Azalea Growing Care and Pruning Guide

Azalea Growing Care and Pruning Guide

Free-blooming Shrubs for Lime-free Soils and Pot Culture

 (Aza’lea). Botanists now classify all plants they once called Azaleas as Rhododendrons. Garden lovers still use Azalea for deciduous or leaf-losing kinds and for a few that are not, and the name Rhododendron for evergreen kinds which have large, leathery leaves. In the treatment that follows, Azalea is used as a common name and Rhododendron as the scientific name, thus, when a species is named it is written, for example, R. calendulaceum instead of A. calendulacea.

Azaleas are hardy and tender spring and early summer-flowering shrubs, natives chiefly of North America and eastern Asia. They belong to the Heather family, Ericaceae. The name is derived from azaleos, meaning dry or arid, an allusion to their habitat as first described by the earliest.

General Care

Azaleas need ample supplies of moisture but will not survive if the soil is constantly waterlogged or is subject to flooding. In dry weather the soil should be thoroughly soaked at about weekly intervals.

Regular (which normally means once a year, in spring before new growth begins) fertilizing promotes good growth. Good fertilizers for Azaleas include cottonseed meal, soybean meal, tankage, old well-rotted manure and commercial fertilizers prepared especially for Azaleas, Rhododendrons, Camellias and other acid soil plants.

The maintenance of a three-inch deep mulch of organic material over the surface of the soil beneath Azaleas is very important. These are surface-rooting shrubs and such a covering protects the roots from excessive heat in summer and excessive cold in winter as well as from harmful drying. Suitable mulch materials are leaf mold, coarse peat moss, coarse compost, bagasse (sugar cane refuse), peanut hulls, ground corncobs, decayed sawdust and wood chips. If any of these are applied in a fresh rather than in a well-decayed condition it is important to add at the same time a fertilizer that provides readily available nitrogen.

Under no circumstances should one disturb the soil around Azaleas by digging, cultivating or hoeing. Weeds that come through the mulch should be hand pulled. Ordinarily, these will be few in number and will not present much trouble.

The old flowers of Azaleas should be removed as soon as they fade, except a few that may be left to yield seeds, if these are wanted; if many seed pods are allowed to form, flower buds may not develop for next year’s blooms.

Azaleas are subject to a number of pests and diseases. They are also subject to a yellowing of the foliage caused by iron chlorosis


Azaleas require little pruning. Most are best if this is confined to the removal of dead, dying or broken branches and to the cutting out of branches that obstruct paths or driveways. The occasional removal of some of the older and less thrifty branches of Ghent, Mollis, Knap Hill and similar leaf-losing (deciduous) Azaleas encourages new vigorous growth from the base (especially if the plants are fertilized as well as pruned). Azaleas of the Kurume type may be (and frequently are) sheared annually to more or less formal shapes immediately after flowering, but under this treatment, they do not develop their natural and more beautiful habits of growth.

When to Take Cuttings

The half-ripe or semi-woody shoots of the current year’s growth provide the best cuttings. The cuttings, 2-3 in. long, are taken off with a very thin heel or piece of the old branch during July and are planted in a mixture of sand and peat moss in a frame or greenhouse. If a little bottom heat is available it is an advantage. The frame is kept close for three or four weeks to encourage the cuttings to form roots; the cuttings are watered after insertion and are afterward kept moist by syringing daily. Not all Azaleas root satisfactorily from cuttings; the Kurume types are particularly easy, many Native American kinds notoriously difficult.

Layering and Grafting

Propagation is also carried out by layering the lower branches in summer. This is a method adopted for increasing the named varieties of Mollis and Ghent Azaleas as these do not come true from seeds. The named varieties are also increased by grafting, the common yellow Azalea pontica being used as a stock. This method is declining in favor because strong suckers not infrequently grow from the stock and, unless pulled off in time, eventually overpower the named variety and kill it.

Native American Azaleas

The natural flora of North America includes a number of Azaleas, many of great beauty and splendid for garden purposes. Most of these are natives of the East but one, R. occidentale, is found naturally from southern Oregon to southern California, and another, R. albiflorum, in the Rocky Mountain region. The latter is generally not satisfactory in cultivation away from its native habitat, but R.occidentale is one of the finest of native Azaleas. Unfortunately, it is not hardy where winters are severe; it is planted in West Coast gardens.

R. occidentale is a variable plant, its flowers, which are larger than those of other American natives, are usually white with a yellow blotch but sometimes are pink. They are borne in spring just as the new foliage is developing. In fall the leaves color handsomely. This Azalea attains a height of about 8 ft.

