Difference between revisions of "Rhododendron"

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Latest revision as of 22:19, 22 September 2013


Rhododendron ponticum


Plant Characteristics
Habit   shrub

Lifespan: ⌛ perennial
Poisonous: ☠ toxins in pollen and nectar
Cultivation
Features: ✓ flowers, fire resistant
USDA Zones: on"on" is not a number. to on"on" is not a number.
Scientific Names

Ericaceae >

Rhododendron >


Rhododendron (from the Greek rodo, meaning "rose", and dendro, meaning "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 1000 species and most have showy flower displays. It includes the plants known to gardeners as azaleas. It is the national flower of Nepal & State flower of Uttrakhand ,India.

The Rhododendron is a genus characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm tall, and the largest, R. giganteum, reported to over 30 m tall.[1] The leaves are spirally arranged; leaf size can range from 1–2 cm to over 50 cm, exceptionally 100 cm 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 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.


Read about Rhododendron in the Standard Cyclopedia of Horticulture 

Rhododendron (Greek, rhodon and dendron, rose-tree; the Rhododendron of the ancient writers is Nerium). Ericaceae. Including Azalea which most botanists consider inseparable from Rhododendron, but horticulturists may be inclined to retain Azalea for the deciduous species and to use the Azalea names given in parentheses. Highly ornamental woody plants, chiefly grown for their beautiful flowers and many species also for their handsome foliage.

Evergreen or deciduous shrubs, rarely trees: lvs. alternate, short-petioled, entire: fls. pedicelled, in terminal umbel-like racemes, rarely lateral, in 1- to few-fld. clusters; calyx 5-parted, often very small; corolla rotate, campanulate or funnel-shaped, sometimes tubular, with 5-, sometimes 6-10-lobed limb; stamens 5-10, sometimes more; anthers opening with pores at the apex; ovary 5-10-loculed; style slender with capitate stigma: caps. separating into 5-10 valves containing numerous minute seeds.—About 350 species are known, distributed through the colder and temperate regions of the northern hemisphere; in Trop. Asia they occur in the mountains and extend as far south as New Guinea and Austral., the greatest segregation being in Cent. and W. China and the Himalayas; several species closely allied to those of the Malayan Archipelago are found in the Philippine Isls, but are not yet intro.; 16 species occur in N. Amer. The rhododendrons possess but few economic properties. The hardy close-grained wood of the arborescent species is used for fuel; also for construction and for turnery work; the lvs. of some species are used medicinally; those of R. arboreum and other species are believed to be poisonous to cattle. In India the fls. of various species are sometimes made into a subacid jelly. The honey obtained from the fls. is believed to be poisonous.

The rhododendrons belong to our most ornamental and most beautiful flowering shrubs and are often completely covered with their showy trusses of brilliantly and variously colored flowers and the evergreen species are attractive throughout the whole year with their handsome usually large foliage. They grow best in a peaty or porous loamy soil, which does not contain lime and always retains a sufficient amount of moisture; they like as a rule half-shaded positions. In regard to their culture and particular ornamental qualities, they may be divided into three horticultural groups: Evergreen rhododendrons, hardy deciduous azaleas, and Indian azaleas.

Evergreen rhododendrons.

Here belong the largest number of species comprising the two sections Lepidorhodium and Leiorhodium. Although most of the species are hardy only in warm temperate regions, there are many which are hardy at least as far north as Massachusetts. They are R. maximum, R. catawbiense, R. caucasicum, R. brachycarpum, R. Metternichii, R. Smirnovii, R. mucronulatum, R. dahuricum, R. micranthum, R. lapponicum, R. ferrugineum, R. hirsutium, R. Kotschyi, R. carolinianum, R. minus, and probably also R. chrysanthum, R. Przewalskii, R. campanulatum, R. californicum, R. Ungernii, and most of the small-leaved Chinese species. Somewhat more tender are R. ponticum, R. niveum, R. Hodgsonii, R. Thomsonii, R. Anthopogon and many of the recently introduced Chinese species, as R. discolor, R. oreodoxa, R. decorum. South of Philadelphia such species as R. cinnabarinum, R. glaucum, R. ciliatum, R. Fortunei, R. lepidotum, R. Collettianum, and the Yunnan species, as R. yunnanense, R. irroratum, and R. racemosum, are probably hardy; also R. arboreum, R. barbatum, R. Falconeri, R. Keysii, R. triflorum, and R. Wrightii in very sheltered positions. Species like R. Dalhousiae, R. Edgeworthii, R. Griffithianum, R. formosum, R. Maddenii, R. Nuttallii, and R. pendulum stand only a few degrees of frost. The Javanese species, as R. javanicum, R. jasminiflorum, R. Brookeanum and R. Lobbii grow and bloom continually and stand no frost at all.

Variation in height.—Most of the species are shrubby; a few only, and these mostly Himalayan species, grow into small or medium-sized trees, attaining 60 feet in the case of R. barbatum, 40 feet in R. grande and R. arboreum, 30 feet in R. Falconeri and R. maximum. A number of northern and alpine species always remain dwarf, as R. ferrugineum, R. hirsutum, R. lapponicum, R. virgatum, R. lepidotum, R. racemosum, and others. A few Himalayan and Chinese species and most of the Malayan species are often epiphytal and grow on branches of large trees like orchids; e.g., R. Dalhousiae, R. pendulum, R. Nuttallii, R. moupinense.

