- Description of rhododendron flower
- Planting of rhododendron
- Rhododendrons: care
- Rhododendron after flowering
- Species and varieties of rhododendron
- Rhododendron properties
- Growing of rhododendron in a cold climate zone
- References and links
Rhododendron plant (lat. Rhododendron) is a genus of semi-deciduous, deciduous and evergreen trees and shrubs in the heath family that according to various sources contains from 800 to 300 species, including such popular indoor plants as azaleas that are nicknamed “indoor rhododendrons”. The word “rhododendron” is made up of two parts: “rhodon” that means “rose”, and “dendron” that means “tree”. So literally rhododendron is a rosewood or a tree with roses. And azaleas really look like roses. In nature rhododendrons mainly grow in the Northern Hemisphere – in Southern China, Japan, the Himalayas, North America and Southeast Asia.
Mostly they can be found in the coastal zone of rivers, seas and oceans, in the half-shade of the underbrush and on the northern slopes of the mountains. Some of the rhododendrons grow 12 inches in height, and some species are creeping shrubs. The flowers of this genus differ in size, color, and shape. The smallest of them are very tiny while the largest of them reach 8 inches in a diameter. Today a garden rhododendron has about 3,000 forms, species and varieties.
Description of rhododendron flower
The garden rhododendron is represented by bushes with leaves of various size and shape – annual, biennial and perennial, sessile or petiolate, alternate, smooth-edged or serrate, ovate or obovate. The rhododendron flower is popular all over the world because of the ornamentality of its foliage, and, above all, the magnificence of its flowers that can be white, pink, red, purple, violet, collected in clusters or brushes resembling a chic bouquet. Depending on the variety and species, the flowers can be bell-shaped, funnel-shaped, wheel-shaped or tubular. The flowers of some species exhale a pleasant aroma. The fruit of rhododendron is a multi-seeded quinquevalve capsule with seeds up to 0.08 inch in size. The root system of rhododendron is compact, superficial, consisting of a multitude of fibrous roots. A superficial location of its roots makes transplanting of rhododendron easy without causing much troubles either to a gardener or to the plant. Rhododendron is an excellent early spring honey-plant.
Planting of rhododendron
Where and when it is better to plant rhododendron
In our climate zone it is better to grow exclusively winter-hardy rhododendrons in the garden. Planting of rhododendron in the ground is carried out from April till mid-May, as well as from September till November. Actually, if necessary, it can be done at any time during the vegetative period, except for the time when rhododendron blooms, and within one-two weeks after its flowering. Rhododendron should be planted in the shade (the north side of the building), in a loose, well drained, rich in humus, acidic soil. If on your site the groundwater lies at a depth of less than 3.3 ft, rhododendron should be planted in the elevated flower bed. The neighbors of rhododendron can be pine, oak, larch, that is, the trees with a root system that goes deep into the ground. Such tree species as linden, chestnut, alder, maple, willow, elm or poplar will deprive rhododendron of the necessary nutrition, since their roots will be at the same depth as the roots of rhododendron. If there is no way to avoid such a neighborhood, it will be necessary to protect the root system of rhododendron by digging a ruberoid, slate or polyethylene into the soil. Good neighbors for rhododendron are garden trees such as apples, pears.
How to plant rhododendron
Dig out a planting hole, about 2 ft in diameter and 1.3 ft in depth, fill it with a carefully mixed mixture of 8 buckets of high-moor peat and 3.5 buckets of clay loam (you can replace the clay loam with two buckets of clay). The mixture is carefully tamped at the bottom of the hole, and then dig out a hole in it that corresponds to the size of the root of the seedling. Before planting, immerse the rhododendron seedling into water and keep it there until the air bubbles cease to appear. Then place the roots of the seedling in the hole, fill the hole with the substrate, tamp it so that there are no voids left. The root neck of rhododendron should be at the level of the ground surface. If you plant rhododendron in a dry soil, water it so that the soil becomes soaked at the depth of 8 inches, and mulch the area near the trunk with 2-2.4 inch layer of peat, oak leaves, moss or pine needles. If the bush has many flower buds, it is better to remove them all to direct resources on successful rooting, but not on rhododendron blooming. When planting is carried out in a spacious area, for the wind not to shake a just-planted tree, you need to arrange the support, tilting it toward the direction of the most frequently blowing winds, and tie a seedling to it. Once the bush is rooted, the support can be removed.