Of the Azaleas native to eastern North America the following are the most important from the gardener’s point of view:

R. arborescens grows wild from Pennsylvania to Georgia and Alabama, where it occurs in upland and mountain woods. It is hardy in the North and is a very excellent shrub. It attains a height of 10-12 ft. and sometimes up to 20 ft. and, in its typical form, bears white, fragrant flowers in late spring. This kind is quite variable and some of its variants are likely to be confused with R. viscosum (discussed later), although it usually blooms two or three weeks earlier than the latter and has flowers with somewhat wider throats. It is normally a more showy plant in bloom than R. viscosum and under favorable conditions attains a greater height. R.arborescens prefers moist soil.

The Flame Azalea, R. calendulaceum, is a native from Pennsylvania to Georgia and Kentucky in the mountain regions. It is a magnificent kind, very variable in color of flower, shape of flower and in other details. This kind is one of the parent species of the Ghent Hybrid Azaleas. It attains heights of 10-12 ft. or more. Color of bloom varies from clear light yellow to almost blood-red, with every intermediate shade represented, and with the flowers of some plants showing mixed color patterns. The flowers may be in comparatively dense heads or in more loosely arranged clusters. They are scentless, or nearly so, and are borne in late spring, somewhat earlier than those of R. arborescens and R. viscosum. This hardy kind is perhaps the finest of eastern North American Azaleas. In cultivation in most parts of North America where it may be grown, it thrives better and is very much longer-lived than the Ghent Azaleas. It prefers light shade and will grow in drier soil than many Azaleas. In fall its foliage colors in shades of bright yellow and bronze.

R. cumberlandense, a native of the mountains of Virginia and North Carolina, closely resembles the forms of R. calendulacea that have deep red flowers. It blooms about two weeks later than R. calendulaceum.

The Pinkshell Azalea. R. Vaseyi, sometimes called the Pinkshell Azalea, is a native of the Blue Ridge Mountains of North Carolina. It attains a maximum height of 12-15 ft. and is of notably erect habit of growth. Its scentless flowers, which normally are light pink spotted with light brown, are deep pink in bud, and appear in early spring, just before the leaves expand. They are light and graceful appearance, with their stamens very prominently exerted (protruded from the flower). This is the hardiest of native Azaleas and is recorded as withstanding temperatures at least as low as 30 degrees below zero F. It prefers soil that is moderately moist, and is excellent for planting beside a lake or pond. In fall its foliage is beautifully colored.

The White Swamp Honeysuckle. R. viscosum, the White Swamp Honeysuckle, or Swamp Azalea, grows as a native from Maine to South Carolina and Ohio and Tennessee in swamps. It attains heights of 8-10 ft. and bears delightfully fragrant white, or much more rarely pink, flowers which are very decidedly sticky to the touch. Each is about an inch across and they are carried in clusters of 4-9 after the foliage has developed. This is the latest blooming of American Azaleas, and for this reason is of especial value; its flowers are usually bloom in July.

R. viscosum is one of the parents of the Ghent Hybrid Azaleas. It is a variable kind and in its extreme forms it is not always easy to distinguish it from forms of R.arborescens, which see, above. Although a native of swamps and thriving in wet soils, it adapts itself remarkably well to somewhat drier locations and prospers in full sun.

R. roseum, by some botanists considered to be merely a form of R. nudiflorum, which it closely resembles, is, for garden purposes, superior to the latter. It occurs naturally from New England and central New York to Ohio and Virginia. It is a hardy kind and is said to be more tolerant of limestone than any other American Azalea. R. roseum has flowers that are normally clear rose-pink in color, but plants with paler colored flowers occur. The blooms are delightfully fragrant. This shrub ordinarily attains a height of 6-9 ft. The flowers open after the foliage is well developed, and the leaves assume unusual tints of dark purple in fall.

Other native Azaleas of eastern North America include R. alabamense, a low, white-flowering kind that is a native of Alabama and flowers there in mid-May. It is an inhabitant of dry woods and is not hardy in the North. R. atlanticum grows from southern Pennsylvania to Virginia and the Carolinas in the Coastal Plain region. Its fragrant flowers are white or pale pink and the plant is about 18 in. tall and free-flowering. This kind is hardy in the North.

R. austrinum, not hardy in the North, grows along rivers in southern Georgia, Alabama and northern Florida and bears yellow to orange and orange-red, scented flowers in early spring. R. canescens, not suitable for planting in the North, inhabits Coastal Plain regions from South Carolina to Florida. It somewhat resembles R.roseum but is less beautiful. Its blooms tend to be of washy pink coloring.

