Rhododendron

This huge group of evergreen and leaf losing trees and shrubs are found wild in Asia, Europe, and North America. Hundreds of wild kinds have been found and numerous hybrids have been developed. These plants vary greatly in growth. Some grow into moderately sized trees and some into little shrubs or bushes that are great for the rock garden. The leaves of some may be as small as Thyme leaves and on the other extreme may be 20 or 24 inches long and 6 to 8 inches wide. Some flowers are shaped like saucers, 5 or more inches across, others resemble Lilies, and some are quite flat when expanded; some are fragrant. The flowers usually grow in clusters on the past year's shoots. The clusters may be loose bunches with few flowers, or they may be 8 or 9 inches high and compact, with many blooms. In ideal climates, many varieties will bloom from late winter well into the summer. Seedpods, which contain many seeds, follow the flowers. These will ripen the following winter. When the seeds are ripe, the cases split open and eject them. In the wild, these plants grow in many different conditions; some cover hills or form thin woods, some are found mixed in the trees, others are epiphytes on the branches of trees and some grow among the rocks or on moorlands. R. amagianum (Mt. Amagi Azalea) forms a medium- to large-sized, deciduous shrub with broad leaves in clusters of three at the ends of the branches.

The orange-red, funnel-shaped blossoms are marked with red and produced in late spring. R. augustinii (Augustine Rhododendron) is a large, evergreen shrub clothed in small, dark green leaves. In mid- to late spring, blue to rosy-lavender flowers are produced. R. ciliatum (Fringed Rhododendron) is a beautiful, evergreen shrub that reaches a height from 4 to 5 feet. Its attractive leaves have bristly edges. In early to mid-spring, gorgeous, bell-shaped flowers are produced. These fragrant blossoms may be white to rosy-lilac. R. kaempferi (Torch Azalea) is another beautiful, medium-sized shrub that may be deciduous or semi-evergreen. The funnel-shaped flowers are produced in clusters of 2 to 4, in late spring or early summer. They vary from pink to scarlet, salmon-red, and orange-red. The foliage takes on a reddish hue in the autumn. R. luteum (Pontic Azalea) is a medium-sized, deciduous shrub that can grow up to 11 feet high and wide. In late spring, sweetly scented, funnel-shaped, yellow flowers are borne. The foliage turns gorgeous colors of orange, crimson, and purple in the fall. R. schlippenbachii (Royal Azalea) is a superb, medium- to large-sized, deciduous shrub covered in whorls of large, bright green leaves. The foliage is mottled with a purplish-red color when young, turning crimson, orange, and yellow in the fall. The fragrant, saucer-shaped blossoms grow with or before the leaves in mid- to late spring. The 3-inch flowers are light to rose-pink or white. More species and hybrids will be mentioned below in the varieties section.

Pot Cultivation

Rhododendrons and Azaleas vary in their tolerance to light and cold; however, they both need light, well-drained, acidic soil (pH 4.5 -6) and a cool, moist atmosphere. It may be difficult to grow these plants in certain parts of North America, especially the Midwest. They will flourish in the Pacific Northwest and the East and some Azaleas do well in the South. Care must be taken selecting locations for your plants. They need shelter from sweeping winds, especially during the winter. Areas near the sea or atop hills are usually not good unless they have protection from a windbreak of some sort. The best spots in which to plant them are valleys and dells or partly down hillsides. If planted in an exposed location, they will need shade and shelter against winter sun and wind.

This can be done by inserting pine boughs in the ground around the plants or by a burlap bag that is supported with stakes or chicken wire. Barrels or boxes aren't a good idea, because the plants need good air circulation. They don't need to be kept warm, just shaded. Most kinds of Rhododendrons need a little shade from the strongest rays of the sun, especially the kinds with large leaves, or else their foliage will burn. The varieties with brilliantly colored flowers, such as scarlet, should have the brightest spots and those with flowers that are mauve or white should have the shadiest spots. Too much shade, however, will cause leggy growth and poor flowering. Too much sun is especially harmful in the winter. Their soil should be moist, but well-drained, and high in organic matter or humus such as pine needles, decayed oak leaves, leaf mold, peat moss (a high grade peat, not one that is so fine it prevents good soil aeration), rotted compost or very old and well-rotted manure.

