Rhododendron

Azalea, Rhododendron, Rosebay

A classic shade garden plant, rhododendrons are prized for their glossy green foliage and showy clusters of blooms. Available in a variety of hues, the most common flower colors are purples and pinks, as well as white and cream. The Rhododendron genus contains an option for every landscape, from the giant rhododendrons of East Asian mountainsides to the rosebay rhododendrons native to Eastern U.S. woodlands. In areas where dry winters tend to desiccate evergreen types, deciduous varieties of rhododendrons can fill in the gap. Many deciduous types also boast bright yellow and orange hues that work wonders in brightening up shady corners of the garden.

I. Appearance and Characteristics 

Rhododendron is a genus of shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm (4–40 in) tall, and the largest, R. protistum var. giganteum, reported to 30 m (100 ft) tall. The leaves are spirally arranged; leaf size can range from 1–2 cm (0.4–0.8 in) to over 50 cm (20 in), exceptionally 100 cm (40 in) in R. sinogrande. They may be either evergreen or deciduous. In some species, the undersides of the leaves are covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. A recently discovered species in New Guinea has flowers up to six inches (fifteen centimeters) in width, the largest in the whole genus. The accompanying photograph shows it as having seven petals. There are alpine species with small flowers and small leaves, and tropical species such as section Vireya that often grow as epiphytes. Species in this genus may be part of the heath complex in oak-heath forests in eastern North America.

They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote). These scales, unique to subgenus Rhododendron, are modified hairs consisting of a polygonal scale attached by a stalk.

Rhododendron are characterised by having inflorescences with scarious (dry) perulae, a chromosome number of x=13, fruit that has a septicidal capsule, an ovary that is superior (or nearly so), stamens that have no appendages, and agglutinate (clumped) pollen.

II. How to Grow and Care

Rhododendron plants, like many other plants in the Ericaceae family, prefer acidic soils. Many of the evergreen types can be susceptible to burn in winter where they are exposed. To prevent this, plant evergreen types in sheltered areas, avoiding southern exposures where warm, sunny winter days can be fatal. Keep them sheltered from drying winter winds as well.

Sunlight

Most rhododendrons prefer a shade-filled existence, but newer varieties are available for sunny areas as well. Deciduous varieties can hold up much better to more sun.

Watering

Due to their shallow, fine roots, rhododendrons require regular watering through dry periods. They will show signs of drought stress much sooner than plants with deeper roots. Keep the soil moist, but don’t let it become soggy.

Soil

The ideal soil pH for rhododendron plants is somewhere between 4.5 and 6.0. If you have had problems growing rhododendrons in the past, perform a soil test. You can amend the soils with peat moss, compost, and other soil acidifiers to keep them happy.

Fertilizing

Fertilize rhododendrons and azaleas soon after blooming ends. Add a slow-release granular fertilizer to the soil or mulch around the plants. Use a fertilizer especially labeled for rhododendrons or a high-quality compost.

Mulching

Mulch with compost, bark chips, or pine needles to prevent weeds, since hoeing can easily damage the surface roots. Mulching also helps retain moisture. Replenish the mulch annually, or as needed.

Pruning

Pruning may be necessary to create a more desirable shape and overall more visually appealing plant. After the plants have bloomed, spent blossoms can be cut back to the new growing tips. After bloom is also the ideal time to do any other pruning. Damaged or diseased growth should always be removed to prevent the spread of disease. You can also do rejuvenation pruning by cutting older plants back more severely to encourage better branching.

Propagation

The easiest method for a home gardener to propagate a rhododendron or azalea is by layering an existing plant. Weigh a lower branch down until it touches the soil. Cut a slit in the branch where it touches the soil and apply a rooting hormone. A couple of years later, cut the rooted branch to separate it from the parent and transplant it.

Another propagation method is by stem cuttings taken from new growth in the early fall. Dip the cuttings in rooting hormone, plant them in a planting medium and keep them in a warm place. Rooting occurs anywhere from 6 weeks to 4 months later.