Growing of rhododendron
Care of rhododendron implies the usual procedures: watering, sprinkling, weeding, feeding, forming a bush and fighting with diseases and pests, if necessary. Do not loosen and dig the soil around rhododendron since the plant roots are located too close to the surface. For the same reasons, you need to remove the weeds manually without using a chopper. Rhododendron requires more soil and air moisture than other plants, especially during the formation of buds and flowering. The adequate watering also affects the formation of the flower buds of the next year. Watering is carried out with soft water – sedimented water or rainwater. You can soften and acidify the water for rhododendron by adding a few handfuls of high-moor peat into it a day before watering. The frequency of watering is determined by the state of the leaves: if they turn dull and lose the turgor, then they are thirsty. When moistening, the soil should get wet to a depth of 8-12 inches. However, it is very important not to overwater the rhododendron roots, since the plant is sensitive to excess moisture. In case of waterlogging it behaves exactly as during drought – it lowers and folds the leaves. So if you want rhododendron not to mislead you, during dry and hot periods you should spray the rhododendron leaves with soft water as often as possible without increasing the amount of water while watering the plant.
Pruning of rhododendron
Pruning of rhododendrons should be minimal, since the bushes form the right shape by themselves. However, sometimes it is necessary to cut too high bushes, remove frozen shoots or rejuvenate the old rhododendron. How to prune properly an adult bush? The shoots should be cut out in the early spring before the beginning of the sap flow. Those cuts where the thickness of the branches reaches 0.8-1.6 inches are processed with tree wax. In a month the buds emerge on the shoots, and the process of renewal begins and lasts for a year. Very old or heavily frozen bushes are cut at a height of 12-16 inches from the ground: one half of the bush is pruned this year, another half of the bush is pruned the next year.
Rhododendrons have one peculiarity: if they bloom and bear fruit very abundantly this year, it means that the next year both flowering and fruiting will be much more modest. To get rid of this periodicity, you need to remove the wilted inflorescences immediately after flowering, so that rhododendron uses strength and nutrition to form flower buds for the next year.
Fertilizing of rhododendron
You need to fertilize even those rhododendrons that were planted the current year, and the first dressing is carried out in early spring, and the last one should be done after flowering at the end of July, when young shoots begin to grow. Rhododendrons prefer liquid fertilizers made of half-rotted manure, horn powder. Add water into manure at a ratio of 1:15 and let it brew for several days, and only then you can use it as a fertilizer. Before applying of the fertilizer, rhododendron is watered. Since rhododendrons grow in acidic soils and in order not to destroy chemical reaction, it is preferable to use such mineral fertilizers as ammonium sulphate, superphosphate, potassium nitrate, potassium sulphate or potassium phosphate, calcium and magnesium sulphate at a very low concentration of 1.2: 1000, and potassium fertilizer solution can be even weaker. The optimal feeding regime assumes the applying of organic or mineral nitrogen fertilizers at the rate of 1.8 oz of ammonium sulphate and 1.8 oz of magnesium sulfate per 11 ft² in spring, and after flowering, in early June, you should apply 1.4 oz of ammonium sulphate and 0.7 oz of superphosphate and potassium sulfate per 11 ft² of the site with rhododendrons. In July only 0.8 oz of superphosphate and potassium sulfate per 11 ft² are applied.
Pests and diseases of rhododendron
The pests that attack rhododendrons are mealybugs, scales, spider mites, bed bugs, weevils, rhododendron flies, as well as snails and slugs. Gastropods are collected by hand, and as a preventative measure you should apply a fungicide. Spider mites, rhododendron bugs, as well as weevils are destroyed by treatment with diazinon, and in the case of rhododendron damage by weevils, you should also process the top layer of the soil with an insecticide. The remaining insects are killed with malathion prepared in accordance to the instructions.
Rhododendron can get affected by fungal diseases – leaf spot, cancer, chlorosis, rust. Usually they develop as a result of poor aeration of the roots. Spots and rust are treated by drugs with copper sulfate, in particular, Bordeaux mixture. Chlorosis that makes rhododendron yellow requires the adding of an iron chelate to water for irrigation. With regard to cancer, it is necessary to remove diseased shoots or cut them to a healthy tissue, in addition, you need to carry out preventive treatment of rhododendron with Bordeaux mixture in early spring and late autumn.