Their soil should not dry out, but it cannot be saturated constantly. They will not thrive in soil containing lime. R. hirsutum is an exception, though, because it's found growing on limestone formations. Spade two feet deep into the spot that you chose for your plant and remove a bit of the subsoil. In the top 12 or 15 inches, mix a 3- to 6-inch layer of peat moss, leaf mold or good compost. If the soil is very sandy, double the amounts of these materials. In heavy soil, add gypsum (calcium sulfate) to provide adequate drainage. If the soil is slightly alkaline, the spot should be dug up to a depth of 18 inches and filled with new soil made by mixing good topsoil with acid peat moss and leaf mold or commercial humus in equal proportions. This treatment is advisable when planting against buildings where the builders may have mixed alkaline materials into the soil. If the soil is very alkaline, new beds can be built over the soil surface. With adequate watering during dry spells, these plants may adapt to living in such beds, but it is usually best not to attempt to raise Rhododendrons in soil that has high alkalinity. Early fall and early spring are the best times for planting. Dig up these plants when the soil is moist. A good ball of soil should be attached to the roots when they are dug up. Wrap and secure it in burlap. Never let the roots dry.

Holes that are much wider than the root balls should be prepared. Never plant Rhododendrons and Azaleas deeper that what they were growing. Their root ball should be even with or an inch or two above the surface of the ground. Compost should be packed around the root balls and they should be watered thoroughly with a fine spray. Peat moss, leaf mold, pine needles, oak leaf mold, shredded pine bark, or compost should be spread around the plants; these are acid forming materials that will keep the soil moist. Their roots grow very shallowly so stirring the surface of the ground up isn't necessary and is actually very harmful. Care should also be taken when removing weeds. If they are provided with mulch yearly, weeding will seldom be required. Water them generously during dry weather, this is especially important in the late summer and fall because if they're too dry during this time they will most likely suffer during the winter. These plants will benefit from an annual application of an acid-forming fertilizer. Young plants that tend to become leggy should be cut back to encourage bushy growth. Once a good foundation is laid, regular pruning isn't necessary. If necessary, straggly branches may be cut back in the spring. Azaleas can be pruned anywhere along the stem; Rhododendrons should be pruned just above a leaf cluster or growth bud along the stem. Plants that have become too tall and leggy may be pruned back hard in the spring just before new growth begins. Cut the entire bush down within a foot of the ground. Add fertilizer and mulch to the soil and in dry weather soak the soil weekly. When they are cut back this hard it usually takes 2 or 3 years before they flower again.

On grafted plants, make sure to check for suckers that grow below the graft union. If they aren't removed, they'll grow quickly and the grafted variety will die. Immediately remove faded flower heads unless, of course, seeds are needed. Javanese and Malayan Rhododendrons can only grow outdoors in mild climates. A greenhouse that maintains a minimum winter temperature of 45�-50� F is necessary for these plants. They can be grown in pots or planted in a border of soil in the greenhouse. The border should be raised a little above the paths and be well drained. A compost of light, sandy loam and peat, or sandy peat alone, placed among large pieces of sandstone is good to use. These Rhododendrons have a straggling habit; they don't branch well after pruning. When they're grown in pots, the shoots are sometimes trained around stakes.

Propagation

Seeds: Seeds should be planted as soon as possible after they are ripe in pots, flats or pans that have good drainage and placed in a cool greenhouse or cold frame. It's necessary to have a light sandy soil. A mixture of loam and sand with either leaf mold or peat is good. Sifted sphagnum moss is also a fine medium to use. Fill the containers with the mixture within a half of an inch from the top, press it down lightly and evenly. Water thoroughly with a fine spray or stand the containers in water almost to the rims and allow it to soak in. Let the pots or flats drain, then scatter the seeds thinly over the soil and press them lightly into the compost. Scatter a little sand over them, but don't try to cover them completely. Cover them with sheets of glass and shade them from bright sunlight. Water them very carefully so the seeds won't be disturbed. The soil should never become dry or constantly saturated. They'll usually sprout in two or three weeks. Gradually expose them to air and light, but never strong, direct light. When they are large enough to handle, transplant them into flats 2 inches apart and use one of the composts already described. When they start to get crowded, move them to a cold frame and space them four inches apart. The sandy soil needs a lot of leaf mold or peat moss and a bit of very decayed manure. Let them grow for about a year before moving them to an outdoor nursery bed. Never let them become dry at the roots and always shade them from strong, direct sunrays. During good weather, they should be sprayed morning and evening. If the plants become leggy, remove the tips of the shoots to induce branching. Plants of the more vigorous kinds that are grown from seed take a long time to flower, maybe over ten years, but seedlings grown from the small alpine kinds may flower in two years or less.

