When to plant rhododendrons?

In mild climates rhododendrons and azaleas can be planted almost any time of the year with reasonable success. In colder areas, early spring planting is recommended, with early fall planting being a second choice. In hot areas, fall planting is recommended, as this allows the plant’s root system to get well established during the colder fall and winter months.

Rhododendrons and azaleas do well with direct light for at least part of the day. Excessive shade normally results in very limited flowering. In hot areas, northern exposures are preferable to southern exposures. Exposure to constant wind is not desirable, especially the salty winds of marine environs. Generally large-leaf rhododendrons are less tolerant of sun and wind than small-leaf rhododendrons or evergreen and deciduous azaleas.

Proper soil conditions are very important. (See Soil Conditions for more information.) Rhododendrons are acid loving plants. As such they perform best when the soil is acidic (with a pH between 4.5 and 6.0). They need well-draining soil with an abundance of organic matter. Rhododendron and azalea roots also need oxygen for healthy growth. Many materials can be used to amend the soil. Compost or decomposed pine bark are very effective. Heavy clay soils collect and retain water so it is recommended to plant rhododendrons and azaleas above the base clay soil in a mound of desirable soil. (See Figure below.) If you dig a hole in heavy soil and fill it back with a light soil mixture, you may be creating a bucket which will hold significant water.

The top of the root ball should never be below the level of the surrounding soil. The top of the root ball should be planted several inches above the surrounding soil. Planting rhododendrons and azaleas too deep can eventually lead to plant death.

Planting near concrete foundations or other concrete materials is to be avoided as the concrete creates alkaline conditions (a pH of 6 or above) that are harmful for healthy rhododendron and azalea growth. People often use aluminum sulfate to lower the pH of soil, i.e. make it more acidic, for growing hydrangeas. Using aluminum sulfate to acidify rhododendron soil is not recommended as aluminum is toxic to rhododendron and azalea roots. To lower soil pH use wettable sulfur or ferrous sulfate.

In hot climates, root rot organisms flourish in wet soils and can kill rhododendrons and azaleas. Under these conditions, raised planting beds that incorporate 50% or more fine pine bark can be helpful in suppressing Phytophthora root rot. Extreme cases may require the use of fungicides such as Subdue or Aliette. Always read and follow label instructions when using chemicals. Soil from plants that have succumbed to Phytophthora should not be reused to plant more rhododendrons.

Plants should be thoroughly watered prior to planting. The roots should be loosened. Root bound plants that have been in containers for a lengthy time should be thoroughly loosened, and some of the outer roots cut. With a knife make vertical cuts 2″ or more deep, equally spaced around the sides of the root ball. Use your hands to gently loosen the roots where cuts were made and pull the roots outward. This will stimulate new root growth and allows water and nutrients to penetrate into the root mass.

Field grown plants are dug up with a ball of soil around their roots, and the ball is then wrapped in burlap or a synthetic material and tied with twine or wire. Such balled-and-burlapped plants can be damaged if handled roughly. Always support the bottom of the root ball when moving the plant; avoid dropping the plant which might shatter the root ball. The burlap may be left on the root ball unless it is plastic or otherwise non-biodegradable. Open up the biodegradable burlap and lay it well back from the trunk, remove any cord or wire. Field grown plants typically are grown in heavy soil that holds the ball together when dug. Loosen the rootball and cut some of the roots to stimulate the growth of new roots. The texture of the soil surrounding the root ball should match that of the rootball.

Rhododendrons and azaleas rely upon their shallow root structure for their water and nutrients. It is very beneficial to mulch around the plant at least out to the drip line. Do not pile the mulch right up to the trucks, leave 5″ – 6″ free of mulch. Mulching helps accomplish several important functions. It helps in keeping the soil moist and cool. As it decomposes it provides nourishment for the plant. Competing plants and weeds have a more difficult time getting established so they are not taking the moisture and nutrients. Many materials are useful as mulch, such as fir bark, pine needles, wood chips, composted materials, etc. Ideally, the mulch should have loose texture to allow water and air to reach the root zone. Chips, bark, compost materials, etc. should be allowed to age at least for a season so that they do not use up the available nitrogen to decompose, thus leaving less for the rhododendrons. They are not sufficiently decomposed if they heat up.

