Rhododendrons

Some of the most magnificent of flowering evergreen shrubs and trees are the Rhododendrons. These spectacular shrubs and small trees are native mostly to the temperate and subtropical regions of the Northern Hemisphere.
 
Most of the hardiest varieties come from Europe and many from the Americas while others are Chinese. The name Rhododendron is Greek meaning “Rose Tree” which aptly describes their beautiful blooms that usually appear, dependent upon species and variety, from Late Autumn, Winter and Spring in temperate varieties and year-round in the tropical Vireyas.

Flower colours cover almost every shade in the rainbow. They range in height from low spreading ground-cover species less than 1m/3t tall to small spreading trees which can easily attain 10m/33+ft. Most are rather open and much branched with broad, often glossy simple spear-shaped leaves clustering near the end of each branch. Flower buds develop well ahead of their blooming and are almost always at the end of each branch.

The hardy, cool-climate Rhododendrons are very hardy throughout New Zealand with the majority preferring the cooler conditions of the south. But many of the cool climate varieties will thrive in Auckland provided they receive the proper conditions. This amounts to them being exposed to cool air so that buds can set and mature properly. Because these are mostly forest and woodland natives, they prefer a partly shaded or dappled sun position.

They are one of the only shrubs that will flower in full shade. But most do much better when their heads are in the sunshine. This is especially true for the much more tender subtropical Vireya cultivars and species.  Almost all Vireya species are quite frost tender and are killed by freezing weather. The hardy species can tolerate some frosts or occasional freezing provided this is not severe and they have been planted in a protected environment.

But severe cold often results in damage to exposed foliage and developing buds and flowers will often be damaged or destroyed by the cold. In such situations, it is often best to shelter them from the early rays of morning sunlight that could magnify within the droplets and ice crystals causing spotting of the blooms.

All Rhododendron shrubs and trees must never dry out and need a constant supply of moisture but good drainage. They will not survive long in sodden wet soils that might become sour. In their native woodland habitat, their shallow roots feed off of a continuous supply of moist leaf litter which is of an acid pH.

Thus wherever they are planted they require an acid soil that is rich in humus and leaf mould. When preparing their site for planting dig in plenty of compost, well-aged manure, peat, crushed pine needles or untreated acidic sawdust (cedar, oak, pine, etc.) In limy land also dust the ground with powdered Sulphur. Water this in and let it settle into the soil for a week or more before planting. This will help lower the soil pH.

Mulching around the plants is important to maintain the cool, moist, constant conditions they prefer. Mulch with peat, compost, well-aged sawdust or oak leaves, pine needles, bark, or chopped bracken or pea straw.

The mulch should be at least 8cm/4 inches deep and kept constant, replacing it in Spring and Autumn. During periods of drought or dry and windy weather, make sure that the soil within the mulch remains continually moist. Often if the soil does dry out, the shrubs' sturdy evergreen foliage will show little sign of stress until suddenly the leaves begin to wilt.

By then developing flower buds will likely have been damaged and may even fail to open properly later on as they should. Vireya Rhododendrons, being somewhat epiphytic species, respond very well to frequent misting over their foliage almost as much as they do to a good soaking. Most Rhododendrons prefer rather high humidity. Low humidity combined with dry weather often invites predation by Mites and Thrip which soon transmit other infectious diseases.

While Rhododendrons will need little feeding provided they are constantly surrounded by a generous organic mulch, it is possible to boost growth and flowering with commercial plant foods made specifically for them or by spreading even and light sprinklings of potash, superphosphate, and sulphate of ammonia around the shrubs in early Spring and Autumn.

Autumn and Winter (in mild and moderate climates) is the best time to plant Rhododendrons. First dig out a planting hole twice the size of the root ball. It is more important to make the planting hole wide rather than deep as Rhododendrons produce only limited deep anchoring roots but mostly spreading, surface-feeding roots. Then fill the hole with aged-compost, peat, very well-rotted manure and/or a generous barrow load of the acid mulching materials.

This is especially important in heavy soils and very sandy land.  Avoid all fresh manures that could be too caustic or mushroom compost that has a limy content so is of a higher soil pH.  Dig this in well adding sand, gravels, bark or peat if the soil is extra heavy. Mix these ingredients thoroughly so that the final product is a fluffy, light and loose mixture with good organic content. Then remove this mix or push it to the sides of the planting hole so there is ample room to place the shrub within.

Removing the shrub from it’s bag  or container, spread the roots outward rather than down as these are surface rooting plants. If the site is prone to drying out plant in a shallow basin. For heavy clay soils or those that poorly drain, plant on raised mounds of peat or the aforementioned soil mixture. Fill in the hole, adding only a few centimeters of soil over the top of the root ball. Water in thoroughly and add the mulch immediately and keep moist. If the site is at all prone to winds, stake the young plants at the time of planting.

Vireya Rhododendron, being somewhat epiphytic plants, respond very well to growing in containers.  This allows them to be shifted about to capture the light and warmth they require and shelter them from frosty nights or freezing. They also can be planted throughout the Autumn and Winter, or almost anytime with suitable care. Most traditional cool climate hardy Rhododendrons have such a large and spreading root system and need such constant supplied of moisture that they often struggle and eventually fail when planted into containers unless these are quite large landscape planters or raised beds.

Rhododendrons are highly compatible and effective when planted with shrubs and trees like deciduous and sometimes evergreen species of Magnolia and Viburnum. Also plant them amongst Maples, especially Japanese Maples, upright and weeping Cherries, Plums and most Prunus species, Cornus (Dogwood), Birch and Acacia (Wattles). Also plant them with compatible companion shrubbery like Adenandra, Azalea, Camellias, Chaenomeles japonica, Choisya, Pieris japonica. They make a spectacular display when mass planted on banks, slopes and hillsides in the woodland garden or in an open forest/bush setting

Many ground-covers also are highly compatible with Rhododendron plantings. These are often established as plantings on top of the mulch. This includes a wide variety of low growing perennial species like Epimedium,Vinca minor and most Violet species and many varieties of Primula. Myosotis, the Forget-Me-Not is a wonderful annual ground-cover. to allow to spread beneath them. The entire planting can be even further enhanced when inter-planted with Spring Flowering bulbs.

All of these plant species are well planted now in Autumn through Early Winter so that they will be well established for a memorable Spring display with many more years of beauty to come. Considering that there are over 1000 species and 10,000 named varieties and hybrids to choose from small trees to shrubby ground-covers, there’s something for everyone in the world of Rhododendrons.

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