Hybrid rhododendrons.—Many hybrids have been raised and they are now more extensively cultivated than the original species. The first hybrid was probably the one raised from R. ponticum, fertilized by a hardy azalea, probably A. nudiflorum; it originated about 1800, in the nursery of Thompson, at Mileend, near London, and was first described and figured as R. ponticum var. deciduum (Andrews, Bot. Rep. 6:379). Many hybrids of similar origin were afterward raised for which the name Azaleodendron has been proposed by Rodigas. The first hybrid between true rhododendrons was probably a cross between R. catawbiense and R. ponticum, but it seems not to have attracted much attention. It was by hybridizing the product of this cross with the Himalayan R. arboreum introduced about 1820 that the first plant was raised which became the forerunner of a countless number of beautiful hybrids. From the appearance of this cross, obtained about 1826, at Highclere, in England, and therefore called R. altaclarense, the era of rhododendron hybrids is to be dated. Figs. 3378 and 3379 are common hybrid forms. A second era in the history of the rhododendron may be dated from the introduction of a large number of the beautiful Sikkim rhododendrons about 1850, among them species like R. Griffithianum which entered into the parentage of many of the most striking tender hybrids, and from the introduction of the Javanese species shortly afterward. A third era will perhaps be traced from the recent introduction of the Chinese rhododendrons.

Their place in ornamental planting.—Rhododendrons are equally effective and desirable as single specimens on the lawn as when massed in large groups, and are especially showy when backed by the dark green foliage of conifers. which at the same time afford a most advantageous shelter. The dwarf species, which are mostly small-leaved and flower at a different time, should not be grouped with the large-leaved ones, as they do not harmonize with them; however, they are exceedingly charming plants for rockeries or in groups with other smaller evergreens. It is certainly true that the rhododendrons have not yet received the attention they deserve. They are still far from being as popular as they are in England. The beautiful Himalayan species and their numerous hybrids are still almost unknown in this country, although without doubt they could be grown as well outdoors in the Middle and South Atlantic states as they are in England, if the right situation were chosen. Formerly it was considered impossible to grow the beautiful hardy hybrids in the New England states, and it was first shown by the splendid collections of H. H. Hunnewell at Wellesley, Massachusetts (see A.F. 13:24-31 and Gng. 5:375-7), that, even in a trying climate, they can be grown to perfection if the proper situations are found and the right way of cultivation is followed.

Outdoor cultivation.— The selection of a suitable situation is of foremost importance. If possible the beds should be sheltered against drying winds and the burning sun by tall conifers, but the shelter should be always light and natural, as too much shelter by dense hedges or walls close to the plants is worse than no shelter at all. Any open well-drained soil which does not contain lime or heavy clay and has a moist and fresh subsoil will prove satisfactory. Where limestone or heavy clay prevails, beds must be specially prepared and filled with suitable soil. They should be at least 2 to 3 feet deep, or deeper when the subsoil is not porous, and in this case the bottom should be filled in about 1 to 2 feet high with gravel or broken stones for drainage. A mixture of leaf-mold or peat and sandy loam will make a suitable soil. In dry spells during the summer, watering is necessary if the subsoil is not very moist; it is most essential that the soil never becomes really dry. In autumn the ground should be covered with leaves, pine needles, hay, or other material to protect from frost. This mulch should be allowed to remain during the summer, especially when the plants are not large enough to shade the ground. An occasional top-dressing of well-decayed stable- or cow-manure will prove of much advantage. The ground should never be disturbed, as the roots are very near the surface. After flowering, the young seed-vessels should be removed. The rhododendrons are easily transplanted either in spring or in fall, especially if they grow in peat or turfy loam, and if a good ball of earth can be preserved in moving. They should be planted firmly, especially in porous, peaty soil, and thoroughly watered after planting. If they are carefully handled they are not much affected by transplanting, and tender kinds may be dug in fall, heeled-in in a frost-proof pit, and planted out again in spring. Potted and well-budded plants transferred in January into a temperature not exceeding 60° will develop in about six to eight weeks into very attractive and showy specimens for decoration.