Index of Contents

Classification

Landscape Use

Plant Selection

What To
Plant Where

Weather
Protection

Soil
Conditions

Irrigation

Fertilization

Pruning & Spent
Flower Removal

Propagation &
Hybridizing

Transplanting

Insect & Disease Control

Rhododendron

Rhododendron

Rhododendron ponticum
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Ericales
Family: Ericaceae
Genus: Rhododendron
L.
Subgenera
  • Azaleastrum
  • Candidastrum
  • Hymenanthes
  • Mumeazalea
  • Pentanthera (Azaleas)
  • Rhododendron
  • Therorhodion
  • Tsutsusi (Azaleas)
  • Vireya

Rhododendron is the common and genus name for a large and diverse group of woody shrubs and small (rarely large) trees in the flowering plant family Ericaceae. It is a large genus with over 1000 species, and subdivided into various subgenera (commonly eight, nine, or five, depending on the taxonomic scheme). The trees may be deciduous or evergreen and the spirally arranged leaves can vary significantly in size according to species, from one to two centimeters (0.4-0.8 inches) up to 100 centimeters (39 inches). Members of two of the traditional subgenera, Pentanthera (deciduous), and Titsushi (evergreen), are known as azaleas.

Rhododendrons, including azaleas, generally have showy flower displays. Indeed, the name rhododendron comes from the Greek rhodos, meaning “rose,” and dendron, meaning “tree.” As such, and combined with their often being evergreen, they often are cultivated as ornamental plants, such as around the foundations of houses or in woodland gardens, and even as specimen plants.

They also provide important ecological values. They provide food for a number of insects and an important source of pollen and nectar for many bumblebees, who in turn pollinate the plants. However, rhododendrons also are poisonous to many grazing animals, such as horses.

Some species (e.g. Rhododendron ponticum in the United Kingdom) are invasive as introduced plants, spreading in woodland areas replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

Overview and description

Rhododendron decorum subsp. diaprepes

Ericaceae, the family to which the genus Rhododendron belongs, is known as the “heath family,” and consists of herbs, shrubs, and trees with leaves that are alternate, simple, and estipulate, and mostly lime-hating or calcifuge plants that thrive in acid soils. In addition to rhododendrons, well-known examples include cranberry, blueberry, heath, heather, and huckleberry.

The Rhododendron genus is characterized by shrubs and small to (rarely) large trees, the smallest species growing to ten to 100 centimeters (3.9-39 inches) tall, and the largest, R. giganteum, reported to over 30 meters (98 feet) tall (Francis 2006). The leaves are spirally arranged; leaf size can range from one to two centimeters (0.4-0.8 inches) to over 50 centimeters (20 inches), exceptionally 100 centimeters (39.4 inches) in R. sinogrande. They may be either evergreen or deciduous. In some species the underside of the leaves is covered with scales (lepidote) or hairs (indumentum).

Some of the best known species of rhododendrons are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as section Vireya that often grow as epiphytes.

Some rhododendrons are known by the common name of azalea. These are flowering shrubs that are characterized by the lack of scales on the underside of the generally thin, soft, and pointed leaves, and typically having terminal blooms (one flower per stem), flowers with five or six stamens, and with long straight hairs parallel to the leaf surface and along the midrid on the ventral surface of the leaf. Originally, azaleas were classed as a different genus of plant, but now they are recognized as part of the rhododendrons. In the common classification of rhododendrons into eight or nine subgenera, azaleas comprise two of the subgenera: Pentanthera (deciduous) and Titsushi (evergreen).

Distribution

Rhododendron is a very widely distributed genus, occurring throughout most of the Northern Hemisphere except for dry areas, and extending into the Southern Hemisphere in southeastern Asia and northern Australasia.

The highest species diversity is found in the Himalayan mountains from Uttarakhand, Nepal, and Sikkim to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan, and Taiwan. Rhododendron, also known as “Lali Guras” in local language, is the national flower of Nepal and the state tree of the state of Uttarakhand in India. In Korea, Rhododendron schlippenbachii is particularly common (called 철쭉 (choltjuk)). In addition, there are a significant number of tropical rhododendron species from southeast Asia to northern Australia, with 55 known species in Borneo and 164 in New Guinea (Argent 2006).

Relatively fewer species occur in North America and Europe. Rhododendrons have not been found in South America or Africa.

Classification

The genus Rhododendron was created by Linnaeus in 1753, but he placed the azaleas in another genus. However, it was later recognized that a separation between azaleas and the members of Rhododendron could not be justified and they were placed together. The Rhododendron genus was subdivided into eight sections in 1834 by George Don and these eight subcategories have been commonly recognized until today, with the azaleas comprising two of the sections, subgenus Pentanthera (typified by Rhododendron nudiflorum) and subgenus Tsutsusi (typified by R. Tsutsusi) (Henning 2007).

However, other subcategories also have been recognized. It is not uncommon for nine subgenera to be recognized, with section Vireya within the large subgenera Rhododendron being raised to the subgenus level (Callard 2008). Furthermore, in 2004, based on genetic and chemical data, Goetsch et al. (2005) called for a new realignment in which the genus Rhododendron was subdivided into five subgenera (Rhododendron, Hymenanthes, Azaleastrum, Choniastrum and Therorhodion). Essentially, they call for the elimination of three subgenera and two sections that are present in the taxonomic system of Chamberlain et al. (1996) and the inclusiong of inclusion of section Pentanthera within subgenus Hymenanthes.