When growing from seeds, sow them indoors in early winter on top of a moist potting soil. Maintain a high humidity, warm environment with no direct sunlight. Transplant the tiny seeding to individual pots. It will be about two years before they can be planted in the ground.

Pests and Diseases

Rhododendrons and azaleas are not particularly susceptible to insects, but they sometimes are afflicted by lace bugs, borers, or weevils. Prevent an infestation by choosing resistant species or hybrids that show resistance and by keeping the area around the plant cleaned up.

Gardeners who live in areas with a large deer population may have a problem growing rhododendron. Deer love to munch on the leaves of the plant. In these areas, gardeners need to be willing to share, erect fencing or plastic mesh barriers, or use a spray that repels deer.

Evergreen types may begin to curl their leaves during the winter. This is a physiological response to dry winter weather. By curling their leaves, they are protecting themselves from cold temperatures and winds to prevent potential winter burn.

III. Uses and Benefits 

Pharmacology

Rhododendron species have long been used in traditional medicine. Animal studies and in vitro research have identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of flavonoids or other phenolic compounds and saponins the plant contains. Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB in rats.

Symbolism

Rhododendron arboreum (lali guransh) is the national flower of Nepal. R. ponticum is the state flower of Indian-administered Kashmir and Pakistan-controlled Kashmir. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron arboreum is also the state tree of the state of Uttarakhand, India. Pink Rhododendron (Rhododendron campanulatum) is the state flower of Himachal Pradesh, India. Rhododendron is also the provincial flower of Jiangxi, China and the state flower of Nagaland, the 16th state of the Indian Union.

Rhododendron maximum, the most widespread rhododendron of the Appalachian Mountains, is the state flower of the US state of West Virginia, and is in the Flag of West Virginia.

Rhododendron macrophyllum, a widespread rhododendron of the Pacific Northwest, is the state flower of the US state of Washington.

Literature

The nineteenth-century American poet and essayist Ralph Waldo Emerson in 1834 wrote a poem titled “The Rhodora, On Being Asked, Whence Is the Flower”.

In Joyce’s Ulysses, rhododendrons play an important role in Leopold and Molly’s early courtship: Molly remembers them in her soliloquy – “the sun shines for you he said the day we were lying among the rhododendrons on Howth head in the grey tweed suit and his straw hat the day I got him to propose to me”. Jasper Fforde a British author, also uses rhododendron as a motif throughout many of his published books. See Thursday Next series, and Shades of Grey. Amongst the Zomi tribes in India and Myanmar, “Rhododendrons” called “Ngeisok” is used in a poetic manner to signify a lady.

In Daphne Du Maurier’s novel Rebecca, the character of Rebecca is associated with “blood red” rhododendrons throughout the novel, perhaps due to the toxic roots of the plant mirroring the poisonous character of Rebecca. On the other hand, azaleas (a type of rhododendron) represent the second Mrs. De Winter.

In the young adult novel Walk Two Moons by Sharon Creech, the protagonist, Sal, and her friend, Phoebe, construct an elaborate story in which they suspect Phoebe’s neighbor of murdering her husband and burying his body beneath a rhododendron in her yard. Chapter 14 of the book is entitled “The Rhododendron”.

Culinary

The rhododendron is the national flower of Nepal, where the flower is considered edible and enjoyed for its sour taste. The pickled flower can last for months and the flower juice is also marketed. The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones. The juice of rhododendron flower is used to make a squash called burans (named after the flower) in the hilly regions of Uttarakhand. It is admired for its distinctive flavor and color.

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 on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might want, such as resistance to root rot when overwatered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.

Rhododendron Details

Common name Azalea, Rhododendron, Rosebay
Botanical name Rhododendron
Plant type Houseplant
Hardiness zone 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b
Growth rate Slow
Harvest time Fall
Height 6 ft. 0 in. - 10 ft. 0 in.
Width 6 ft. 0 in. - 10 ft. 0 in.
Sunlight Dappled Sunlight (Shade through upper canopy all day)
Soil condition High Organic Matter
Flower color Gold/Yellow
Leaf color Green