R. nudiflorum, the Pinxterbloom or Pinxter Flower, grows as a native from Maine to Florida and Texas. Its flowers, borne in spring, just before the developing foliage, are pale pink or rather impure white and are inferior to those of R, roseum, which they resemble. It is fragrant but less so than R. roseum. This species is so like R. roseum that by some authorities it is regarded as a variety of the latter. Intermediate forms between R. nudiflorum and R. roseum occur.

R. prunifolium, a species native in southwestern Georgia and eastern Alabama, is not hardy in the North. It has sizable red flowers and grows about 8 ft. tall.

The Chief Kinds for Planting Out of Doors.

Assuming that all soil requirements are met, the factor that determines the varieties or kinds to be planted is climate. Certain Azaleas will not tolerate extremes of cold and others will not thrive in protracted summer heat, particularly in areas where there is no dependably continuous period of winter chilling, for all Azaleas require some winter rest.

All the species native to North America are worth growing in gardens. All are deciduous; many have fragrant flowers, some markedly so. There is a wide range of seasonal bloom, with the earliest coming into flower as soon as growth starts, and the last blooming at midsummer. Species from elevations will grow farther north than those from the southern coastal plains or northern Florida. The one species native to the Pacific Coast, like most of its progeny, is not vigorous in the East, but is a splendid plant for the West Coast.

The European species, Rhododendron luteum (the name Rhododendron is used because it is the correct botanical designation for all Azaleas), is excellent on the Pacific Coast but not always dependable in the East. It has brilliant yellow flowers and is delightfully fragrant.

The Asiatic deciduous species, R. japonicum from Japan, and the somewhat similar but less cold-resistant R. molle from China, are useful for yellow and orange to orange-red colors, have fine large flowers and have interbred well. They are practically useless in the South. The Korean, R. Schlippenbachii, is one of the most magnificent of all Azaleas, reasonably cold resistant, but untested as yet in the South at low elevations. The color is the purest of pinks. Its foliage has good autumn color. Two other species, R. reticulatum from Japan and R. Mariesii from China, flower, the first before and the second after R. Schlippenbachii, but are not so spectacular. The first, however, is fine when grown with early-blooming Cherries and Redbud, and is of the same general lavender-pink tone. The more southern R. Weyrichii is more of a salmon-red color but, while hardy in cold climates, has not bloomed freely. The related R. amagyanum has bloomed only on the Pacific Coast.

R. dauricum, from Siberia and Japan, is semi-evergreen where winters are not very severe. It grows 5-6 ft. high and bears rose-purple flowers in winter in mild climates, but blossoms in very earliest spring where winters are harsh. The nearly related R. mucronulatum, from China, is showier than R. dauricum. It forms a dense bush, 3-4 ft. or more high, and bears pale, rosy-purple flowers freely in winter or early spring, depending upon the climate.

Relatively few evergreen species are grown in their wild forms. With the exception of R. indicum, usually known as Azalea macrantha, and its several color forms and variants with modified corollas, and more rarely the Formosan R. oidhamii, few are found in gardens. R. scabrum is probably the most spectacular, especially in its pure red form, but it is sensitive to cold and does not survive even in parts of the South. R. serpyllifolium, particularly in its white-flowered form, is charming when it has made its growth and is covered with lacelike small flowers. The semi evergreen R. poukhanense, from Korea, is very cold-resistant and blooms freely especially when grown in sun, but because of its lavender-pink flowers, it needs careful placement. It is good for autumn color. R. Simsii and R. obtusum Kaempferi aresemi evergreen and in small gardens are out-classed by their progenies. R. mucronatum and its color variants are evergreen in the South, becoming increasingly more deciduous the further north it is used. It is largely outshone by its offspring in modern hybrids.

Rock Garden Azaleas

There are relatively few Azaleas that make creeping mats of twiggy branches such as one expects of rock garden materials. If one wants an accent plant in the rock garden, however, any of the Kurume Azaleas will serve for this purpose. The one striking Azalea that does meet rock gardening requirements is R. indicum balsaminaeflorum, which is slow-growing, prostrate, and covered in June with double salmon-pink balsam-like flowers. In regions where they are hardy, the Japanese clones (varieties propagated vegetatively, not sexually) known as Gumpo, Bunkwa, Gunrei and Gunbi are almost its equal and have even more spectacular though single flowers, in white, tinted whites and a pale salmon. These last are sometimes listed among the Chugai hybrids, but not all varieties so listed have this habit of growth. Do not use Kurume Azaleas and expect them to retain the low-growing forms that were described from the original high-mountain locations in Japan, unless the garden is also on a mountain.