Rhododendron wardii var. puralbum

Overall, the species today are organized by subgenus, section, subsection and series. The traditional recognition of eight subgenera divides the genus into four large and four small subgenera:

  • Subgenus Rhododendron L.: small leaf or lepidotes (with scales on the underside of their leaves); several hundred species, type: Rhododendron ferrugineum.
    • The tropical rhododendrons (sect. Vireya, about 300 species) are usually included as a section in this subgenus, but sometimes split off as a ninth subgenus.
  • Subgenus Hymenanthes (Blume) K.Koch: large leaf or elepidotes (without scales on the underside of their leaves); about 140 species, type: Rhododendron degronianum.
  • Subgenus Pentanthera G.Don: deciduous azaleas; about 25 species, type Rhododendron luteum.
  • Subgenus Tsutsusi: evergreen azaleas, about 15 species; type Rhododendron indicum.
  • Subgenus Azaleastrum Planch.: five species; type Rhododendron ovatum.
  • Subgenus Candidastrum (Sleumer) Philipson & Philipson: one species; Rhododendron albiflorum.
  • Subgenus Mumeazalea: one species, Rhododendron semibarbatum.
  • Subgenus Therorhodion: one species, Rhododendron camtschaticum.

Recent genetic investigations have caused an ongoing realignment of species and groups within the genus, and also have caused the old genus Ledum to be reclassified within subgenus Rhododendron. As noted, further realignment within subgenera is currently proposed by Goetsch et al. (2005).

Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. There are over 28,000 cultivars of rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental bark or stems.

A garden with tall rhododendrons in Lynnwood, Washington Sample species

  • Rhododendron atlanticum
  • Rhododendron canadense
  • Rhododendron catawbiense
  • Rhododendron chapmanii
  • Rhododendron ferrugineum
  • Rhododendron groenlandicum
  • Rhododendron hirsutum
  • Rhododendron lochiae
  • Rhododendron luteum
  • Rhododendron macrophyllum
  • Rhododendron maximum
  • Rhododendron moulmainense
  • Rhododendron occidentale
  • Rhododendron ponticum
  • Rhododendron schlippenbachii
  • Rhododendron spinuliferum
  • Rhododendron tomentosum

A sample hybrid:

  • Rhododendron ‘President Roosevelt’

Toxicology

Some rhododendron species, including azaleas, are poisonous to grazing animals. These rhododendrons have a toxin called grayanotoxin in their pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behavior of Greek soldiers after having consumed honey in a village surrounded by rhododendrons. Later, it was recognized that honey resulting from these plants have a slightly hallucinogenic and laxative effect.

The suspect rhododendrons are Rhododendron ponticum and Rhododendron luteum (formerly Azalea pontica), both found in northern Asia Minor. Eleven similar cases have been documented in Istanbul, Turkey during the 1980s (Sütlüpmar et al. 1993).

Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage.

Use in landscaping

Both species and hybrid rhododendrons, including azaleas, are used extensively as ornamental plants in landscaping in many parts of the world, and many species and cultivars are grown commercially for the nursery trade. Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen (Huxley 1992). Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings and woodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such as R. arboreum and R. falconeri eventually growing to ten to 15 meters (33-49 feet) or more tall (Huxley 1992).

Commercial growing

Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States most of them are located on the west coast (Oregon, Washington, and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might, such as resistance to root rot when over-watered, ability to be forced into budding early, ease of rooting or other propagation, and saleability (Cox 1993).

Planting and care

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5 to 5.5. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly-drained or alkaline soils, rhododendrons are often grown in raised beds using mediums such as composted pine bark. Mulching and careful watering are important, especially before the plant is established. Some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids.

Insects and diseases

There are a number of insects that either target rhododendrons or will opportunistically attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will attack rhododendrons. Major diseases include Phytophthora root rot, stem and twig fungal dieback. Rhododendron species are used as food plants by the larvae of some members of the Order Lepidoptera (butterflies and moths).

Gallery

  • Wild rhododendrons in Kashmir by Edward Molyneux; painted before 1908

  • Flowering rhododendrons in Whitwick Leicestershire

All links retrieved July 28, 2019.

  • Flora of China: Rhododendron
  • Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.