Hardy varieties.—The following varieties have proved in the vicinity of Boston and may be recomended for planting in similar climates and for experimental trial farther north. They are mostly hybrids of R. with R. maximum, R. ponticum, R. caucasicum and with some infusion of R. arboreum and perhaps a few other species. As in most of them the parentage of R. catawbiense is the most predominant, they are all usually called "catawbiense hybrids." Choice kinds are (those marked with an asterisk have proved the hardiest): Album elegans, blush, changing to white; Album grandiflorum, blush, changing to white, flowers larger, less spotted; Alexander Dancer, bright rose, paler in center; Atrosanguineum, rich blood-red; August Van Geert, bright carmine, spotted dark purple; Bacchus, crimson, large flowers; Bicolor, purplish pink, spotted; Blandianum, rosy crimson (H.F. 1859:153); Bluebell, blush, with light purplish margin; Boule de Neige, white, early; Caractacus, deep crimson; Charles Bagley, cherry-red.; Charles Dickens, dark red, spotted brown, one of the most striking red ones; Caerulescens, pale lilac; Coriaceum, white, spotted yellow, dwarf and free-blooming; Crown Prince, carmine, spotted greenish yellow; Delicatissimum, blush, edged pink, changing to almost white, late (Gn. 63, p. 415); Edward S. Rand, rich scarlet; Everestianum, rosy lilac with crisped edges, excellent habit and very free-flowering (G. 26:103); F. L. Ames, white center, edged pink; F. L. Olmsted, pink; Giganteum, bright rose, large clusters; Glennyanum, white, suffused with pink (G.M. 44:355; 48:565); Gomer Waterer, blush-pink; Grandiflorum, clear rose; Guido, deep crimson; Hannibal, rosy carmine; Henrietta Sargent, pink; Henry W. Sargent, crimson, large clusters; H. H. Hunnewell, rich crimson; John Waterer, dark crimson; J. D. Godman, carmine, distinctly spotted; Kettledrum, rich crimson; King of Purples, purple, spotted dark brown; Lady Armstrong, rose-red, paler in center, distinctly spotted; Lady Clermont, rosy scarlet; Lady Frances Crossley, salmon-pink; Lady Gray Egerton, delicate lilac, spotted greenish brown; Lee's Purple, purple; Madam Carvalho, blush, changing to pure white; Melton, rich purple; Mrs. C.S. Sargent, similar to Everestianum, but pink; Mrs. Milner, rich crimson; Norma, pink; Old Port, plum-color; Princess Mary of Cambridge, white with purple margin; Purpureum crispum, lilac-purple, spotted greenish; Purpureum grandiflorum, purple, large clusters; R. S. Field, scarlet; Ralph Sanders, rich purplish crimson; Rosa mundi, white slightly flushed with yellow spots, dwarf (Gn. 63, p. 369); Roseum elegans, rosy lilac, dwarf; Scipio, rose with deep spot; Sefton, deep maroon, large clusters; Wellsianum, blush, changing to white.

For greenhouse culture, the most successful way, especially with the taller-growing species, like R. arboreum, R. Griffuhianum, R. barbatum, and R. Falconeri, is to plant them out in a porous peaty soil provided with good drainage. If grown in pots, a sandy compost of leaf-soil and peat, with an addition of some fibrous loam, will suit them. The pots, which should never be too large, must be well drained and the plants freely watered during the summer, while during the winter water must be carefully applied. The Himalayan species and their hybrids will do well in a cool greenhouse, where the temperature is kept a few degrees above freezing-point during the winter. The Javanese species and hybrids, however, on account of their continual growing and blooming, require a warmer greenhouse and must have a minimum temperature of 50° during the winter. They like a moist atmosphere and should be freely syringed in warm weather. In potting them, their epiphytal habit must be borne in mind, and the soil should consist mainly of good fibrous peat broken into pieces, with a liberal addition of sand and broken charcoal. The soil should never be allowed to become dry. They are readily propagated by cuttings with bottom heat in the warm propagating-house. The Javanese rhododendrons are especially valuable for their continual blooming during the winter and the brilliant color of their flowers. A large number of beautiful hybrids have been raised; the following are a small selection of them: Balsaminaeflorum, with double pink flowers (Gt. 37, p. 265. G.C. II. 18:230; III. 12:769. J.H. III. 43:151. G.Z. 27:241); Balsaminaeflorum album, with double white flowers (Gn.W. 5:373); Balsaminaeflorum aureum, with double yellow flowers; Brilliant, brilliant scarlet; Ceres, tawny yellow (Gn. 41:845); Diadem, orange-scarlet; Duchess of Connaught, vermilion-red; Duchess of Edinburgh, scarlet with orange-crimson (F.M. 1874:115); Eos, scarlet-carmine (G.C. III. 19:327); Exquisite, large light fawn-yellow flowers (Gn. 56:62); Favorite, satiny rose; Jasminiflorum carminatum, deep carmine (Gn. 41:328); Little Beauty, flowers small, but bright carmine-scarlet (Gn. 56:242); Lord Wolseley, bright orange-yellow, tinted with rose at the margins; Luteo-roseum, flowers satiny rose, suffused with white, center light yellow (G. 33: 313); Maiden's Blush, blush, with yellowish eye (Gn. 16:394); Princess Alexandra, white, faintly blushed; Princess Frederica, yellow, faintly edged rose; Princess Royal, pink; Rosy Morn, bright pink (Gn. 42:164); Taylori, bright pink, with white tube (F. M. 1877:242); Triumphans, crimson-scarlet. Propagation.—All rhododendrons are easily propagated by seeds, which are very small and are sown in spring in pans or boxes well drained and filled with sandy peat. Pots should be well watered previous to sowing. The seeds should be covered only a very little with fine sand or finely cut sphagnum, or merely pressed in and not covered at all. To prevent drying, a glass plate may be placed over the pan or some moss spread over the surface; this, however, must be taken off as soon as the seeds begin to germinate. The seeds also germinate very readily if sown on fresh sphagnum, but in this case they must be pricked off as soon as they can be handled. In any case, it is of advantage to prick off the young seedlings as soon as possible, but if they are not sown too thickly they may remain in the seed-boxes until the following spring. The seedlings of hardy rhododendrons should be placed in coolframes and gradually hardened off; those of greenhouse species remain under glass. Rhododendrons are also sometimes increased under glass by cuttings of half-ripe wood taken with a heel, and if gentle bottom heat can be given after callusing it will be of advantage. They root, however, but slowly, except those of the Javanese kinds, which are mostly propagated in this way, since they grow very readily from cuttings. Layering is sometimes practised, especially with the dwarf and small-leaved species, but the layers usually cannot be separated until the second year. For the propagation of the numerous varieties and hybrids of hardy and half-hardy rhododendrons grafting is most extensively employed. R. catawbiense or seedlings of any of its hardy hybrids may be used as stock; R. maximum is also probably as good. In English and Belgian nurseries R. ponticum, which is inferior in hardiness, is mostly employed as a stock, but this often proves fatal if the grafted plants are transferred to colder climates. R. arboreum may be used for strong-growing varieties intended for cultivation in the greenhouse or South. Veneer- or side-grafting is mostly practised, and sometimes cleft- and saddle-grafting (see G.C. III. 24:425 and Figs. 3380, 3381). The leaves should be removed only partly and the stock not headed back until the following year. The grafting is usually done late in summer or early in spring in the greenhouse on potted stock without using grafting-wax, and the grafted plants kept close and shaded until the union has been completed. If large quantities are to be handled the plants are sometimes not potted, but taken with a sufficient ball of earth, packed close together and covered with moss. Covering with moss to keep the atmosphere moist is also of much advantage if the plants are potted.