Propagation of rhododendron
Rhododendrons are propagated generatively by seeds and vegetatively by dividing the bush, layers, cuttings and grafting. The simplest way is to propagate rhododendrons with layers, and we are going to tell you about this method, as well as how to sow seeds properly and make cuttings. The seeds of rhododendron are sown in an earthen saucer with a well-moistened heath or peat soil with an admixture of sand at the rate of 3: 1, the seeds are sprinkled with washed sand, the saucer is covered with glass and put in a bright place for germination. Care for seedlings implies moistening of the substrate as needed, daily airing and removing of condensate from the glass. The sprouts usually appear in a month, and when leaves appear on the shoots, they are planted out according to the scheme of 0.8x1.2 inches, deepening into the soil at the level of cotyledons for the root system of seedlings to form. For the first year the shoots are kept in a cool greenhouse, and the next year they are planted out in open ground on training beds with garden soil and with an admixture of sand and peat. Seedlings grow very slowly and start blooming only in 6-8 years.
Propagation of rhododendron by cuttings is not simpler. In order to do this you will need semi-lignified shoots and make cuttings 2-3 inches long. The lower leaves of the cuttings are removed, and the lower sections are kept in a root growth stimulant solution, for example in heteroauxin, for 12-16 hours. Then cuttings are placed in a mixture of peat and sand in a ratio of 3:1 and covered with a transparent glass. The process of rooting is long and difficult: deciduous species need a month and a half, and evergreen species need 3-4.5 months. Then the cuttings are grown in boxes with a mixture of peat and pine needles in a ratio of 2:1, in winter they are placed in a cool bright room, where the temperature is kept within 46-54 ºF, and in spring they are planted in the garden with the boxes and are grown another year or two before transplanting to a permanent place.
Propagation by layers is the simplest and most natural way of rhododendron reproduction. In spring, a young, flexible shoot growing at the very bottom of the bush is bent and put in a pre-made groove at least 6 inches deep, with the middle part of the shoot being pinned to the groove, and then the shoot is covered with garden soil mixed with peat. The top of the shoot remains on the surface and is attached to a peg stuck vertically. During the whole season, the layer and the bush are moistened, and in autumn or next spring the rooted layer is separated from the mother plant and transplanted into a permanent place. It is the best way of propagation of deciduous rhododendron.
Rhododendron after flowering
Rhododendron in autumn
If autumn is dry, rhododendron should be watered abundantly – 2.2-2.6 gallons of water per bush. If fall is regular and with rains, then there is no need to water rhododendron. By November each bush is insulated in the root zone by putting a layer of peat along the trunk.
Rhododendron in winter
If winter is moderate, before the first frosts place spruce and pine branches between the branches of rhododendron, slightly tie the bush with a string and cover the bush with burlap. The cover is removed on a cloudy day in early spring, as soon as the snow melts. In warmer areas, rhododendrons hibernate without any cover.
Species and varieties of rhododendron
There are lots of rhododendron species and we can talk about them endlessly. So we just list those that are grown in culture, and also introduce you to the most popular garden varieties of rhododendrons.
Daurian rhododendron (Rhododendron dauricum)
grows in nature in Maritime Territory, Northeast China, Korea, Eastern Siberia and Northern Mongolia, preferring coniferous forests and rocks. This is an evergreen, medium-sized, strongly branched shrub, 6.5- 13 ft in height, with a gray bark and upward branches. Its shoots are thin, reddish-brown, the tips are covered with short hairs. Small leathery leaves are up to 1.2 inches long, the upper side of the leaf is smooth, the underside is with scales. Young leaves are light green, adult leaves are darker, and in autumn they are either brown or red-green. With the onset of winter the bush does not shed all leaves, many of them remain on the branches all winter long. The abundant flowering of the daurian rhododendron that lasts for about three weeks occurs before the appearance of the leaves. It blooms with funnel-shaped large flowers of violet-pink hue, reaching 1.5 inches in diameter. Sometimes in autumn dahurian rhododendron starts blooming again. This species is very winter-hardy. It is easily propagated with green cuttings. It has two varieties:
- an evergreen variety with dark green leaves and purple-lilac flowers;
- a garden early hybrid, low growing, with abundant, bright, early blooming bluish-red flowers up to 2 inches in diameter. This form is not so winter-hardy as the main species.
White wing, or Sagan-Dale (Rhododendron adamsii)
is an evergreen rhododendron growing in the Far East and on the northeast foothills of Tibet, preferring mountain forests and stony slopes. It is a branchy shrub up to 20 inches in height with shoots covered with glandular down. Dense matte green leaves are oblong-elliptical, up to 0.8 inch in length and up to 0.8 inch in width, the upper side of the leaf is smooth, scales make the underside red. Flowers are up to 0.6 inch in diameter of different shades of pink. 7-15 flowers form corymbose inflorescences. This rhododendron is included in the Red Book of Buryatia.