Credits

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  • Rhododendron history

The history of this article since it was imported to New World Encyclopedia:

  • History of “Rhododendron”

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Rhododendron minus

  • Attributes: Genus: Rhododendron Species: minus Family: Ericaceae Life Cycle: Woody Recommended Propagation Strategy: Seed Stem Cutting Country Or Region Of Origin: North Carolina, South Carolina and Tennessee mountains Wildlife Value: Flowers attract hummingbirds. Members of the genus Rhododendron support the following specialized bee: Andrena (Andrena) cornelli. Dimensions: Height: 3 ft. 0 in. – 6 ft. 0 in. Width: 3 ft. 0 in. – 6 ft. 0 in.
  • Whole Plant Traits: Plant Type: Native Plant Poisonous Shrub Leaf Characteristics: Broadleaf Evergreen Deciduous Semi-evergreen Habit/Form: Dense Erect Irregular Rounded Spreading Growth Rate: Slow
  • Fruit: Fruit Type: Capsule
  • Flowers: Flower Color: Green Pink White Flower Value To Gardener: Showy Flower Bloom Time: Spring Flower Shape: Tubular Flower Size: 1-3 inches Flower Description: Small magenta-pink (rose-pink) flower in large trusses of 5 to 10. Greenish spots on the petals. Blooms in mid-May after new leaves develop.
  • Leaves: Leaf Characteristics: Broadleaf Evergreen Deciduous Semi-evergreen Leaf Color: Green Purple/Lavender Leaf Feel: Glossy Leathery Leaf Value To Gardener: Fragrant Deciduous Leaf Fall Color: Purple/Lavender Leaf Type: Simple Leaf Arrangement: Alternate Leaf Shape: Elliptical Hairs Present: No Leaf Length: 1-3 inches Leaf Width: 1-3 inches Leaf Description: Small, glossy, leathery, elliptical leaves 2 to 3 in. long and .75 to 1.5 in. wide. Undersides are scaled with brown spots, petioles reddish-brown. Aromatic when crushed. Purplish tinge in winter, some older leaves turning yellow and falling off.
  • Bark: Bark Color: Dark Brown Dark Gray Bark Description: Usually hidden by foliage.
  • Stem: Stem Color: Green Purple/Lavender Red/Burgundy Stem Is Aromatic: No
  • Landscape: Landscape Location: Houseplants Naturalized Area Woodland Landscape Theme: Native Garden Pollinator Garden Shade Garden Design Feature: Border Foundation Planting Mass Planting Attracts: Bees Hummingbirds Pollinators Specialized Bees Problems: Poisonous to Humans
  • Poisonous to Humans: Poison Severity: High Poison Symptoms: HIGHLY TOXIC, MAY BE FATAL IF EATEN! Salivation, watering of eyes and nose, abdominal pain, loss of energy, depression, nausea and vomiting, diarrhea, weakness, difficult breathing, progressive paralysis of arms and legs, coma. Poison Toxic Principle: Andromedotoxin Causes Contact Dermatitis: No Poison Part: Bark Flowers Fruits Leaves Roots Seeds Stems

Rhododendron Species R. calendulaceum

There are more than 1000 different natural species in the Genus (group) Rhododendron. These wild types, called species (as differentiated from hybrids), are native to the temperate regions of Asia, North America, and Europe, as well as to the tropical regions of southeast Asia and northern Australia. No Rhododendron species are indigenous to Africa or South America. By far, the largest number of wild species rhododendrons are native to Asia. Wild rhododendrons are found from sea level to 16,000 feet in elevation, and they occur in a variety of habitats, including alpine regions, coniferous and broadleaved woodlands, temperate rain forests, and even tropical jungle conditions.

Rhododendrons exhibit an enormous diversity of size and shape, from prostrate ground covers growing no more than a few inches high to trees more than 100 feet tall. Between the prostrate alpine forms and large trees are a variety of shrubby forms in all shapes and sizes. Leaf sizes range from less than 1/4 inch to almost three feet long, and they also appear in a variety of shapes: rounded, lance-shaped, and elliptical. The flowers may be white, red, pink, yellow, almost blue, purple, magenta, orange, and shades and mixtures of most of these colors. There is diversity, too, in bark texture and color. And while March, April, and May represent the peak months for flowering, some rhododendrons can flower as early as January in an ideal climate and others as late as October. The actual beauty of many is supreme -in flower, in decorative new growth, in foliage, in bark, in structure, and even in fall color, the latter is noted particularly with deciduous azaleas.

R. praeterui

Rhododendrons, not surprisingly, are among the most popular shrubs that people grow where conditions are suitable. They grow best in climates that avoid extremes in temperature and have substantial rainfall. They also require a slightly acid soil. Hence, the western coast of Britain and Scotland and the coastal Pacific Northwest of North America have close to ideal conditions. In the Pacific Northwest the rhododendron is ubiquitous in both public and private gardens. Indeed, because the winter weather west of the Cascade Mountains is relatively mild, the blooming season is often quite long at the Rhododendron Species Botanical Garden, where flowers of a few species often begin to appear in early January. The majority bloom from March through May with a smaller number of species flowering in June and July. One of the very last species to flower is called Rhododendron faithiae and is especially attractive with its large white trumpet flowers in late September which are fragrant! The last azalea to flower is Rhododendron prunifolium, with salmon-red flowers opening in late June or July.

