing right at the outset will ensure that plants do not grow into each other and will avoid the need to move plants around later, like musical chairs. Sylvester Christie of Blackhills, near Elgin, wrote one of the best articles on the planting of rhododendrons which he entitled ‘Rhododendrons in the Space Age’ (Rhododendron with Magnolias and Camellias: Royal Horticultural Society 1974). In it he makes the obvious, but often forgotten point, that if rhododendrons are not given sufficient space in their youth there will be problems in old age, not only for the plants, which will be drawn up or intertwined with near neighbours, but also for the ageing gardener who will be less able to deal with the problems.
Gardeners, like the rest of the world, often want it both ways; they seek advice on how to grow rhododendrons but are looking to control the process so that they are not too large or too high. But plants live in an ever expanding environment and this problem, if it is a problem at all, can only be addressed by distinguishing between and selecting rhododendrons that either grow into trees, or into large shrubs, or small plants, or dwarf rhododendrons (although on the west coast of Scotland some dwarfs rarely remain small for any length of time).
Given the need of the larger rhododendrons for space and protection they may be best placed at a distance from a house and part of, for instance, woodland. The smaller, more exotic shrubs might be positioned in intermediate areas of the garden, possibly as features, with a backdrop of trees and other plants, or within spaces, glades, or ‘rooms’ created in the garden. Close to the house is often an appropriate location for dwarf rhododendrons. This is by no means a fixed rule but is a useful framework. Another way of looking at composition is to place the sturdy species plants at a distance from, and the more exotic and hybrid rhododendrons nearer to the house, creating a graduation from the ‘wild’ to the ‘man made’. But this is getting rather too close to the selection of particular rhododendrons to suit the different areas of the garden, a huge subject that has to be left to another day.
Michael Thornley, (SRS Yearbook No.15 Winter 2013-4)