Hybrid Azalea Races or Groups.

These are many, and most of them contain plants with even more spectacular flowers than the species. Their origins are various and their uses follow the recommendations as given. Several have become standard forcing varieties in the North and excellent garden plants in the South and on the Pacific Coast.

The Rutherfordiana Azaleas. These again contain tender species in their ancestry and in the South and on the Pacific Coast properly replace the Kurume Azaleas and many of the Glenn Dale Azaleas. There is not so wide a color range in the Rutherfordiana clones as in the Glenn Dales, but there are single, hose-in-hose and double-flowered clones, all excellent.

The Glenn Dale Azaleas were bred for the Middle Atlantic States only, but some are proving useful further north near the coast, and many are excellent in the South, where, as on the Pacific Coast, many are autumn and winter-blooming without detracting from the spring display. They are not uniform in breeding and embrace a wide range of color, form and season of bloom, with some colors and patterns not found in any other hardy race. On the Pacific Coast, where Rutherfordiana and Belgian types can be grown, the Glenn Dales are not needed. Most are evergreen.

The Gable Azaleas represent another regional breeding project, for an area further inland and more northerly. They should take the place of Glenn Dales in more northerly uses, though they do not have so wide a color range nor so great a seasonal range. They do include some superb doubles. Not all are completely evergreen.

Knap Hill Hybrids. These are modern equivalents of the Ghent and Mollis hybrids. They are hybrids of R. molle, R. calendulaceum, R. occidentale and R. arborescens. The strain was originated by Waterer’s at Knap Hill Nursery in England and was further developed by other breeders, notably by Slocock (England), Rothschild (Exbury, England) and Stead (Ilam Estate, New Zealand). The four “substrains” raised by these breeders are known respectively as the Knap Hill, Slocock, Exbury and Ilam Azaleas. Varieties belonging to these strains have wide open flowers of excellent texture and come in a broad range of colors. They are deciduous.

Indian or Indica Azaleas of the South. These are the surviving representatives of the earliest Azalea breeding in Europe, chiefly in Belgium. In their modern forms they are grown for forcing in the East and as garden plants in some parts of the Pacific Coast. There are perhaps about forty clones in cultivation, including some for which there is no name extant. They are evergreens, some perhaps species, many complex hybrids. Not all have R. indicum in their parentage. The color range is limited, a small series in the lavender to purple series, and a shorter series in the salmon to scarlet series. There are few whites, some of them striped or flaked with color. There are a few doubles. In most cases, the quality is second rate, but the effects secured from well-grown specimens can be very fine.

Ghent Azaleas. These are deciduous Azaleas bred mostly in Europe near Ghent, and represent the progenies obtained by crossing the European yellow Azalea with various American deciduous Azaleas. The color range is wide, from white through all pink to deep rose, some with yellow tints and many with yellow blotches on the upper lobes. Many are fragrant. There is a double-flowered race that is usually listed as R. rusticum flore-plenum. Many are not much better than the best forms of our own native Azaleas.

Mollis Hybrids. Originally these were hybrids between the American species, the European R. luteum, and the Chinese R. molle. They varied from the above race chiefly in larger flowers. Many were less cold-resistant. With the introduction of the more cold-resistant R. japonicum, there appeared hybrids between R. molle and R. japonicum, most of which are excellent and some of which appear to have been raised with R. japonicum on the existing R. molle hybrids. There is a wide color range, but these hybrids are chiefly valued for the yellow, orange and scarlet tonalities. The flowers are large and funnel-shaped as compared with the Ghent hybrids, which have tubular flowers with smaller open faces. There is little fragrance in this group and they are generally earlier flowering than the Ghents. Useless in the South, except at elevations.

Kurume Azaleas. These originally were natural variants from the Japanese R. obtusum, and the original location was at high elevations in southern Japan. As garden plants in Japan, they come from lower elevations and are widely grown. There is a wide range of color from pure and tinted whites through pure pinks to rose, through lavender-pinks to purple, and through salmon-pinks to scarlet. There are both single and hose in-hose types of bloom. The paler colors appear to be less cold-resistant than deeper-colored clones. There is no complete report for the South but there seems to be no difficulty generally. Some of the more modern varieties listed as Kurumes appear to include other ancestry.

Pericat Azaleas. This is a small group of Azaleas said to have been bred between Kurumes and Florist’s Azaleas (Belgian or Indian). They vary in cold-resistance, and the larger-flowered forms are the more striking.

