Azaleas prefer loose, moist, well-drained soil for their roots. If the soil is heavy, mix it with as much as 50% organic matter, such as fine pine bark or rotted leaves, before using it to plant the azalea. When choosing such soil amendments, avoid materials which may be alkaline or “hot” (containing fresh manure), such as the “mushroom compost” sold for use with perennials.
If the plant is wilting, soak it in a tub of water for a few minutes, or water it slowly and thoroughly with a hose before planting it.
If the plant is in a container, remove it. Avoid pulling it by the stems, but instead turn the container upside down and lift it off the plant. Any visible roots wrapped around the rootball will strangle the plant when they grow, instead of growing out into the soil. With a sharp knife, cut these roots by making slits about 1/2″ deep from the top to the bottom of the rootball, about every 2″ or 3″ around the rootball. Cut any matted roots off the bottom of the rootball. While it may seem harsh, cutting or untangling the roots is very important o help them become establishedd after planting.
In good soil, dig a hole at least a few inches wider than the rootball and just as deep as the rootball, and plant the top of the rootball even with the top of the soil. Avoid disturbing the soil at the bottom of the hole. If it is disturbed or soil must be returned to the hole, tramp it firm before planting. The goal is to avoid the azalea from sinking more deeply as the soil settles. In heavy soil, plant high, with the top of the rootball several inches above the ground level, and mound the amended soil up to the rootball. In very poorly drained soil, plant on top of the soil, or in a very shallow depression.
Put the root ball into the hole, and rotate and tip the plant to its best appearance. If it was wrapped in burlap, optionally remove it. Real burlap can safely be untied and stuffed down beside the root ball, as it will rot away in a few months. You must remove plastic burlap (usually yellow or orange), as it will not rot and will impede root growth. Add soil to fill the space under and around the root ball, tamping it firmly with your fingers, and continue until the fill soil is at the same level as the top of the root ball and the surrounding soil. The goal is to avoid any airspaces without compacting it so much that water will not enter.
Mulch the plant with 2 to 4 inches of pine straw, leaves, pine bark, wood chips or whatever is available, but leave an inch around the stems without any mulch. Then water it slowly and thoroughly. Water it again the next day, and at least once a week for several weeks. The goal is to settle the soil and remove any air spaces, and to make sure the disturbed roots have ready access to water until they can grow into the surrounding soil. Remember to watch small plants for a month or more, and large plants for a year or more, and water them deeply whenever they look wilted.

Azaleas have shallow roots, and prefer moist, well-drained soil with a pH between 4.5 and 6.0. If in doubt, you can submit soil samples through your County Extension Service to have the pH and nutrient levels tested, usually for little or no cost. The test results will also have information about changing the pH if needed, which will be appropriate for your soil type and climate.

Check how well your soil drains by filling a hole with water. If it is empty in a few hours, it drains well. If not, you have so-called "heavy" soil. In heavy soil, don't plant azaleas into small holes filled with amended soil, since the holes will hold water and be death traps for the plants. Instead, you can build raised beds, or amend a large area, or "plant high", ranging from an inch or so above ground level to planting on top of the ground, and mounding amended soil around the root balls.

Azalea roots need access to both moisture and oxygen. In wet soil, the roots will grow closer to the surface to get oxygen. In dry soil, the roots will go deeper to get moisture. In reasonably well drained soil, the roots of evergreen azaleas tend to stay in a well defined mass of fine feeder roots from the surface down to around 12 inches deep. They seldom extend beyond the width of the plant, and usually stay within a foot or so of the trunk. Roots of deciduous azaleas may range deeper and further from the trunk in their search for water.

Sand and clay are the soil extremes, and neither are suitable for growing azaleas. Sand has large particles with large spaces between them. Gravity quickly drains excess water from it, there is rather little surface area to hold a film of moisture for roots, and there is plenty of space for oxygen. Clay has small particles with a lot of surface area. Gravity takes a long time to drain excess water from clay, there is a lot of surface area to hold moisture, and almost no space for oxygen. The ideal soil, called loam, has a mixture of large and small particles. That mix allows excess water to quickly drain out by gravity, and has a lot of surface area moisture and spaces for oxygen. Sand and clay can be improved by mixing them together, to provide the desirable mix of particle sizes. They can also be amended by adding organic matter and humus (decomposed organic matter), as much as half by volume, to improve their fertility.

About an inch of rainfall each week is ideal for azaleas. Supplemental water may be needed if the rainfall is much less than that, especially if there is no rainfall for extended periods. Especially watch for signs of dryness on recent transplants, on azaleas which have been planted high, and on azaleas which are in full sun or locations exposed to drying winds. Fortunately, drooping leaves show the need for water well before the plant dries out completely, and watering it slowly and deeply usually restores it within hours.

Mulch insulates the soil to even out soil temperature extremes (which may undesirably lower the air temperature at the base of plants in the fall). It also helps to keep the root zone moist, reduces water runoff, and reduces weeds. Perhaps even more importantly, decomposing organic mulch adds humus to the soil. Humus improves the ability of the soil to accept and hold water, it helps get oxygen to the azalea roots, it maintains the acidity of the soil, and it supports the very beneficial mycorrhizal fungi that absorb nutrients from the soil and move them into the azalea roots.

