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Growing and Caring for Rhododendron decorum: Expert Tips

The Great White Rhododendron is an evergreen shrub or small tree, standing 1-3 meters tall, but occasionally reaching 6-7 meters. The bark is gray-brown or gray-white. Young branches are green and hairless, while the older ones turn brown. Winter buds are apical, ovate, hairless. The leaves are thick, leathery, ranging from oblong, oblong-ovate to oblong-obovate. […]

The Great White Rhododendron is an evergreen shrub or small tree, standing 1-3 meters tall, but occasionally reaching 6-7 meters. The bark is gray-brown or gray-white. Young branches are green and hairless, while the older ones turn brown. Winter buds are apical, ovate, hairless.

The leaves are thick, leathery, ranging from oblong, oblong-ovate to oblong-obovate. The upper surface is dark green while the underside is pale green. The plant bears apical umbellate inflorescences with 8-10 fragrant flowers.

The corolla is wide, funnel-shaped, varying considerably, measuring 3-5 cm in length with a diameter of 5-7 cm. Flowers are pale red or white. The capsule fruit is cylindrical, transitioning from yellow-green to brown, with prominent ribs.

Rhododendron decorum

The blooming period is from April to June, while the fruit matures between September and October. With its dense branching and lush foliage, the Rhododendron is picturesque and sprouts strongly. It’s tolerant of pruning, and its peculiar root stump makes it an excellent choice for bonsai.

Ideally planted in gardens by the forest edge, streamside, beside ponds, or near rocks, it can also be scattered under sparse forests. Its qualities make it a fine hedge material, and it can be trimmed to various shapes.

Morphological Features

The Great White Rhododendron is an evergreen shrub or occasionally a small tree, growing between 1-3 meters but can reach up to 6-7 meters. The bark is gray-brown or gray-white.

Young branches are green and hairless, while the older ones turn brown. The apical winter buds are ovate, measuring 9-10 mm, and are hairless.

Rhododendron decorum

Leaves are thick and leathery, ranging from oblong, oblong-ovate to oblong-obovate, with a length of 5-14.5 cm and a width of 3-5.7 cm. The tips are blunt or rounded, with the base either wedge-shaped, blunt, or occasionally nearly round.

Edges of the leaves curl back, with a dark green upper surface and a pale green underside. The midrib is slightly depressed on the top, yellow-green, and protrudes beneath.

The apical umbellate inflorescences consist of 8-10 fragrant flowers. The inflorescence axis is 2-2.5 cm long, pale red-green with sparse white glands. The flower stalks are stout, measuring 2.5-3.5 cm, pale green with purple-red hints, featuring stalked white glands.

The flower’s calyx is small and shallow dish-shaped, 1.5-2.3 mm long with 5 irregularly cut teeth. The corolla, wide and funnel-shaped, varies significantly, measuring 3-5 cm in length and 5-7 cm in diameter, in shades of pale red or white. The base of the inner surface has fine white hairs, while the outside has a few white glands.

The capsule fruit is cylindrical, 2.5-4 cm long, slightly curved with a diameter of 1-1.5 cm. Its color transitions from yellow-green to brown, with evident ribs and glandular traces. The flowering period spans from April to June, and the fruiting period is from September to October.

Growth Environment

Azaleas are native to high-altitude areas and prefer a cool, moist climate. They do not thrive in harsh, hot, and dry conditions. They thrive in acidic soil that is rich in organic matter, loose, moist, and with a pH between 5.5 to 6.5.

Some species and horticultural varieties are adaptable, drought-resistant, and can grow even in soil with a pH of 7-8. However, they don’t grow well in heavy clay or soil with poor drainage. Azaleas have specific light requirements. They do not tolerate direct sun exposure.

During summer and autumn, they should be shaded by deciduous trees or canopies to block the intense sun, and the ground should be frequently sprinkled with water. Their main growing period is during spring, although they also sprout in autumn.

The ideal growing temperature is between 15-20°C. They bloom between March and May and tolerate pruning. Pruning is generally done before May. New shoots that grow post-pruning can form flower buds within the same year, but late pruning can affect flowering.