Choosing Rhododendrons and Azaleas

Rhododendrons and azaleas come in all shapes and sizes from a few inches tall to giant forest trees.

There are over 900 species of rhododendrons and over 25,000 named hybrids of rhododendrons and azaleas.

Glendoick have around 5000 varieties in their collection and propagate around 1000 species and hybrids.

Our online catalogue or 56 page paper catalogue can be daunting. But don’t worry, it is easy to narrow down what you want to a few sections. If you know broadly what you want, then go to our online catalogue arranged in categories.

Rhododendron and Azalea Hardiness

We use divisions of hardiness which have now been adopted by R.H.S. In the UK, it is rarely extreme cold that causes damage so don’t pay too much attention to minimum temperature ratings. Damage from late or early season frosts when plants are soft or not hardened off is much more common and can be very damaging or fatal. In continental Europe the minimum temperature ratings are more relevant.

You can search our online catalogue by hardiness
H6 Hardiest, -24C (-10F) and below suitable for inland areas in N. Europe:
H5 Very hardy: -18C (0F) and below, coldest UK
inland areas, amongst hills, much of N Europe.
H4 Hardy: Suitable for most of UK areas, including fairly well inland, coastal Europe.
H3 Fairly hardy: Low elevations and sheltered gardens,
fairly near east coast. Limit of hardiness at Glendoick. Such plants are damaged in our severest winters or by late or early frosts.
H2 Rather tender. Low elevations fairly near west coast and on east coast. Not reliably hardy outdoors at Glendoick but reliably hardy in Argyll, Cornwall, West Wales, Ireland and similar climates.

H1 Tender (mild frosts only). Mildest U.K. west-coast islands and south/western coastline. Greenhouse culture for colder areas.

Size: how large will rhododendrons and azaleas get?

Spacing and Size

Rhododendrons have no ultimate size, they can carry on growing indefinitely. The key is the annual growth rate, up to 60cm per year for the most vigorous and less than 1cm for the slowest growing. Moisture and length of growing season greatly affect rate of growth: mild wet areas may have twice the growth rate of cold dry ones.

We recommend planting the larger growing and large leaved species 3-4m apart as they look best as individuals rather than blocks. Plant larger hybrids at 2m spacing for quick effect to fill in 5-8 years and 3 m apart for more long term plans. Deciduous azaleas can be planted 2m apart and are fine if they grow into oneanother. They can be pruned. Medium rhododendrons will grow to 1/2m x 1/2 in 6-10 years so 1 1.5m spacing is good. Dwarf rhododendrons and evergreen azaleas are best planted in clumps 60cm apart so that they carpet the ground.

Can I prune rhodendrons and azaleas to keep them small?

Yes! Most varieties can be cut back if they get too big, but some (especially species with smooth bark) will not regenerate. All lepidotes (small leaved rhododendrons) and deciduous and evergreen azaleas can be cut back or pruned. After flowering is the best time. Beware of pruning old (pre 1950s) plants grafted on R. ponticum which tend to sucker. These R.ponticum suckers are easy to see at flowering time (with purple-magenta flowers) Break the suckers off. The Picture > shows some bushes of ‘Fastuosum Flore Pleno’ which have been cut back hard and regrown.

Sizes used in Glendoick Catalogue

(online and paper catalogue)

Dwarf: to 40cm in 10 years, usually spreading, wider than high.
Semi-dwarf to 40-80cm in 10 years, often spreading wider than high.
Low 80-1.3m in 10 years, will eventually reach 2m x 2m or more (20 years)
Medium 1.3-2m in 10 years up to 3-4m+ in time.
Tall 2m+ in 10 years, ultimately 4-6m x 3-6m or more.

Flowering Time

In mild and maritime climates, in most of the UK, France etc, the flowering season can cover 7 months or more, from Christmas onwards, while in severe climates, Sweden and Northern Germany for example, the flowering season is compressed into a 3 month period starting in April. We find with the vagaries of the British climate, there is considerable variation from year to year, especially with early-flowering varieties.

