Rhododendron obtusum: The Enchanting Bell-Shaped Flowers

The Rhododendron obtusum is originally from Japan. It is a renowned cultivated species within the Rhododendron genus and is cultivated in eastern and southeastern China, with numerous horticultural varieties.

The plant has umbrella-shaped inflorescences, typically with 2-3 flowers; the corolla is funnel-shaped and bell-shaped, ranging from red to pink or light red. It is most suitable for planting in clusters or patches by the edge of forests, riversides, ponds, and rocks in gardens.

It can also be scattered under sparse woods and is excellent material for hedges. It can be pruned and cultivated into various forms.

I. Basic Introduction

The Dull-leaf Rhododendron (Scientific name: Rhododendron obtusum (Lindl.) Planch.) is a dwarf shrub, 1 meter high, rarely reaching 4 meters.

The twig is slender, highly branched, often in a pseudo-whorl shape, sometimes nearly prostrate, and densely covered with rust-colored rough hairs.

The leaves are membranous, often clustered at the end of the branches, varied in shape, from oval to ovate or oblong to inverted lanceolate or inverted ovate, bright green on the upper side, pale green on the underside.

The umbrella-shaped inflorescence, typically with 2-3 flowers; the corolla is funnel-shaped and bell-shaped, ranging from red to pink or light red, 1.5 cm long, 2.5 cm in diameter, with 5 petals, oblong. The capsule is conical to broad elliptical, 6 mm long, densely covered with rust-colored rough hairs.

This species is originally from Japan, a renowned cultivated species within the Rhododendron genus, and is cultivated in eastern and southeastern China, with numerous horticultural varieties.

II. Growth and Distribution

This species is originally from Japan, a renowned cultivated species within the Rhododendron family, and is cultivated in eastern and southeastern China, with numerous hybrids and horticultural varieties.

III. Morphology and Features

The Dull-leaf Rhododendron is a dwarf shrub, 1 meter high, rarely reaching 4 meters. The twig is slender, highly branched, often in a pseudo-whorl shape, sometimes nearly prostrate, and densely covered with rust-colored rough hairs.

The leaves are membranous, often clustered at the end of the branches, varied in shape, from oval to ovate or oblong to inverted lanceolate or inverted ovate, 1-2.5 cm long, 4-12 mm wide, blunt or rounded at the tip, sometimes with a short point, wide wedge-shaped at the base, margins with fine hairs, bright green on the upper side, pale green on the underside, both sides scattered with light gray rough hairs, more obvious along the midrib, the midrib is recessed on the upper side, protruding on the underside, lateral veins are prominent on the underside; the petiole is about 2 mm long, covered with gray-white rough hairs.

The bud is oval, the scale is ovate, pointed at the tip, the edge has rough hairs, along the midrib with light gray rough hairs.

The umbrella-shaped inflorescence, typically with 2-3 flowers; the flower stem is 4-8 mm long, densely covered with flat rust-colored rough hairs; the sepals are 5, green, ovate, up to 4 mm long, covered with rough hairs; the corolla is funnel-shaped and bell-shaped, ranging from red to pink or light red, 1.5 cm long, 2.5 cm in diameter, with 5 petals, oblong, 1 cm long, about 5 mm wide, blunt at the top, with 1 petal with dark spots; there are 5 stamens, about the same length as the corolla, the filament is hairless, the anther is light yellow-brown, the ovary is densely covered with brown rough hairs, the pistil is 2.5 cm long, hairless.

The capsule is conical to broad elliptical, 6 mm long, densely covered with rust-colored rough hairs.

IV. Habitat Preferences

Rhododendrons originate from high-altitude regions, thriving in cool and humid environments, while withering under harsh heat and dryness. They require humus-rich, loose, moist, and acidic soil with a pH between 5.5 and 6.5.

Some species and horticultural varieties are highly adaptable and can tolerate drought, poor soil, and pH levels between 7 and 8. However, they grow poorly in heavy or poorly drained soils.

Rhododendrons have specific light requirements, but they do not tolerate direct sunlight. During the summer and autumn, deciduous trees or shade structures should be used to block the sun, and the ground should be regularly sprinkled with water.

Rhododendrons typically sprout in the spring and autumn, mainly in spring. The optimal growth temperature is 15-20℃. They bloom from March to May and are tolerant of pruning.

Pruning should generally be done before May, as the new shoots formed will develop flower buds in the same year. Late pruning can affect flowering.