Cuttings: Cuttings from the alpine kinds with small leaves can be rooted in the summer in sand and peat moss in a closed cold frame or under a bell jar. Those that have large leaves, including the hybrids of R. catawbiense, can be rooted in sand and peat moss in a greenhouse propagating case that has a little bottom heat. They should be inserted towards the end of summer. Cuttings of single leaves (with a small piece of stem attached at the base) can also be used to produce more plants. They should be inserted at the same time and taken care of the same way as regular cuttings. Cuttings of these large-leaved kinds are not easy to root so treat them with a root-inducing hormone. Rhododendrons that are natives of Malaya and Java can be propagated by cuttings inserted in a warm, closed propagating case.

Layering: This can be done in the spring. Peg the branches firmly into sandy soil, leaf mold or peat or sandy loam mixed with peat or leaf mold. It isn't necessary to notch the shoots, although this is sometimes done. A heap of peat moss and sand should be placed over the layers where they are touching the ground to help keep them moist. Roots form slowly so leave them alone for two years, after which they probably can be moved safely.

Grafting and Budding: This can be accomplished on healthy seedling understocks that are established in pots in March or April. Plants raised from layers shouldn't be used as understocks because they tend to produce suckers. Seedlings that produce suckers shouldn't be used either. The plants that are going to be used as understocks should be taken into a warm greenhouse a few weeks beforehand to advance the flow of sap more than that of the scions. The method called saddle grafting is the best technique to use. Plants raised this way are much more stable than those raised from side grafts. As soon as they are grafted set them in a closed, warm frame. Budding may be done in the summer in a greenhouse. Do this in June or July when the bark separates easily from the stock. As soon as the union between the stock and scion is complete, remove them from the propagating case and gradually harden them off in a cold frame until they can be planted in a nursery bed.

Varieties

There are so many varieties of Rhododendron that they are not all listed. The following are a selection of natural species (kinds that grow wild in some parts of the world). F = flowers L = leaves.
R. maximum (hardy as far north as Nova Scotia, Quebec & Ontario & south to Alabama, grows up to 30 ft. or more in the south, but not more than half this height in the north, F. grow in sm. heads, pale pink, naturally thrives in damp woods, ravines, river banks, margins of bogs);
R. catawbiense (far north as New England, F. magenta-pink);
R. catawbiense album;
R. carolinianum (inhabits the Blue Ridge Mt. in North Carolina, South Carolina, & Tennessee, F. abundant, beautiful, white, soft pink, pale pink, rose or lavender pink);
R. minus (lower elevations of the Blue Ridge Mt. of North Carolina to Georgia & Alabama & the inner Coastal Plain, F. purplish-rose, magenta-pink or bright magenta);
R Chapmanii (near the Gulf Coast of Florida, lives in sandy pine lands, resembles R. carolinianum);
R. macropnyllum (lives in the mountains of California to British Columbia, F. rosy-purple or pink spotted w/ yellow or brownish color);
R. lapponicum (Arctic tundras and alpine tops of northern mountains. It occurs in Labrador and on Mt. Washington & northern Europe & northern Asia, F. bright purple).
These are natives to Europe and the Caucasus and are reasonably hardy in the northeastern U.S. and definitely in the Pacific Northwest, with the exception of R. ponticum:
R. caucasicum (L. up to 4" long, F. pink or yellowish white dotted w/ green, about 2" across, June or July);
R. Smirnowii (very hardy shrub, L. 4-6" long, F. in trusses, pale to dark rosy-purple, frilled edges, individual flowers are 2-3" wide, May);
R. ferrugineum (Alpine Rose - F. dark pink or rose-scarlet, about �" across, L. glossy green above, rusty & scaly below, 1-2" long);
R. Kotschyi (F. clear rose-pink, L. quarter or three-fourths of an inch long);
R. ponticum (F. in clusters about 5" across, lavender-purple or pinkish purple spotted w/ brown, L. 4-6" long & paler below than above);
R. Ungernii (L. 4-7" long, F. white or light rose, in trusses about 6" across, usually blooms in July).
R. lapponicum; R. ponticum and R. hirsutum (it will grow in limey soil).
These are hardy Himalayan varieties, but none can be expected to make it through very cold winters. Several are described:
R. campanulatum (L. oblong, 3-5�" long, dark green above, covered w/ thick brown or gray-brown fuzz beneath, F. in med. bunches, purplish, rosy-purple or lilac);
R. campylocarpum (F. clusters of bell-shaped, soft yellow, waxy);
R. ciliatum (L. fuzzy, F. white stained w/ rose);
R. Edgeworthii (L. dark green, fuzzy underneath, F. fragrant, white, 4" across, wavy petals);
R. Falconeri (L. 6-12" long, 3-6" wide, dark green surface, underneath covered w/ thick reddish-brown felt, F. lg., dense clusters, 6-9" high and wide, creamy white);
R. grande (L. grow in whorls, dark green above & silvery beneath, 6-12" long, 3-5" wide, F. cream w/ reddish blotch on one petal);
R. Griffithianum (L. 6-9" long & 3" wide, F. loose heads, white, saucer shaped, fragrant, 5-6" across);

