Sun & Shade
"All rhododendrons and azaleas will grow well in light shade; the Carolina rhododendrons will bloom more abundantly in full sun if the soil is kept moist, but in hot climates or in windy places partial shade is usually mandatory. Also full sunlight tends to bleach the flowers. In cold climates, most rhododendron do better on the north side of a building or on a northwest slope. All need some sun for best flowering but in general require partial shade.
Cold resistance
In catalog listings, the cold resistance of a hybrid rhododendron is indicated by a code that indicates the lowest temperature the flower buds can tolerate during the winter and still open perfectly in the spring. Plants bearing the code designation H-1 survive to -25°, H-2 to -15°, H-3 to -5°, H-4 to 5°, H-5 to 15°, H-6 to 25° and H-7 to 32°. Most varieties grown in this country range between H-1 and H-4 in hardiness. Also, many catalogues refer to hardiness zones. Zone 4 corresponds to H-1, Zone 5 to H-2, Zone 6 to H-3, Zone 7 to H-4, Zone 8 to H-5, etc. Other factors are important. If your plants are subject to desiccating sun and wind in the winter, then you should probably use plants that are one zone hardier. For example, I live in Zone 5, H-2, but try to use plants that are good for Zone 4, H-1. Plants with a H-2 rating need winter protection. Rhododendrons may be harmed in winter by drying winds and bright sun; protect their shallow roots with a mulch of oak leaves or pine needles and their foliage with a loose blanket of evergreen boughs or specially built screens. Keep the mulch away from the trunk of the plant.
Soil
The most important factor in achieving vigorous growth is an acid soil mixture high in organic content. They need an acid soil with a pH of 5.0 to 6.0, well mulched with organic material. Rhododendron thrive in a moist, well-drained, humus-filled soil, enriched with peat moss or leaf mold. Prepare the soil by thoroughly mixing equal parts of loam, coarse sand and ground oak leaves or redwood before planting. Many commercial growers root rhododendrons and azaleas in pure peat moss, or in a 50-50 mixture of peat moss and coarse sand or perlite. A favorite mixture on the West Coast is 1/2 peat moss and 1/2 ground bark dust, but in such mixtures, plants must be fed regularly. Soil around the rhododendrons' shallow roots must be kept cool and moist but well drained.
Transplanting
Because the roots grow near the surface, a bed prepared especially for rhododendrons and azaleas need not be more than 12 inches deep; deep planting keeps the roots from getting the air they need. In fact, it is a good idea to set rhododendron about 1 inch higher than they grew at the nursery. Balled-and-burlaped plants may be transplanted in blossom but it is better to transplant them early in spring in areas where their hardiness is questionable, and in spring or fall if mild winter weather does not damage the shallow-rooted plants. All except leathery leaved rhododendron transplant best in the spring.
Fertilizing
Do not fertilize at the time of planting, as this might injure the roots, but water deeply. Plants that have been given a soil mixture rich in organic matter probably will not need feeding for several years. Do not stimulate fast growth because it produces long weak stems and few flowers. But if a plant seems weak or sickly, use cottonseed meal or a special rhododendron-azalea-camellia-holly fertilizer, dusted on the soil early in early spring. Supplemental feeding later is not normally needed, but phosphorus and potassium may be applied any time. Do not mistake the normal wilting action caused by extreme heat or cold as an indication of a problem. It is normal and will go away when milder temperatures return. Desiccation of the roots can be serious in cold or hot conditions. Watering may be needed in winter or summer .
Cultivating
Do not cultivate around the shallow roots of rhododendrons and azaleas. Cultivating the soil around rhododendrons and azaleas could damage their roots. Instead, keep the roots cool and moist with a permanent 2- to 3-inch mulch of wood chips, oak leaves, chunky peat moss or other light organic material to conserve moisture and discourage weeds. Keep the mulch from touching the trunk.
Mulching
Soil around the rhododendrons' shallow roots must be kept cool and moist but well drained. To keep the soil cool and moist, mulch it with a 2-inch layer of wood chips, ground bark, pine needles or coarse peat moss. A year-round mulch of rotted oak leaves will also provide natural nutrients and will help keep the soil cool and moist. Keep the mulch from touching the trunk.
Walnut trees and Rhododendrons
The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut trees are sometimes grafted onto black walnut rootstocks. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil. [from Ohio State University Extension Factsheet HYG-1148-93 by Richard C. Funt and Jane Martin]
Pruning
For maximum flower production, pinch off faded flowers or the seed capsules that follow [deadheading]. Pruning is seldom needed except for removal of faded flowers, but if it is, branches may be trimmed immediately after flowering. Rhododendrons may be pruned after the flowers have faded to induce new growth. Prune out dead, diseased or damaged branches, and cut the oldest branches back to the soil level to encourage growth in younger branches. Pruning in the fall, will remove the buds for next years flowers.
A good technique to contain size is the remove terminal leaf buds from branches in the late fall or early spring. This can be done by carefully breaking the buds off [pinching]. This technique minimizes disease and insect damage. After pruning, spraying with a fungicide may prevent infection.
If necessary, you can remove a great deal of material. It is a general rule to not remove over 1/3 of the leaf area each year. Pruning is generally used to control unsatisfactory height or width of a plant. I don't prune very often and try to limit myself to plants which have a natural shape that is unsatisfactory. If I want to cut trusses for bouquets, I always cut the tallest flowers since this helps keep the plant within bounds.
Sanitation
It is important to remove diseased material from the vicinity of a planting. This means removing dead material that harbors spores of fungal disease and dying material that contains eggs of insects. Dying portions of a plant should be cut off and removed. The ground should be cleaned of dead leaves or flower parts. Pruned parts should be removed. Deadheading flowers involves removing the structure left after blooming before seed pods form. Usually they can be broken off at the spongy region at the base. In general all dead or dying plant material may harbor disease and should be removed. All such material should be burned or bagged and removed.
Propagation
Most evergreen rhododendrons may be propagated from stem cuttings. Cuttings are usually taken in the early fall of new growth that is just hardening off. Cuttings are taken in the morning when full of moisture. The cuttings are usually terminal cuttings with one whirl of leaves with the leaves cut in half (to reduce the leaf area) and any flower bud removed. The cutting has the end cut off just before placing in hormone powder (containing a fungicide). Then the cuttings are placed in a flat of sterile media containing a mix of peat moss, pearlite, and vermiculite. The flat is placed in a polyethylene bag and placed in a light area with no direct sunlight. The flat is rotated every couple days to compensate variations in light and temperature. Rooting usually takes from 6 weeks to 3 months. After 3 to 4 months the cuttings are potted up.
Most rhododendrons and evergreen azaleas root fairly easily. Deciduous rhododendrons are propagated by seed, grafting or cutting. Deciduous azaleas are very tricky to propagate from cuttings. Usually bottom warmth of 75°F is used to encourage root growth and artificial light is used to prevent the cutting from going into dormancy. Tissue culture is used to propagate varieties that are difficult to root."