V. Cultivation Method

Fertilizer and Water Management

Rhododendrons require acidic soil to grow and develop. As northern soils are often alkaline, the potting soil should be a mixture of decomposed pine needle soil and other humus soils.

Rhododendrons have fibrous, fine roots and have strict requirements regarding fertilizer concentration and water quality. When fertilizing, it is important to apply it on time and in appropriate amounts, following the principle of frequent application of thin fertilizer.

Before they bloom in the spring, a monthly application of phosphorus fertilizer can promote the growth of leaves and flower buds. After flowering, apply a mixed fertilizer, mainly of nitrogen and phosphorus, once or twice.

During the bud-forming period in September and October, apply phosphorus fertilizer once or twice. During the growth period and flowering period, more fertilizer and water are required.

During the winter dormancy and slow summer growth periods, control the amount of fertilizer and water to prevent root rot. Rhododendrons prefer a humid and cool environment.

In the dry climate of the north, it is necessary to water and spray in time to maintain high air humidity. The best water for watering flowers is alum fertilizer water and rain water. If tap water is used, a small amount of ferrous sulfate and vinegar should be added.

You can also cut watermelon or tomatoes into small pieces and apply them to the soil, which can also improve the soil quality and flower quality.

Shaping and Pruning

To accelerate the blooming of the plants, rhododendrons are often topped to promote the growth of new branches. For flowers that are crowded and affect the shape of the flower, early thinning can be done.

This not only makes the flowers larger and brighter in the current year, but also benefits the growth and flowering of the plants in the coming year. Rhododendrons have strong buds.

Branches that severely affect the growth and development of the plant and reduce its ornamental and commercial value should be pruned. Pruning is generally done after the flowers fade in spring and in autumn.

Cut off dead branches, slanted branches, overgrown branches, and some overlapping branches to avoid nutrient consumption and ensure the plant blooms fully.

Flowering Control

Rhododendrons undergo flower bud differentiation in autumn. Through refrigeration and heating treatments, the flowering period can be artificially controlled.

To make rhododendrons bloom earlier, they can be moved to a greenhouse for cultivation, keeping the temperature between 20-25℃, and frequently spraying water on the leaves to maintain a relative humidity of over 80%.

This way, they can bloom in about a month and a half. To delay the flowering of rhododendrons, move them to a low-temperature environment with temperatures between 2-4℃. Water them when the pot is dry, then move them outside in summer and autumn, and they can bloom in about two weeks.

VI. Propagation Methods

Cuttings

Cuttings propagation is the most commonly used method in the cultivation of rhododendrons.

Typically, in May or June, healthy semi-woody new branches of about 5-8 cm in length are cut, the leaves at the lower part are removed, and only 2-3 leaves at the top are kept as cuttings.

The base of the cuttings should ideally be treated by dipping in solutions such as indolebutyric acid or ABT rooting powder, and then planted in loose, breathable, and humus-rich acidic soil.

The temperature needs to be maintained at 20-25 degrees Celsius, and shading and frequent misting are necessary to promote the growth of new roots.

Grafting

Grafting is used for valuable varieties that are difficult to survive, such as Western rhododendrons. First, a tender stem of about 3-4 cm is cut as a scion. The base is sharpened into a wedge shape with a sharp knife, and a common rhododendron is used as a rootstock.

The graft is then placed under a shade and bound with plastic film, and a plastic bag is used to cover the scion and rootstock together to maintain moisture.

Seeding

Seeding is a form of sexual reproduction. As the natural hybridization rate is very low, artificial pollination is usually conducted to increase the fruiting rate of rhododendrons and obtain more seeds.

The general method is to select a vigorous and beautiful mother plant when the rhododendron is in full bloom, move it to a greenhouse, and keep 3-4 large, early-blooming flowers per pot.

The stamens and new buds near the flowers are removed to concentrate nutrients. When sticky liquid appears on the pistil head, pollen from the father plant is applied several times with a fresh brush.

A week after pollination, the flower pot is moved outdoors, and water and fertilizer management are intensified. After about 5-6 months of growth and development, fruits gradually change from green to brown from November to December, indicating that the seeds are mature.

They are then harvested, allowed to dry in the shade, cleaned, and stored in a cool and ventilated place.

Rhododendron seeds are very small and should not be stored for long periods, otherwise, the germination rate will be low.

If they are evergreen rhododendron seeds, they should be sown as soon as they are harvested, in pots in the greenhouse; if they are deciduous rhododendron seeds, they can be sown in the spring of the following year.