Other experience with the evergreen rhododendrons. (B. M. Watson.)—Rhododendrons, in this article, mean more particularly R. maximum and the hybrid varieties of R. catawbiense; in the main, however, the directions for the various operations apply to the azalea group and to many other members of the heath family.

Rhododendrons as a class are increased by seeds, layers and grafts, and occasionally by cuttings. Seeds should be sown under glass, between January 1 and March 15, in soil one-half peat and one-half pure fine sand, with good drainage. The seeds are small and require no covering, the usual watering after sowing being quite sufficient. A thin layer of sphagnum over the surface of the seed-pan is good protection from the sun and keeps the soil evenly moist; it should be removed when germination begins. Seeds may also be sown on growing sphagnum, a thin layer being compactly spread above the seed-soil and drainage, and an even surface being secured by clipping. Seed-pans or flats of convenient size are used and they should be plunged in sphagnum still further to insure even moisture; the temperature of the house should be 45° to 50° F. Seedlings are prone to damp-off and should be pricked off into fresh soil as soon as they are big enough to handle; wooden pincers, made from a barrel hoop, are handy for this work. They are slow growers, and must be tended carefully. Keep under glass, well shaded until the weather is settled. Frames with lath screens make good summer quarters. Winter in pits and plant out in frames in peaty soil when large enough. Never let them suffer from dryness. It has been suggested that the seed of R. maximum might be planted on living moss under high-branched trees in swamps where the water does not collect in winter. (See Jackson Dawson, on the "Propagation of Trees and Shrubs from Seeds," in Transactions of the Massachusetts Horticultural Society, 1885, part I, page 145.) Layers probably make the best plants, and in the best English nurseries layering is the common method of propagation. In the United States layering in spring is preferable, but abroad it is practised in both spring and autumn. It is a slow process, but desirable for the hardy hybrids of R. catawbiense. Roots form on wood of almost any age; when removed the layers should be treated as rooted cuttings and carefully grown in well-prepared soil where water and shade are easily furnished. See Layering. See, also, G.F. 6:63 (1893) for an interesting account of layering large plants by burying them to the top.-Grafting is the common method of propagation, and is employed almost universally in continental nurseries. R. ponticum is the usual stock, a free grower and readily obtained from seeds. Attempts have been made to use R. maximum in American nurseries, because of the tenderness of R. ponticum, but no great progress has been made. It is asserted that the rat growth is somewhat slower than that of the hybrids; this seems hardly possible, and it is to be hoped that further experiments will be made.R. should be established in pots in spring and grafted under glass in autumn and early winter, using the veneer-graft (see Grafting, page 1362, Vol. III). Graft as near the root as possible ancf plant the worked parts below the surface when planting in the nursery or permanently. With these precautions, and an extra covering of leaves until the plant is established on its own roots, the defect of tenderness in this stock can be overcome. Nurse carefully the young grafted plants in frames until of sufficient size to be planted in the nursery rows. Figs. 3380 and 3381 illustrate two common methods of grafting rhododendrons and other woody plants. The details of the unions are shown in Fig. 3380, and the completed work in Fig. 3381. Statements are made that cuttings of half-ripened wood will strike, but it is not likely that this will ever prove a practical method of propagating R. maximum or the R. catawbiense hybrids; it might be worth while to experiment with wood grown under glass,particularly with some of the smaller-leaved evergreen kinds.

As to cultivation, the point on which successful American growers of rhododendrons now insist is that the water-supply shall be sufficient. (See H. H. Hun- newell, in G.K 3:201, 1890.) To effect this: (1) make the soil deep and fine, using materials like peat, leaf- mold, well-rotted manure and yellow loam, all of which are retentive of moisture; (2) plant in masses, at any rate while young, so that they may protect each other and prevent evaporation; (3) give the bed a northern exposure or a situation where the force of the midday sun is broken; (4) do not plant under or near trees like elm, oak, or maple, which make undue inroads on the natural water-supply, nor so near buildings that the border is sheltered from rain or overdrained by cellar walls; (5) mulch with leaves summer and winter, protect from wind and sun with evergreen boughs in winter and in summer give heavy watering whenever the weather is excessively hot or dry.