Growth and Propagation

  • CuttingsCuttings are the most commonly used propagation method for cultivating azaleas. Typically, in May or June, one cuts healthy semi-woody new branches that are about 5-8 cm long. The leaves from the lower part of the cutting are removed, leaving 2-3 leaves at the top to serve as the cutting’s shoot. It’s recommended to dip the base of the cuttings in a solution containing indole butyric acid (IBA) or ABT rooting powder to promote root growth. These cuttings are then planted in loose, well-aerated acidic soil that’s rich in organic matter. The temperature should be maintained between 20-25°C. The cuttings should be kept in a shaded area and frequently misted to maintain moisture, facilitating the growth of new roots.
  • Softwood GraftingSoftwood grafting is employed for valuable azalea varieties that are challenging to propagate, such as certain special types. First, a softwood section about 3-4 cm long is cut to serve as the scion. The base of this section is carved into a wedge shape using a sharp knife. Another variety of azalea, commonly referred to as ‘hairy azalea’, is used as the rootstock. The softwood scion is then grafted onto this rootstock. They are placed in a shaded area and bound with a thin plastic film. To maintain humidity, both the scion and rootstock are covered together with a plastic bag.

Disease and Pest Control

  • Leaf Gall DiseaseBefore the disease appears, especially during the leaf expansion stage, it’s recommended to spray a Bordeaux mixture at a 1:1:200 ratio. Infected leaves should be removed promptly. Before budding, one can spray 0.3-0.5 Beauveria bassiana sulfur-lime mixture or a 1:1:200 Bordeaux mixture 2-3 times, typically once every 7-10 days. After the disease onset, one can spray a 65-80% Mancozeb solution at a 500x dilution or 0.3-0.5 Beauveria bassiana sulfur-lime mixture 3-4 times, once every 7-10 days.
  • Leaf Spot DiseaseFor controlling Leaf Spot and Brown Spot Diseases, between May and August, one can spray a 70% Methylobenzene solution at a 1000x dilution, a 20% Rust Ning solution at a 4000x dilution, or a 50% Mancozeb solution at a 500x dilution. It’s recommended to spray once every 10 days, for a total of 7-8 times, to effectively control the disease’s progression. To prevent leaf yellowing, one can also apply ferrous sulfate.
  • Mite Gall DiseaseThe affected young branches and leaf tips are densely covered with a white or pink mite-like layer. Sometimes, mite galls appear on the leaves, often resulting from aphid-induced leaf injuries. For control, while removing the diseased leaves, it’s recommended to spray a copper sulfate solution. Spraying Oxymatrine emulsifiable concentrate or placing Furadan directly in the pot can control aphids and other piercing-mouthpart pests.

Primary Values

  • Medicinal UseFlower: The properties are pungent and sour, and they are warm in nature. They help in relieving coughs, itching, securing essence, and killing parasites. It is used for kidney deficiency.Function and Treatment:
    • It clears damp-heat from the body.
    • It promotes blood circulation and relieves pain.
    • It is mainly used for leukorrhea, vaginal discharge, rheumatic pain, and injuries from falls or blows.
    Pharmacological Effects:
    • Toxins named “Large White Flower Toxin I” and “Large White Flower Toxin II” are isolated from the Large White Rhododendron.
    • Large White Flower Toxin II, when administered to rats at doses of 1-100μg/kg, has a notable hypotensive effect. The effect is rapid and short-lived. Blood pressure drops to its lowest point 1-2 minutes after administration, typically returning to normal within 15 minutes.
    • The median lethal dose (LD50) of Large White Flower Toxin II, when injected into the abdominal cavity of mice, is 1.05mg/kg.
  • Gardening:Rhododendrons have abundant branches and leaves and display a variety of beautiful forms. They have strong sprouting ability, are tolerant of pruning, and have unique root shapes, making them excellent materials for bonsai. In gardens, they are most suitable for planting in clusters along forest edges, beside streams, ponds, or rocks. They can also be scattered under sparse woods. They are good materials for hedges and can be pruned to take on various forms. During their flowering season, Rhododendrons always give a lively and bustling feeling. Outside of their flowering season, their deep green leaves are also suitable for planting in gardens as low walls or barriers.
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Peggie

Peggie

Founder of FlowersLib

Peggie was once a high school mathematics teacher, but she set aside her chalkboard and textbooks to follow her lifelong passion for flowers. After years of dedication and learning, she not only established a thriving flower shop but also founded this blog, “Flowers Library”. If you have any questions or wish to learn more about flowers, feel free to contact Peggie.

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