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Monthly Archives: October 2016

Oct 26

How to care for and when to fertilize Rhododendrons

By chadchaffin | Gardening

There’s nothing shy and retiring about the rhododendron family – the colours are vibrant, the flowerheads large and voluptuous – and the delicious fragrance of Rhododendron luteum (commonly known as the yellow azalea) drifting in the air will stop you in your tracks. To see them in their full glory, visit the woodland garden at Bowood House in Wiltshire during May and early June, when the mature glades of rhododendrons are in full and magnificent bloom. Immense undulating drifts, some reaching ten metres high, blend brilliant- and subtle-hued shades along pathways that wind through the 70 acres, where carpets of bluebells fringe the paths.

Successive generations of the resident Lansdowne family have been planting this hillside with rhododendrons since 1854. The third Marquis established the original planting, while the fifth expanded the garden and added many more varieties. The current Lord Lansdowne (and ninth Marquis) continues this tradition in a very hands-on manner. He loves nothing more than escaping his other duties and disappearing across the parkland in his battered four-wheel drive for a bit of woodland management: it may be clearing land with the help of a specially adapted digger, pruning or planting. Tucked well away, there’s a shed where he can get on with repairing tools and machinery, and a modest greenhouse for propagating seed he has gathered and growing-on cuttings with the help of a misting unit. Rhododendrons root readily from layering (making a shallow cut in a low-growing branch and pinning it to the ground), so he also uses this technique on selected specimens to increase the collection.

How to Identify Rhododendron Species

From Visually.

Fortunately, there’s no shortage of things to keep the Marquis happily occupied for the foreseeable future and even in such an established setting there are opportunities for planting new areas. Most recent is the four-acre Jubilee Garden, which was begun in 2006 in a small valley with a stream at its centre. Compared with the rest of the woodland garden, it is still in its infancy, but it has allowed Lord Lansdowne to make his own mark as he has planted it with favourite varieties and extended the season of interest by adding other shrubs, including hydrangeas, magnolias, cornus and eucryphia. Few of us have the acreage or the legacy of planting that exists at Bowood, but for anyone who has more modest inclinations to grow rhododendrons, a visit here will provide colourful inspiration.

Where to grow Rhododendron

As a general guide, the smaller the leaf of the rhododendron, the more sun-tolerant it will be. Large-leafed specimens are woodland plants and, to grow well, they need the shade and shelter provided by a canopy of mature deciduous trees. Compact and dwarf varieties and evergreen and deciduous azaleas are all suitable for container- growing in ericaceous compost. They must not be allowed to dry out (or become waterlogged) and should be watered with rainwater.

The menace of the wild rhododendron

There’s no doubt that the sight of a hillside covered in the wild purple Rhododendron ponticum in full bloom is very attractive, but it comes at a great cost to the native flora and fauna. Since it was planted as cover for pheasants in Victorian times, it has thrived in the damp conditions in the north and west of Britain and taken over great tracts of countryside, squeezing out other plant species and proving very unwelcoming (and poisonous) to wildlife. It is also incredibly difficult to eradicate.

A single large bush is able to produce up to one million seeds in a single year, so even if the parent plant is removed, it still requires several years of controlling the seedlings. If all of this was not worrying enough, in recent years it has also been established that it is host to the plant disease phytophthora, which threatens to kill many of our favourite native shrubs and garden trees. However, two things can be done to help – firstly, buy rhododendron cultivars (never R. ponticum) from reputable specialist growers and, secondly, work as a conservation volunteer assisting organisations including the National Trust and the Woodland Trust with clearance programmes

Planting tips for Rhododendrons

Acid soil is essential for most varieties and it is important that it has moisture-retentive qualities. The evergreen foliage provides structure in the garden throughout the year, but can be dull once the blooms have finished. To extend the flowering season, mix rhododendrons with other acid-loving shrubs, such as Japanese maples, magnolias, cornus and pieris. The dense canopy and shallow roots of mature rhododendrons prevent other plants establishing nearby.

Choose varieties carefully to fit the scale of your garden: left to their own devices, some become very large. There are many lovely pastel-hued cultivars if vibrant combinations are not to your taste. Buy plants when they are in flower so you can be sure you like the colour of the blooms. To plant, prepare a large hole and incorporate plenty of leaf mould and well-rotted compost into it. Rhododendrons are shallow rooters, so ensure the rootball is level with the surrounding soil. Water well and mulch with leaf mould, composted bark or pine needles (not nutrient- rich animal manure or lime-rich spent mushroom compost), but keep the stem free of mulch.

Types of rhododendrons

  • R. augustinii  Striking violet-blue funnel-shaped flowers on a small-leaved shrub; 1.4m-1.6m high.
  • R. ‘Loder’s White’  Upright clusters of mauve-pink buds opening to trusses of slightly fragrant trumpet-shaped white flowers up to 10cm across; 1.5m-2.5m high.
  • R. luteum (yellow azalea) Scented blooms on a deciduous shrub with vivid-hued autumn foliage; 2.5m-4m high.
  • R. campanulatum x pictum  Natural hybrid bearing pale lilac-pink flowers with speckled throats; attractive large glossy leaves; reaches 1.5m in ten years.