The planting-bed should be prepared by excavating to the desired dimensions and at least 3 feet deep. The poor material should be discarded, but the good soil can be replaced, adding enough peat and the like (see above) to make good that which was rejected; all should be thoroughly and carefully mixed. Peat, although excellent, is not necessary. Yellow loam or hazel loam, if not too sandy, is equally good and is improved by additions of humus. To nearly pure peat an admixture of sand is beneficial; the essential point is that all soils for these plants must be fine. The beds should be prepared in autumn and left to settle all winter, due allowance being made for shrinking. In spring level off to the grade of the adjacent land and do not leave "rounded up." A bed higher at the center than at the sides perhaps makes a better display of the plants, but is more likely to dry up and does not catch all the water possible from occasional showers. It is generally conceded that lime soils and manures containing lime, e.g., wood- ashes and bone-meal, are injurious to rhododendrons; in limestone regions it is undoubtedly advisable to substitute, for the natural soil, others which are free from this objectionable element.

Plant rhododendrons in spring when weather is settled and the March winds have passed. If the ball of roots is dry, soak well before setting. Plant closely, so that the tops are only 10 to 12 inches apart and pay particular attention to "facing" them, i. e., see that the best side is facing the most important point of view, and that all are faced alike. Grafted plants should, if possible, have the worked portion below the surface. Do not plant in autumn. Plants grown on the premises may be transplanted in favorable weather in summer if great care is taken to prevent the roots suffering from dryness. In planning the original border it is well to leave room for extension: when planted, as described above, the beds can be enlarged at intervals of four or five years, or new beds made from the old stock. Place the beds so that the glare of the midday sun is screened both summer and winter, and avoid situations where there is any interference, owing to trees or buildings, with a naturally good condition of the soil in respect to moisture. If permanent protection is desired, use conifers, particularly the hemlock, in preference to deciduous trees. Good positions for beds may be found along the edges of ponds and streams, and in reclaimed meadows, with their cool moist soil, but keep aloof from any ground where the water collects in summer or winter. Beds, or even single plants, if sizable, may be introduced into open spaces in woodlands if the precautions noted above are observed and plenty of air and light are obtainable. It is somewhat difficult to combine rhododendrons and many deciduous shrubs, among which are the azaleas, their near relatives. A background of dark green conifers seems most appropriate. Mountain laurels, Pirns floribunda, Leucothoë Catesbsrí, and Daphne Cneorum are proper companions, but at times these seem better apart. Our native lilies, L. superbum and L. canadense, are good associates and thrive under the same conditions. In hot, dry weather water should be given, not daily in driblets, as lawns are sprinkled, but in quantity, enough at one time to soak the border to the depth of the soil, but at comparatively infrequent intervals, once a week or so. The bed should also be mulched with leaves, or other material, to prevent evaporation; grass clippings are serviceable, but should not te used m large quantities at any one time or else they will heat. Leaves make good winter protection, which should te given just before cold weather,—in eastern Massachusetts, between Thanksgiving and Christmas. Let the ted be covered to the depth of 10 to 12 inches, well worked in beneath the foliage but not over it. In spring dig as much as pos-sible of this material into the ground, reserving a part for the summer mulch. Shelter the tops with evergreen boughs, the butts driven into the earth a foot or more; in windy positions a temporary board fence is useful.

Experience at Rochester, New York (John Dunbar).— About fourteen years ago, rhododendrons were planted in the Rochester City Parks in beds excavated to a depth of 2 to 2 1/2 feet (sandy soil containing lime removed), and filled with humus or soil of a peaty nature from an adjacent swamp. Cow-manure was mixed liberally in the surface. Rhododendrons planted in this preparation began to root immediately, grew with vigor, and flowered splendidly. About 20,000 square feet have been planted to rhododendrons in this way with unfailing success. A 3/4-inch-pipe water-system is connected with all of the beds, with faucets at convenient points, and the plants are thoroughly watered in the growing season, and are never permitted to enter the winter with dry roots. The natural drainage is perfect. Rhododendrons will not tolerate stagnant moisture at their roots. They are thoroughly protected by the lay of the land from the west, northwest, and north winds, but are completely exposed to the south, summer and winter, and with the exception of a heavy mulching of leaves, spread over the beds in the autumn, they receive no other protection. The lace-fly became a very serious pest some years ago. The colonies of nymphs feed on the under side of the leaves and the foliage presents a sickly yellow appearance, as if attacked by red-spider. This is promptly destroyed and kept under control by two sprayings of Ivory soap during the growing season, using it in the proportion of five bars of soap to one hundred gallons of water.