Flowering time

Month

VE

Very Early

Dec-Feb

E

Early

March

EM

Early-Mid

April

M

Mid

Early May

ML

Late Mid

Late May

L

Late

June

VL

Very Late

July-Aug

VVL

Autumn

Growing Rhododendrons in the South

Made In:Jacksonville, Florida Buy It: Blue Limpet Shell Box, $390; karenrobertson.com Photo: Greg Dupree

This is a rhododendron in my back yard. Her name is ‘Caroline.’ Isn’t she purty? Unfortunately, growing rhododendrons in the South can be quite a challenge, unless you follow Grumpy’s expert advice.

But before we get to that, let me dispel some confusion you may have about rhododendrons. People talk about rhododendrons and azaleas as two different groups of plants, but in fact, all azaleas belong to the genus Rhododendron. Our native azaleas, also called wild honeysuckles, are upright, airy, deciduous shrubs, often with fragrant flowers. Their Asian counterparts, like ‘Hershey’s Red’ and ‘Formosa,’ are dense and mounding, non-fragrant, with evergreen leaves from 1 to 2 inches long. They usually grow from 3 to 8 feet tall and wide.

For this story, rhododendrons refers to those plants with large, evergreen leaves up to 6 inches long. They get bigger than azaleas, 8 to 12 feet tall and wide (although I’ve seen our native Catawba rhododendrons towering 30 feet tall in the North Carolina mountains). Rhododendrons have much bigger flowers than azaleas, bloom later, and are open, not dense.

You with me so far? Good. Now here’s how to grow rhododendrons in the South.

2. Pay attention to the soil! Rhododendrons need moist, acid, loose, well-drained soil that contains a good bit of organic matter. This is why they’re hard to grow here. Most Southerners have either acid, clay soil or alkaline, clay soil. Rhododendrons hate both, because clay drains slowly and roots rot. Rhododendrons also hate being dropped into a turkey frier, but I doubt that happens very often. Once burnt, twice shy.

3. Plant a little high. No, I don’t mean you should be a little high. This ain’t Colorado. Grumpy means you should plant your rhododendron so that the top of its root ball sticks about one inch above the soil surface. Then cover the exposed root ball with mulch. This improves drainage and aeration around the root ball. No weed required.

4. Provide light shade, especially in the afternoon. Plant rhododendrons in the dappled light beneath tall pines and hardwoods. Don’t plant in deep shade or the plants won’t bloom.

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Wednesday – September 10, 2008

From: Brooklyn, NY
Region: Northeast
Topic: Trees
Title: Rhododendrons for afternoon sun
Answered by: Nan Hampton

QUESTION:

Thanks for your suggestion that I plant rhodedenrons in my Brooklyn garden. In fact, the only bushes I’ve planted in the past that have survived are rhodedenrons so your definitely right! Here’s my problem- the 2 places I want to plant bushes get afternoon sun – one gets about 3 hours of mid noon sun, the other, 2-3 hours of late noon sun and I know rhod. prefer morning sun. Is there a particular variety that you would recommend for afternoon sun? Thanks.

ANSWER:

Although morning sun is preferable, Mr. Smarty Plants doesn’t think your 2-3 hours of mid-noon and early afternoon sun will be a serious problem for any of the rhododendrons below, except for possibly, R. prinophyllum (early azalea) which prefers shade (less than 2 hours of sun per day). The others do well in part shade (defined as 2-6 hours of sun per day) and R. canadense (rhodora) will grow in full sun. However, there are other factors such as soil pH and soil moisture that could make a difference in your success. Since I don’t know what your soil pH and soil moisture is, I can’t really recommend one of these species over another. I can, however, highlight those factors in each of the species:

Rhododendron arborescens (smooth azalea)—part shade and moist acidic soil. This is a very hardy azalea.

Rhododendron calendulaceum (flame azalea)—part shade and dry acidic soil.

Rhododendron canadense (rhodora)—sun, part shade and wet acidic soil.

Rhododendron lapponicum (Lapland rosebay)—part shade and dry limey (alkaline) soil.

Rhododendron maximum (great laurel)—part shade and wet acidic soil.

Rhododendron periclymenoides (pink azalea)—part shade and wet acidic soil.

Rhododendron prinophyllum (early azalea)—shade and moist neutral soils. This is a hardy azalea.

Rhododendron viscosum (swamp azalea)—part shade and wet acidic soil.

You might check in our National Suppliers Directory for a nursery in your area that specializes in native plants to see what they have available. They could most likely advise you further about which species or cultivar would best suit your shade condidtions, soil type and moisture.

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Caring For and Planting Rhododendrons

In order to understand the culture of rhododendrons it is helpful to visit mountainous areas where they flourish. If you take a fistful of soil at their roots you will find that it is not dirt, but dead and dying plant debris. The fallen leaves, twigs, and flower parts decay into what is called humus. Humus is very porous and with slight water retaining characteristics provides perfect drainage. Observing a plant growing happily in a crevice of a rocky cliff is further evidence of the perfect drainage requirement of rhododendrons. The roots may be found growing many feet away searching for moisture and nutrients in the thin layer of humus.