For the Greenhouse

The evergreen Indian or Belgian Azaleas are decorative greenhouse shrubs. They bear a profusion of attractive flowers which range from white through pink to rose and crimson, and may be single or double. These Azaleas are easily forced into bloom in winter and early spring.

The most popular way of growing them is in the form of mop-headed specimens or low standards on 6-in. stems. They can, however, be trained as pyramids or as tall standards on stems about 4 ft. high.

How to Begin. The best way to begin is to purchase plants well set with flower buds in the autumn. The balls of soil and roots should be pricked over with a pointed stick to remove loose soil, the plants then being potted in flowerpots large enough to provide about an inch of fresh compost round the roots; the soil must be rammed fairly hard with a potting stick. After potting is finished, the plants are set in a green house—temperature 50-55 degrees—and syringed frequently to encourage fresh growth.

It is not advisable to force Azaleas into early bloom the first year, but in the second and subsequent years, when they are well established in pots, they will withstand forcing much better. Plants in full bloom which are purchased from a florist’s shop in the spring often fail afterward because they have been prematurely forced into flower in a warm greenhouse.

When to Repot. As soon as the flowers of Azaleas have faded, they must be removed and the plants turned out of their pots to allow the loose soil to be removed in the way already explained. They are. then repotted as advised, placed in a temperature of about 55 degrees, and syringed daily. This treatment will encourage root action and the formation of new shoots. In May they should be gradually hardened off.

Place Them Out of Doors in Summer. When the weather becomes warm and settled, the plants are set in a slightly shaded position out of doors, to encourage the formation of flower buds, the pots being plunged to the rims in a bed of ashes. A fortnightly dose of a fertilizer will also assist them to produce blooms freely. In October the Azaleas are placed in the greenhouse—minimum temperature 45 degrees—and in the new year they may be forced into early bloom in a temperature of from 50 to 60 degrees.

The reason why many Azaleas grown in pots in the greenhouse do not bloom freely is that they are not exposed fully to air and light during the summer months, and as a consequence, the buds do not mature. It is most necessary to set them out of doors.

Prune After Flowering. These Azaleas are rather slow growing so they do not need repotting every year, but when the pots are full of roots, repotting should be done as soon as the flowers have faded. Very little pruning is required; long branches which have outgrown the others and tend to spoil the symmetry of the plants should be shortened when flowering is over.

When to Take Cuttings. Cuttings are made from half-ripe (half-woody) shoots about 3 in. in length in July. They are inserted in pots of sandy peat placed under a bell jar in a greenhouse or frame—temperature 50 degrees; the inside of the glass must be wiped every morning. The bell jar is removed when, in the course of a month or so, the cuttings are rooted, and a week or two afterward the young plants are potted separately in 3-in. pots. Subsequent treatment consists of cutting back the main shoot of each plant when 6 in. high and shortening subsequent side shoots to ensure a well-balanced specimen.

The Indian Azaleas. A lovely greenhouse kind is the evergreen Indian Azalea, Rhododendron indicum. There are numerous varieties of which these are some of the best: Single-flowered, Mlle. Van Houtte, white flaked with rose; and Reine des Fleurs, salmon. Double-flowered, Deutsche Perle, white; Vervaeneanum, salmon; President Kerchove, salmon-pink, and Empress of India, pink and white.

Other Azaleas for Greenhouses. Kurume Azaleas, Mollis Azaleas, Rutherfordianum Azaleas and some other types are excellent plants for forcing into flower in the greenhouse in early spring.

For this purpose plants well set with flower buds should be obtained in autumn. They are potted in October—November in pots just large enough to accommodate the roots. A compost of equal parts of loam and leaf mold, or lime-free soil from the garden, is suitable. After potting, they are plunged to the rims in a bed of ashes out of doors.

How to Obtain Early Blooms. Early in January, the plants are taken into the greenhouse to force. For the first 3 or 4 weeks a temperature of 50 degrees is high enough, this being increased by 10 degrees when the flower buds commence to swell. By the application of greater heat, flowers can be produced earlier but very little foliage is developed and the blooms fade quickly. A moist atmosphere must be maintained by regularly damping the floor and staging of the glasshouse, and the stems of the plants must be syringed twice a day to assist the buds in breaking. Under this treatment it takes about 6 weeks to force plants into bloom. To keep the flowers in full beauty as long as possible the plants must be removed to a cooler place.

During the period of forcing and while the plants are in bloom the soil must be kept moist. After flowering, the plants are kept in a cool frostproof house until danger of damage by frost is over, when they are replanted in a reserve border out of doors for a year or two to recuperate before being forced again.


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