The following are hardy at Rochester in a normal winter: Catawbiense, Catawbiense album, Anna Parsons, Album elegans, Album grandiflorum, Alexander Dancer, Amarantinora, Atrosan- guinea. Bertha Parsons, Boule de Neige, Caractacus, Charles Dickens, Caerulescens, Daisy Rand, Delicatissimum, Dr. Torrey, Edward S. Rand, Everestianum, F. D. Godman, F. L. Ames, Flushing, General Grant, Glennyanum, Guido, Henrietta Sargent, Henry Probasco, H. W. Sargent, Ignatius Sargent, ,/, R, Trumpy, Kissena, Kettledrum, Lady Armstrong, Macranthum, Maximum, Maximum superbum. Maximum Wellsianum, Mrs. C. S. Sargent, Mrs. Harry Ingersoll, Mrs. Henry 6'. Hunnewell, Mrs. Milner, Old Port, President Lincoln, President Roosevelt, Roseum pictum, Roseum elegans, Samuel B. Parsons, Scipio, Senator Charles Sumner, Sherwoodianum.

The following are liable to severe injury in a very cold winter: Blandyanum, Beauty of Surrey, Charles S. Sargent, Charles Bagelay, Giganteum, Hannibal, H. H. Hunnewell, James Batcman, James Macintosh, J. Marshall Brooks, John Waterer, Lady Clermonl, Lady Gray Egerton, Madame Canalho, Meteor, Marchioness of Lans- doume, Mrs. John Clutton, Ralph Sounders, Rosabel, Stella, The Queen.

The following species and hybrids are hardy: R. arbutifolium, campanulatum, caucasicum pictum, carolinianum, dahuricum, ferru- gincum, hirsutum, Metternichii, mucronulatum, myrtifolium, Smir- novii.

Hardy deciduous azaleas.

These include the species of the sections Pentathera and Rhodora and some of the section Tsutsutsi, and the hybrids known as Ghent azaleas. Most of them are hardy, but in the North and in exposed situations a protection with brush, hay, or mats should be given in winter, to protect the flower buds from sudden changes of temperature.

In the open, the flowering period of hardy azaleas extends from April to July. Thirst comes R. canadense. R. rhombicum, and R. Vaseyi; then R. nudiflorum and R. japonicum, followed by R. luteum and R. calendu- laceum and nearly at the same time, R. Schlippenbachii and R. Albrechtii; somewhat later, R. occidentale, and last, R. arborescens and R. viscosum. One of the most beautiful is the American R. calendulaceum, which is hardly surpassed in the brilliancy and abundance of its flowers by any of the Ghent hybrids. There may also be mentioned the few species of true rhododendrons with deciduous foliage, as R. mucronidatum and R. dahuricum, which are the very earliest to bloom, and the hardy deciduous Indian azaleas, as R. poukhanense and R. Raempferi, which flower with R. Vaseyi and R. nudiflorum. Azaleas are cosy to transplant, either in early spring or in early autumn, when the year's growth has ripened. If desired, they may be planted for decorative purposes in early spring, in beds, without injuring the abundance or brilliancy of the flower, and afterward removed to give space for other decorative plants, and planted carefully in nursery beds, where they remain till next spring; and so on every year. Especially the hybrids and varieties of R. japonicum (Azalea mollis) are often and easily forced for winter-flowering. If intended for early forcing, they should be grown in pots, and care taken to allow them to finish their growth as early as possible; for later forcing, after Christmas, they may be potted in fall, or even just before bringing them into the forcing-house. With a temperature of 50° to 55° at night, they will bloom in about six weeks. The Ghent azaleas are grown in great quantities in the Low Countries and in Germany for export to America; it is usually more profitable to buy this stock each fall than to attempt to raise it in this country, where labor is high-priced and the climate dry and hot.

Propagation is usually by seeds sown in early spring in frames or pans, in sandy peat, without covering, and kept moist and shady. When the seedlings appear they should have air and a daily syringing. In autumn they are transplanted into boxes or frames, in sandy, peaty soil. The seeds germinate very readily sown in cut sphagnum, but ought to be pricked into boxes as soon as they can be handled. The second year the seedlings should be planted out in beds, sufficiently wide apart to allow a growth of two years. Long upright branches should be shortened, to secure well-branched plants. The named varieties are grafted on any of the common species, usually by veneer-grafting in autumn in the greenhouse, on potted stock. They may also be increased by cuttings of mature wood 2 to 3 inches long, taken with a heel late in summer, and placed in sand under glass. Layers usually require two years to root sufficiently; they are made in spring, and the buried part inclosed in moss.

Many hybrids, known as Ghent azaleas or Mollis hybrids (R. Morteri, Sweet, Azalea Mortieriana, Spae, A. gandavensis), are in cultivation. They have originated chiefly from crosses of R. sinense, and later R. japonicum, with R. luteum, R. calendulaceum and R. nudiflorum, also in some cases with R. occidentale and R. viscosum. Some good varieties are the following:

Single-flowered varieties: Albicans, Admiral de Ruyter, Altaclarense (B.R. 28:27); Anthony Koster, Comte de Gomer (R.B. 1:9. F.M. 1879:367); Daviesi (Gt. 42:1307); Directeur Charles Baumann, Fragrans (J.H. III: 49:489); Geant des Batailles, Hilda, Louis Hellebuyck (F.S. 19:2019); Marie Verschaffelt, Morteri, Princesse d'Orange, Sanguinea, Tsarine (R.B. 20:277); Van Dyck, Viscosa floribunda.

Double-flowered varieties: Arethusa, Bijou de Gand-brugge (F.S. 19:2024); Louis Aime Van Houtte (F.S. 19:2022); Madame Mina Van Houtte (F.S. 19:2021); Murillo (R.B. 19:232); Phebe (R.B. 19:232); Raphael de Smet, Virgile (R.B. 19:232. G.W. 15, p. 493).