If you remain in the mountains very long you will realize that there is more rainfall than at lower elevations. In fact, very often you will find that you are in a rain cloud. Although it may not be raining, the grass and leaves are wet with dew. Thus you witness the two basic requirements of rhododendrons: (1) abundant water and (2) soil with good drainage. By providing these requirements, we may enjoy in our gardens at lower elevations the exotic species and hybrids of rhododendrons.

Of lesser importance is the plant’s location in your garden. Some rhododendrons will flourish in full sun and some in partial shade. A happy medium is probably the best, with high filtered light from surrounding tall trees. By removing the lower branches of the trees the rhododendrons can enjoy the full early morning and late afternoon sun. Some protection from drying winter winds may be necessary either from a building or a plant windbreak. Since the drying winds come most often from the southwest, planting rhododendrons on a northeast slope or the northeast corner of your house would be ideal.

To provide good drainage, elevated beds are necessary on clay soils. Of course, these beds will quickly dry out and the water needs must not be neglected. On banks or slopes holes may be dug for the planting as long as the bottom of the hole is trenched out. The trench may be filled with rocks and humus so that water will never stand in the bottom of the hole. Before planting it is wise to fill the hole with water and observe complete drainage through the trench.

When making a single planting on level ground in clay soil, the elevated bed should be about 5′ in diameter. The plant should be placed on top of the ground without digging a hole. If a hole is dug it should be very shallow so that water will drain out. The humus may then be placed about the rootball to barely cover the top of the ball. Each year more humus should be added to the bed in the form of a top dressing of composted leaves, bark, grass clippings, peat moss, etc. This will help keep shallow roots cool and moist. The mulch should be composted (well-decayed) and not this year’s crop of leaves or clippings, because they tend to shed water from the bed and take up nitrogen from the soil in the process of decaying.

Most commercial growers today propagate rhododendrons in containers with a soil mix of 2 parts composted pine bark, 1 part peat moss, and 1 part sand. The mixture varies with some using only composted pine bark. There is much discussion of rhododendrons being “acid loving” and of the need to keep the pH of the growing medium between 4.5-5.5. Actually, if the proper growing mix is used the decaying organic matter will naturally lower the soil’s pH. The home-owner/gardener should be aware of this and when applying alkaline lime to the lawn avoid getting it too close to the rhododendrons.

Very often the containers purchased from a nursery are “root-bound.” The roots grow out to the sides of the container and then circumferentially around the inside of the container and back into the rootball. If planted in this manner it will die in two to three years because the roots cannot escape the ball. When the dead plant is lifted from the bed one finds the rootball to be exactly as it was when planted two to three years before with no roots extending out beyond the ball. The encasing peripheral roots had actually choked the plant to death. This tragic circumstance occurs all too often and is a prime reason for failure with container grown plants. We cannot stress too vigorously the importance of the proper care of the rootball prior to planting. It is folly to purchase an expensive plant that will surely die without proper care.

When the plant is root-bound the peripheral roots should be torn or cut away before planting. The entire ball should be loosened by gently pressing and pulling and breaking its rigid contour. This loosening effect may be aided by directing a forceful stream of water from a garden hose. Another recommendation is to use a sharp knife to slice the sides of the ball from top to bottom. This will help open up the ball and allow the inner roots to escape.

One question that gardeners frequently ask is: “When is the best time to plant rhododendrons?” A container grown plant (that is not rootbound) can really be planted safely anytime. However, since you may not know if the plant is rootbound until you lift it from its container, it would be safer to make plantings in the early spring or fall. April, May, September, and October are the best months for planting. The worst months are June, July, and August, when the plants are in active top growth. Fall plantings are best because the winter rains maintain ground moisture better than in summer. Early fall plantings allow time for the roots to get out of the rootball and into the moist plant bed before ground freeze occurs.

Another frequent question is: “When should I fertilize and what should I use?” New plantings should be fertilized immediately after blooming. Early spring application will stimulate new growth that will obscure the blossoms. Late summer (after July 15th) fertilization should be avoided to prevent late new growth that may be damaged by fall freezing. Established plantings of two or more years should be fertilized at least once every other year. A general recommendation is to use a fertilizer ratio of 2:1:1 or 2 parts nitrogen, 1 part each of phosphorous and potassium. The best and most accurate way to fertilize is first to have the soil tested by your state soil-testing laboratory. This can be facilitated by your agricultural extension office or garden center.

If properly planted and given an extra quota of water, a Rhododendron will outlive the one who plants it by many years.

Rhododendrons are among the most beautiful heath soil shrubs.