Seeds should be sown in shallow clay pots. The soil for sowing can be dug from the surface of a sun-facing slope, which is loose and covered with moss.

Stones should be removed, and it is best to spray once with a dilute solution of 40% formalin and 100 times water for disinfection, then air dry. It can also be sterilized by steaming in steam for 15-25 minutes, then dried to half dry for use.

Before sowing, water the shallow basin to make it moist. Place a layer of about 2 cm of charcoal at the bottom, then lay a layer of 5-6 cm thick sterilized soil, scrape it flat, slightly compact it, then evenly distribute the seeds on top, lightly press them down, and avoid spraying water with a sprayer.

It is best to use the method of soaking the basin in water. Cover it with a piece of glass and move it into the greenhouse. When the surface of the pot soil is dry, you can spray some water in a mist, but don’t spray too much, just keep the pot soil moist.

After sowing, germination usually occurs within 5-6 weeks. When the seedlings have grown 2-3 true leaves, they can be gently picked out with a small bamboo piece and transplanted into other pots.

They should not be planted too densely and should be sparser. After planting, shade them with reed mats, avoid direct sunlight, and do not let the rainwater get on them. When the seedlings are tender, do not fertilize them.

When the second-year seedlings grow to nearly 10 cm high, start watering with thin fertilizer; generally, in late spring to early autumn of the second year, they can be transplanted individually.

By the third year, if the seedlings are well cared for and grow to 20 cm high, a few of them may produce flowers.

VII. Disease Prevention and Treatment

Leaf Swelling Disease

Before the onset of the disease, especially during the leaf expansion period, a 1:1:200 Bordeaux mixture can be sprayed. Diseased leaves should be promptly removed.

Prior to bud burst, 0.3-0.5 lime sulfur or 1:1:200 Bordeaux mixture can be sprayed 2-3 times, typically once every 7-10 days. After the disease onset, 65-80% mancozeb at 500 times dilution or 0.3-0.5 lime sulfur can be sprayed 3-4 times, once every 7-10 days.

Leaf Spot Disease

Leaf spot and brown spot diseases can be controlled effectively by spraying 70% methoxyacrylate at 1000 times dilution, 20% mancozeb at 4000 times dilution, or 50% mancozeb at 500 times dilution once every 10 days for a total of 7-8 times from May to August. To prevent leaf yellowing, iron sulfate can also be applied.

Mite Disease

The disease is characterized by dense white or pink mite layers on the tips of tender branches and leaves, sometimes causing mite galls on the leaves, often caused by aphids.

Its prevention and control methods include: spraying copper sulfate while removing diseased leaves, spraying pesticides containing oxidized dimethoate or placing fenamiphos directly in the pot can control aphids and other piercing-sucking pests.

Crown Shield Bug

Symptoms: Adult and nymph bugs suck sap on the back of leaves, causing the appearance of black, sticky substances resembling splattered dirt.

The entire back of the affected leaf turns rust yellow, forming many pale spots on the front, which may merge into patches when heavily infested, resulting in the entire leaf losing its green color, appearing pale from a distance, and falling prematurely, no longer forming flower buds.

Prevention and Control: Thoroughly remove fallen leaves and weeds from the pots and surrounding garden areas in winter. Apply white paint on the rough trunks of plants. The most opportune time for chemical control is from the emergence of overwintering adults to the hatching of the first generation of nymphs.

Root Rot Disease

Symptoms: Brown, water-soaked spots, soft rot, and bark peeling occur on the roots. The wood turns black-brown, and the bark gradually becomes gray-white and spreads gradually.

Prevention and Control: The fungus survives on plant debris in the soil and can infect suitable hosts for several years. Alkaline soil, high humidity, and high temperatures favor the development and spread of the disease.

Red Spider Mites

Symptoms: Mainly suck the sap of plants causing gray-white spots on the leaves.

Prevention and Control: Remove dead branches and fallen leaves in winter to eliminate overwintering adults.

VIII. Value and Others

Main Values

With lush branches and leaves, strong sprouting power, resistance to pruning, and peculiar root stumps, rhododendrons are excellent materials for bonsai. They are best planted in clusters near the edge of the forest, beside streams, ponds, and rocks.

They are also suitable for scattered planting under sparse forests and are good materials for flower hedges. They can be pruned to form various shapes.

During the flowering season, rhododendrons always give people a lively and bustling feeling, and when it is not the flower season, the dark green leaves are also suitable for planting in gardens as low walls or barriers.