Indian azaleas.

This group contains R. indicum and other species of the section Tsutsutsi and the hybrids of them. They are well-known evergreen shrubs, in the North requiring cultivation in the greenhouse during the winter, but some, as R. Kaempferi and R. poukhanense, have proved perfectly hardy in the neighborhood of Boston; also R. ledifolium and R. linearifolium will stand many degrees of frost in somewhat sheltered positions. Indian azaleas are rarely increased by seeds, which may be sown in the greenhouse in the same way as with the former group. Usually they are propagated by cuttings or grafting. The cuttings root best when made in August from half-ripened wood, and placed in sand under a frame, with gentle bottom heat. Choicer varieties are usually increased by veneer- or tongue-grafting, either in winter or in July and August on vigorous-growing varieties raised mostly from cuttings. Grafting on rhododendron is now used in some German nurseries with very good results. The best soil for azaleas, if grown in pots, is a sandy compost of half peat and half leaf-soil, with an addition of good fibrous loam. It is essential to plant them firmly, and to give very good drainage. The base of the stem should be just above the surface. The best time for repotting is after flowering, when the new growth commences. During the summer, they should be kept in a coldframe or in the open in a sheltered spot, with the pots plunged in the soil, or planted out in prepared beds, where they make a very vigorous and healthy growth. In September they should be repotted and transferred to the greenhouse. They must have plenty of water and free syringing during the hot months. The natural flowering time is from April to June, but in the greenhouse, azaleas may be had in flower from November till June. Against the red-spider and thrips, from which the azaleas are liable to suffer if the air is too dry, free syringing with water is the best remedy. Most of the plants used for forcing in this country are imported from Holland, Belgium, and Germany. Formerly azaleas were kept in summer in shade or partial shade, but now it is the custom of the best growers to give them full exposure to the sun, either planted out or in the pots plunged to the rim in ashes or other good drainage material; in the latter case a top-dressing of 2 or 3 inches of old cow-manure is very beneficial. The only American treatise is Halliday's "Treatise on the Propagation and Cultivation of Azalea Indica," Baltimore, 1880.

Some of the best varieties of Indian azaleas are the following (for a completer account, see August Van Geert, "Iconographie des Azalees," abbreviated here as Ic. Az. 20):

Single-flowered: Antigone, white, striped and spotted violet (R.B. 7:241. Ic. Az. 3); Apollo, vermilion (Ic. Az. 20); Charmer, rich amaranth, very large (F.M. 5:303, 304, 1); Comtesse de Beaufort, rich rose, blotched deep crimson; Criterion, rich salmon-pink, bordered white and blotched crimson (F.S. 8:796. F. 1849:137); Diamond, white, blotched dark crimson (F.S. 21:2233, 2234); Duc de Nassau, rich rosy purple, very free and large; Easter Greetings, small, flower often semi-double, crimson, very free-flowering; Eclatante, deep crimson, shaded rose; Emil Liebig, pink; Fanny Ivery, deep salmon-scarlet, blotched magenta (F.M. 10:542); Fielder's White, pure white, early (A.F. 13:1169); Flambeau, rich, glowing crimson (Gn. 16:242, 4); Fuerstin Bariatinsky, white, striped red (Gn. 16: 242. Ic. Az. 13); Haerens Lorraine, small bright pink flowers, very floriferous; Hexe (Firefly), deep crimson, "hose in hose," small fl. very free-flowering (R.B. 31:49); Jean Vervaene, salmon, striped, bordered white (R.B. 2:145. Ic. Az. 11); John Gould Veitch, lilac-rose, bordered and netted white, striped crimson (F.S. 20:2071, 2072); La Victoire, reddish, white toward the edges, spotted maroon-crimson; Louise von Baden, pure white, sometimes speckled pink (F.S. 17:1796. F.M. 3:158); Madame Charles Van Eeckhaute, pure white, with crisped edges; Madame L. Van Houtte, scarlet-rose, bordered white (F.S. 23:2383. Ic. Az. 5); Marquis of Lorne, brilliant scarlet, very fine; Miss E. Jarret, pure white, with crisped edges (R.B. 14:213); Mrs. Turner, bright pink, bordered white, spotted crimson (F.S. 8:451. Gn. 56, p. 306); Mons. Thibaut, orange-red; Perle de la Belgique, large, pure white; President Victor Van den Hecke, white, striped and speckled crimson, with yellow center (F.S. 15:1567, 1568); Princess Alice, pure white, one of the best; Princesse Clementine, white, spotted greenish yellow; Professor Wolters, pink, with amaranth blotch; Reine des Pays-Bas, rich violet-pink, bordered white (I.H. 13:479); Roi de Hollande, dark blood-red, spotted black; Sigismund Rucker, rich rose, bordered white, blotched crimson, very showy (F.S. 19:2010, 2011. Ic. Az. 31); Stella, orange-scarlet, tinged violet; Wilson Saunders, pure white, striped and blotched vivid red.