A few Rhododendron facts

Name – Rhododendron
Family – Ericaceae
Type – shrub
Height – 1 ⅓ to 16 feet (0.5 to 5 meters)
Exposure – part sun and shade
Soil – acid, heath soil
Foliage – evergreen
Flowering – April-May

Planting, pruning and caring for them are steps that help enhance blooming and avoid diseases.

Planting rhododendron

One of the most important considerations when planting rhododendron is the need to have heath and well drained soil.

If your soil is chalky or limestone soil, it is recommended to dig a much larger hole and fill it in with heath.

Rhododendron tolerates shade, part sun and even sunlit places, as long as it stays cool.

  • We recommend planting rhododendron in fall.
    Planting is tolerated in spring or winter, as long as it doesn’t freeze.
    If you wish to plant in summer, provide for regular watering at the beginning.
  • Choose a partially shaded or sunlit spot that never gets scorching hot.
  • Avoid flood-prone areas at all costs, because this shrub hates sitting water.
  • Follow our advice on planting heath plants and shrubs.
  • Read also: Growing and caring for potted rhododendron

Propagating rhododendron

Making cuttings is the easiest and fastest technique to propagate rhododendron.

Preparing rhododendron cuttings can be done all summer long.

  • Collect 6 to 8 inch (15 to 20 cm) cuttings from non-flowering stems.
  • Remove lower pairs of leaves, keeping only the topmost one or two pairs.
  • If you so wish, dip the base of the cuttings in powdered rooting agents.
  • Place your cuttings in nursery pots filled with cutting soil mix.
  • Keep the substrate moist and put the cuttings near light, but not in direct sunlight.

Pruning and caring for rhododendron

It isn’t really necessary to prune rhododendron, so caring for them is easy.

Pruning is only needed if you wish to maintain the figure, reduce the size of the shrub, or balance growth:

  • Remove dead wood.
  • Remove wilted flowers regularly (deadheading).
  • To reduce shrub size, wait for the end of the summer and cut just above a bud so it will split into multiple branches.

It is important for the soil to remain slightly moist at all times, and for that the best solution is to mulch with pine bark mulch.

Problems with growing rhododendron

Rhododendron leaves and buds turn brown

This is often due to poorly-draining soil, which means water is stagnating around the roots.

  • Rhododendron must never have stagnant water around its roots, water must flow away quickly.
  • It makes sense in this case to mix some sand into the soil. It especially helps to spread a layer of clay marbles or pebbles at the bottom of the hole to help water seep into the ground.

Rhododendron is actually more hardy than they are able to overcome excess water in the ground…

Finally, fertilizing with special heath plant fertilizer at the end of winter strengthens rhododendron, enhances their flowering and helps avoid diseases.

Leaves lose their color and turn yellow

This is generally due to excessively chalky soil, and results in what is called rhododendron chlorosis.

  • Adding a layer of heath the soil in the surface is recommended.
  • A supplement of heath plant fertilizer should also help cure the chlorosis.

Diseases and parasites attacking rhododendron

Rhododendrons are shrubs that resist diseases well when they are well settled in, roots have developed well, and growing conditions are favorable.

Inversely, if there is either excess water or excess heat, or it hasn’t been planted correctly, it is more vulnerable to various diseases, listed below:

Rhododendron withers, looks sad and stunted

This is one of Rhododendron’s most common diseases, and it is often too late when it shows.

  • It is usually due to a fungus that is called Phytophthora cinnamomi. This fungus often is lethal to rhododendrons.
  • Treatment must be swift and merciless, removing and destroying the infested portions of the shrub, but survival chances are slim.
  • Once all infested portions have been removed, treat with systemic fungicide, which is the only effective option against this fungus.

Blisters form on leaves

  • Even though this is not often critical, this fungus-based disease is due to Exobasidium vaccinii, and is more commonly called leaf gall.
  • Usually, it is enough to remove the leaves that host these blisters.

Learn more about Rhododendron

This is probably one of the most beautiful flowering shrubs to decorate your garden.

Rhododendrons boast amazing flowers at the very beginning of spring, and their leaves are gorgeous all year round.

This plant can grow to live very old and can even reach majestic sizes, some shrubs reaching heights of over 16 feet (5 meters).

Rhododendron flowers are grouped in bouquets that can span 8 to 10 inches (20 to 25 cm) across, with hues shifting in ranges of pink to red, including white and violet depending on the species.

Rhododendrons are particularly decorative, and are among those shrubs that are best suited to shaded areas.

Smart tip about rhododendrons

Since it is quite acidic, pine bark mulch is definitely the best mulching option.

Mulch the base of the tree in summer to keep a good level of moisture.

  • Read also: Growing and caring for potted rhododendrons.

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