Japanese azalea (Rhododendron japonicum)
as the name implies, comes from Japan inhabiting the sunny mountains of Honshu. This species is one of the most beautiful deciduous rhododendrons. It is a branchy shrub, up to 6.5 ft high, the shoots are either smooth or with silvery bristles. The leaf of Japanese azalea is green, oblong-lanceolate, with a soft pubescence on both sides of the leaf. In autumn the leaves turn orange-red. Fragrant bell-shaped flowers are up to 3 inches in diameter, collected in racemose inflorescences of 6-12 pieces, the color of flowers is orange and scarlet. In a cold climate zone it would be hard to find more beautiful species than Japanese azalea. In addition, the species is winter-resistant and propagates well by cuttings and seeds.
Georgian Snow Rose (Rhododendron caucasicum)
is native to the Caucasus, as its name implies. It is a low evergreen shrub with creeping branches. The leaves of Georgian snow rose are leathery, oval, oblong, dark green, the upper side of the leaf surface is glabrous, the downside is tomentose and red, placed on long, thick petioles. Fragrant funnel-bell-shaped yellowish flowers with green spots inside the throat are collected in racemose inflorescences of 8-12 pieces and placed on the downy peduncles. The species has several ornamental forms:
- pink-white that blooms earlier than the main species;
- shiny with dark pink flowers;
- golden with yellow flowers, decorated with greenish spots;
- straw-yellow with yellow flowers and reddish specks.
In addition to the described species the following species are also widely cultivated: Albrecht’s azalia, coastal azalea, pinkshell azalea, pink azalea, smooth azalea, yellow azalea, hairy alpenrose, California azalea, rosebay, evergreen azalea, Kamchatka rhododendron, rhodora, Carolina rhododendron, rhododendron myrtifolium, rhododendron racemosum, swamp azalea, rhododendron brachycarpum, rhododendron russatum, great rhododendron, big leaf rhododendron, Catawba rhododendron, Lapland rosebay, rhododendron ledebourii, plumleaf azalea, flame azalea, rhododendron hippophaeoides, Korean rhododendron, rhododendron impeditum, pontic rhododendron, rhododendron keleticum, rhododendron poukhanense, rusty-leaved alpenrose, rhododendron fastigiatum, early azalea, rhododendron sichotense, rhododendron prunifolium, rhododendron obtusum, rhododendron radicans, rhododendron degronianum and many others.
is a so-called combination of varieties and hybrids of rhododendrons grown in culture. In other words, hybrid rhododendron is a garden rhododendron. The most popular varieties of hybrid rhododendron are:
- Alfred, a German cultivar, was bred by crossing Everestianum with Catawba rhododendron. It is an evergreen shrub up to 4 ft in height, the diameter of the crown is about 5 ft. The leaves are oblong-elliptical, dark green and shiny. Bright purple flowers with a yellow-green spot are up to 2.4 inches in diameter and dense inflorescences consist of 15-20 flowers;
- Blue Peter variety was obtained by cross breeding of the pontic rhododendron. The height of the bush is more than 5 ft. The crown is sprawling, up to 6.5 ft in diameter. Flowers are up to 2.4 inches in diameter, the color of flowers is lavender-blue, the edges are wavy with a dark purple spot on the upper petal;
- Jackson is an English hybrid of Nobleanum variety and Georgian Snow Rose (Caucasian rhododendron). The bush is up to 6.5 ft high, the diameter of the crown is about 10 ft. There is a low growing variety reaching up to 2.6 ft in height. The leaves are oblong, leathery, the upper side is matte-green, the underside is brown. 8-12 flowers make up inflorescences. When opening the flowers are pink, and later they turn white with a yellow spot on one petal;
- Rose Marie is a Czech variety obtained through cross breeding of Pink Pearl variety and Great White rhododendron. The height of the bush is 4 ft, the girth of the crown is 5 ft. The leaves are oblong-elliptical, leathery, the upper side of the leaf surface is light green with a wax coating, the underside is blue-green, glossy. Flowers are soft pink at the edges, and at the middle they are densely pink with a purple hue. 6-14 flowers form compact globular inflorescences;
- Nova Zembla is a Dutch hybrid created by crossing of Parsons Gloriosum and Catawba rhododendron. The bush is up to 10 ft in height and has a loose crown up to 11.5 ft in girth. The shoots grow almost vertically, the leaves are large, leathery, shiny. Large flowers are up to 2.4 inches in diameter. The flowers are red with a black spot. 10-12 flowers form dense inflorescences;
- Cunningham is a Scottish cultivar, the most popular variety of Caucasian rhododendron, reaching a height of 6.5 ft with a crown of 5 ft in diameter. Leaves are oblong, leathery, dark green, up to 2.4 inches long and up to 1.2 inches wide. Flowers are white with yellow-brown spots, 10 flowers form dense inflorescences.