Double-flowered: Alice, deep rose, blotched vermilion (I.H. 23:244); Baron N. de Rothschild, rich purple-violet, large (F.S. 23:2477, 2478); Bernard Andre, dark violet-purple, large; Bernard Andre alba, white (I.H. 17:15. Ic. Az. 19); Borsig, pure white; Charles Leirens, dark salmon, blotched dark purple, good form and substance (F.S. 19:1971, 1972); Charles Pynaert, salmon, bordered white (R.B. 10:25); Chicago, deep carmine, bordered white, large; Comtesse Eugenie de Kerchove, white, flaked red-carmine; Deutsche Perle, pure white, early) R.B. 21:85. R.H. 1886:516. Gn. 33:460. Ic. Az. 25); Dominique Vervaene, bright orange;

Dr. Moore, deep rose, shaded white and violet, very fine (R. B. 11:61); Eggebrechtii, bright crimson; Empereur du Bresil (Emperor of Brazil), rich rose, banded white, upper petals marked red (Ic. Az. 15); Ernest Eeckhaute, deep carmine, very double; Francois de Vos, deep crimson (I.H. 14:512. Ic. Az. 14. F.M. 8:443); Frau Herm. Seidel, white, striped red; Helene Thelemann, rosy pink, free-flowering; Imbricata, white, sometimes flaked rose (I.H. 24:281. F.S. 22:2284, 2285. F. 1876, p. 201); Imperatrice des Indes (Empress of India), salmon-rose, bordered white and spotted dark carmine (F.M. 18:357. Ic. Az. 21. F. 1879:97); Johanna Gottschalk, white; John Liewelyn, soft pink; Louise Pynaert, white (R.B. 4:209); Madame Camille van Langenhove, white, striped with rose, very double; Madame Iris Lefebvre, dark orange-carmine, shaded bright violet and blotched brownish red (F.S. 18:1862, 1863); Madame Jos. Vervaene, large, pink and white, similar to Vervaeneana; Madame Petrick, bright rose, very early; Madame Van der Cruyssen, pink, fine form (A.F. 12:1003); Madeleine, white, large, semi-double; Niobe, white, fine form; Pharailde Mathilde, white, spotted cherry-red (R.B. 13:145); President Ghellinck de Walle, bright rose, upper petals spotted yellow and striped crimson; President Oswald de Kerchove, pink, bordered white, blotched carmine; Raphael, white; Sakuntala, white, very free-flowering; Simon Mardner, large, rose, very double and very early; Souv. du Prince Albert, rich rose-peach, broadly margined white, very free-flowering (F.M. 4:201. Ic. Az. 24); Theodore Reimers, lilac, large; Vervaeneana, rose, bordered white, sometimes striped salmon (Gn. 52, p. 137. C.L.A. 5:146; 7:179. J.H. 31:423); Vervaeneana alba, white (R.H. 1908:424); Vuylstekeana, deep crimson, "hose-in-hose." The varieties now chiefly imported and considered the best for forcing are the following (those marked with an asterisk are adapted for early forcing):

Single-flowered: Emil Liebig, Haerens' Lorraine, Hexe, Professor Wolters.

Double-flowered: Eggebrechtii, Empereur du Bresil, Ernest Eeckhaute, Frau Herm. Seidel, Helene Thele-mann, Imperatrice des Indes, John Liewelyn, *Madame Camille Van Langenhove, Madame Jos. Vervaene, *Madame Petrick, Madame Van der Cruyssen, Niobe, Pharailde Mathilde, *Pres. Oswald de Kerchhove, *Simon Mardner, *Vervaeneana.

Plants grown in Germany are preferred for early forcing, because they are grown in pots over summer before shipping, while the Belgian plants are grown in the open field. Generally the Belgian plants are grown more compact, while the German-grown plants are of looser, more naturally graceful habit. Some varieties, though much approved abroad, are rarely imported. because they do not travel well.


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Cultivation

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5-5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. 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.[2] Mulching and careful watering are important, especially before the plant is established.

Propagation

Pests 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; Ohio State University Extension provides information on maintaining health of rhododendronsRhododendrons can easily be suffocated by other plants.

Species

Rhododendron wardii var. puralbum

The species are organized by subgenus, section, subsection and series. These are currently divided into four large and four small subgenera:

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. Further realignment within the subgenera is currently proposed [3][4], including the merging of subgenus Hymenanthes into subgenus Pentanthera.

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.

Some species (e.g. Rhododendron ponticum in Ireland and 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.

A garden with tall Rhododendrons in Lynnwood, Washington
Sample species

A sample hybrid:



Read about Rhododendron in the Standard Cyclopedia of Horticulture 

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Gallery

  • Pink Rhododendron.jpg

  • Rhododendron decorum subsp. diaprepes

  • Rhododendron2.jpg

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

  • Rhododendron 1 Fcb981.JPG

  • Flowering Rhododendrons in Whitwick Leicestershire

  • Rhododendron luteum

  • Rhododendrons in Sōrakuen garden, Kobe

References

External links


  1. ↑ Tree rhododendrons
  2. ↑ Soil information for planting rhododendrons
  3. ↑ Goetsch, L. A., Eckert, A. J. & Hall, B. D. (2005). The molecular systematics of Rhododendron (Ericaceae): A Phylogeny based upon RPB2 gene sequences. Sys. Bot. 30(3): 616-626.
  4. ↑ Summary of Goetsch-Eckert-Hall results
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