These are the others: R. hodgsonii; R. kingianum; R. lepidotum; R. nuttallii; R. edinense; R. tyermanni; R. victorianum; R. thomsonii; R. veitchianum; R. arboreum; R. barbatum; R. formosum; R. fulgens;

These are Chinese, Tibetan and Burmese kinds that are likely to survive in the Pacific Northwest and a few will prosper in the Northeast at least as far as Boston. The following are great for rock gardens: R. racemosum; R. calostrotum; R. campylogynum; R.keleticum; R. saluenense; R. impeditum; R. intricatum; R. fastigiatum; R. hippophaeoides; R. scintillans; R. flavidum; R. williamsianum; R. sperabile; R. orthocladum; R. pubescens; R. spinuliferum; R. repens; R. sanguineum; R. forrestii. The following are larger kinds: R. discolor; R. fortunei; R. decorum; R. praevernum; R. sutchuenese; R. griersonianum; R. neriiflorum; R. adenopodum. These bloom while very small: R. callimorphum; R. yunnanense; R. concinum; R. Davidsonianum; R. chartophyllum; R. ambiguum; R. chasmanthoides; R. lutescens; R. augustinii; R. irroratum; This Rhododendron blooms very late: R. auriculatum. This group has huge leaves and large clusters of flowers: R. sino-grande; R. giganteum; R. praestans; R. protistum; R. fictolacteum; R. falconeri.
These are natives of Java and Malaya and need to be grown in a greenhouse: R. jasminiflorum; R. javanicum; R. malayanum;

HYBRIDS: Less hardy kinds expected to thrive only in the Pacific Northwest in North America: R. kewense and its varieties: Beauty of Tremough (blush pink); Ernest Gill (rose-carmine); Gill's Crimson (dark crimson); Gill's Gloriosa (lg., pink); Gill's Goliath (pink, up to 5�" in diameter); Gill's Triumph (lg., crimson); Glory of Leonardslee (brilliant pink); Glory of Penjerrick (carmine); H.T. Gill (bright red); Loder's White (lg., white); Loderi (huge, shapely truss or bunch of lg., white flowers); King George V (flowers as lg. as Loderi's, but pink); Pink Delight (lg., white stained pink).

Hybrids of R. criffithianum and R. catawbiense are suitable for growing only in the Pacific Northwest: Pink Pearl; Dulcie Daffarn; Daphne Daffarn; The King; Kathleen M. Dallimore.

Hybrids of fragrant flowered R. Fortunei and various garden hybrids that are hardy in mild parts of New England. Most of them produce fragrant, pink flowers in May: Mrs. Thiselton Dyer; Frances Thiselton Dyer; Miss E. A. Boulton.

Hardy hybrids of R. catawbiense with the exception of Boule de Neige (Hybrid of R. caucasicum) and album elegans (Hybrid of R. maximum) for planting in northeastern America and termed the "ironclads": album elegans (lavender fades to white); album grandiflorum (white); atrosanguineum (bright red); Boule de Neige (white); caractacus (purple-crimson); catawbiense album (pure white); Charles Dickens (med. red); Everestianum (rosy-lilac, frilled); Henrietta Sargent (rose-pink); H. W. Sargent (crimson); Lady Armstrong (rose, pale center); Mrs. Charles Sargent (rose-red); purpureum elegans (purple); purpureum grandiflorum (purple); roseum elegans (rosy-pink).

Hybrids of the species from Java and Malaya that need to be grown in a greenhouse: Ajax (orange-red); Amabile (flesh-colored, tinted rose); Aphrodite (blush-pink); aureum (yellow); carneum (flesh-colored); Brilliant (scarlet); Mrs. Heal (white); President (buff-yellow); Triumphans (crimson-scarlet); Sybil (rosy-pink); Primrose (primrose-yellow); Taylori (bright pink, white tube); Balsaminiaeflorum album (double white); Balsaminiaeflorum aureum (double yellow).