In addition to undeniable ornamental advantages, rhododendron has medicinal properties that are widely used in traditional medicine. Such species as Daurian rhododendron, rosebay, white wing, Caucasian rhododendron contain andromedotoxin, ericoline, arbutin and rhododendrin. Rhododendron leaves also contain ascorbic acid, the highest concentration of which is observed in the summer months. Due to the content of substances useful for the human body, rhododendron has antipyretic, analgesic, bactericidal, soothing and sweating action. It removes excess fluid from the body, relieving shortness of breath, swelling, frequent palpitations, lowers arterial and venous pressure, and promotes cardiac activity. However, rhododendron is not so harmless as it seems. Drugs made of rhododendron should be avoided during pregnancy and breastfeeding, as well as by the patients with tissue necrosis and suffering from serious kidney diseases. And in any case before taking such drugs you should consult your doctor.
Growing of rhododendron in a cold climate zone
Sometimes enchanted by a beautiful advertising picture amateur flower-growers are eager to grow an amazing bush called a rhododendron in their garden. But very often, despite money, time and efforts spent, they get disappointed – the bush in their garden looks completely different than in the advertising booklet; moreover, it withers and eventually dies. Is it possible to avoid such a sad ending and grow a heat-loving rhododendron in a cold climate zone? As they say, there is nothing impossible.
Planting of rhododendron in a cold climate zone
First, you need to know exactly what species of rhododendron can survive a cold winter, otherwise the plant can die even if being covered. For this purpose it is better to take deciduous species of rhododendrons: Japanese azalea, yellow azalea, royal azalea, pinkshell azalea, rhodora, Kamchatka rhododendron, rhododendron poukhanense. Among semi-evergreen species, you can choose rhododendron ledebourii, and among evergreen species you can grow Catawba rhododendron (and also its hybrids Alfred, Abraham Lincoln, Nova Zembla, Cunningham White), rhododendron brachycarpum, rosebay, great rhododendron, rhododendron Smirnovii and its hybrids Gabriel, Dorothy Swift, Laika. You can also grow such proven winter-hardy Finnish varieties as Elvira, Hague, and Mikkeli. Hybrids of the Northern Light group such as Rosie Lights, Pink Lights, Spicy Lights and others also overwinter well.
If you have purchased a winter hardy rhododendron, then you need to plant it properly. First of all, you should do this in spring, choosing a semi-shady place and keeping the distance of at least 3.3 ft from any other plant. Secondly, buy a special substrate for rhododendron or prepare a mixture consisting of garden soil, pine needles and peat. You need to add a combined mineral fertilizer into the substrate. Thirdly, the planting hole for the rhododendron seedling should be twice as large as the container with the root system of the seedling, and if the soil is clay, be sure to put a 6 inch drainage layer of broken brick on the bottom of the planting hole. Fourthly, do not bury the root collar – let it be at the same level as it was in the container. After planting, you must water the seedling.
Care for the rhododendron growing in a cold climate zone
Planting and caring for rhododendron in a cold climate zone are almost the same as in regions with warmer winters, but there are some differences. We offer you a list of requirements that if followed will help you succeed despite a cool climate in your area:
- rhododendrons grow in acidic humus soils. In the zone where the roots take nutrients from there should be no dolomite, ash, lime and other substances alkalinizing the soil;
- it is necessary to mulch the area around rhododendron stem since loosening and digging the soil around the bushes are not desirable because of horizontal arrangement of the root system of rhododendron;
- in spring protect rhododendron from the sun rays with a net, gauze or cloth;
- the most important key to success is a deep and balanced watering of rhododendron: it should receive moisture just as much as it needs, no more, no less. In a dry, hot summer watering is carried out twice a week.
Sometimes at the beginning of autumn, due to warm rainy weather, rhododendrons begin to grow, but young shoots do not have enough time to ripen and die in winter. To avoid undesirable late growth of shoots, in dry weather spray the bush with a one-percent solution of sulfate or potassium monophosphate by a finely dispersed sprayer – this measure will stop growth, stimulate the lignification of shoots and the forming of flower buds for the next year. However, after spraying, you should stop watering rhododendron, even if there are still no rains.
If you still worry that your rhododendrons may freeze in winter, arrange a shelter for them by placing a wire frame wrapped with spunbond. Fix the covering material with a rope.
References and links
- Read also about topic at Wikipedia
- Features and other plants of the family Ericaceae
- List of all species on The Plant List
- More information at World Flora Online