The best mulch is naturally decomposing wood and leaf mold from oak or pine woods. Other good materials are leaves, pine needles, wood products such as twigs, wood chips, ground bark, wood shavings, excelsior or sawdust, and shredded crop rubbish such as straw, peanut hulls or corncobs, and well-rotted animal manure (which may have an odor). Other than its appearance, the major factors to look for are the absence of weed seeds, and a mix of different sizes to provide porosity.

Leaves will decompose and become humus in about a year, while other mulches can last several years before they are decomposed. The bacteria and fungi that decompose the mulch use up nitrogen and phosphorus. To maintain the levels of these elements, it is good to add them at the rate of several handfuls of ammonium sulphate or ammonium nitrate and a handful of superphosphate to a bushel of "raw" or fresh mulch. This is more important with sawdust than with other materials, and less fertilizer is needed if the mulch is already somewhat decomposed when applied.

Fertilizer

Established azaleas typically do well without any additional fertilizer if the azaleas are kept well mulched. The humus from the decomposing mulch provides adequate nutrients for the azaleas and promotes the presence of mycorrhizal fungi which bring soil nutrients to the roots. By contrast, applying inorganic fertilizers decreases the presence of mycorrhizae, which increases the need for higher levels of soil nutrients. Thus, using inorganic fertilizers in any significant amounts tends to make the plants dependent upon their continued use. If you do fertilize, it is best to apply it between late fall and early spring when the plants are dormant. Avoid fertilizing after June, to keep from pushing plants into active growth before the winter cold.

Before using fertilizer in your landscape, get your soil tested. The test is usually quite inexpensive, and may be free. Your County Extension Service can probably supply instructions and supplies for the soil test. The results will show the current amount of all the essential nutrients, and will probably give some recommended types and amounts of fertilizer needed for the crop (azaleas) you plan to grow.

Nutritional deficiencies

You may see localized nutrient deficiencies in particular plants. The most common problem by far is chlorosis, seen as a yellowing between the veins of the leaves while the veins remain green. It is caused by a lack of available iron due to a variety of reasons, including compacted soil, too much water, too much fertilizer, or soil acidity outside of the range needed by azaleas (pH between a low of 4.5 and a high of 6.0, where 7.0 is neutral). While spraying the foliage with chelated iron is a quick fix, that does not solve the underlying problem. If the problem is confined to one plant, try moving it. After digging it up, take a close look at the soil and the roots, and check the soil drainage and general looseness.

To loosen up heavy soil, add as much as 50% by volume of organic material such as composted leaves, fine pine bark, or any of the other products mentioned for use as mulch. If the roots have been damaged by overwatering, poor drainage or overfertilizing, there is no quick fix. About all you can do is trim away the rotted roots, remove the problem by replanting it in well drained soil, and give the plant time to grow some new roots.

To lower the pH (increase the soil acidity), sprinkle several handfuls of ferrous sulphate around the base of the plant. Never use aluminum sulphate, as the aluminum is toxic to azaleas.

To raise the pH (decrease the soil acidity), sprinkle several handfuls of ground dolomitic limestone or oyster shells around the plant. Don’t use hydrated lime, which will burn the plant. Wood ashes are good to raise the pH, and add phosphorus, potassium and micronutrients as well.

A nitrogen deficiency appears as yellowish leaves, small new leaves, and stunted plant growth. Use a fertilizer which will not reduce the soil acidity, such as ammonium sulfate, ammonium nitrate, or well rotted manure which has not been treated with lime.

A calcium excess or deficiency appears the same as iron chlorosis to begin with, followed by leaf tip burn and twisted leaf tips. A light application of gypsum, dolomitic limestone or ground oyster shells will correct a deficiency. Gypsum will do that without affecting the soil acidity, while limestone or oyster shells will also raise the pH (less acidity), which may not be desirable.

A potassium deficiency appears the same as iron chlorosis to begin with, followed by leaf tip burn, inward curling of the leaves, and premature leaf drop. Spraying with chelated iron is a quick fix, and sprinkling a small amount of potassium sulphate around the plant will have a longer lasting effect.

A severe phosphorus deficiency produces dull and unusually deep green leaves. They will then turn reddish brown and purplish brown, and eventually fall off. A mild phosphorus deficiency may result in reduced flowering and lighter than normal flower colors. The phosphorus in superphosphate migrates only very slowly in the soil, about one inch per year, so the best way to get it to the roots is to sprinkle a small amount in the bottom of the planting hole, or to mix some with the soil used to backfill the hole. To correct a phosphorus deficiency seen after planting, use diammonium phosphate instead, as its phosphorus migrates more rapidly.

Pruning

To avoid losing next year’s blooms, you can prune small branches when the plant is in bloom (and enjoy them inside the house) or soon after.

For major pruning, the best time is in early spring before the plant is ready to put out new growth, so it has the full growing season to fill out and for the new growth to mature. When pruning azaleas to reduce height, particularly older plants, it is best to do the pruning in stages, to minimize the shock to the plant.

Before you start to prune, look at the plant you intend to work on, remembering that the branches which are shaded out often die back and become dead wood anyway. These should be removed first, as the effect of removing them may alter the way you approach pruning the rest of the branches to maintain the shape of the bush.

Older plants may have a number of tall branches which need to be eliminated. Remove two or three of the tallest branches, taking care to cut back to a side branch which is heading in the desired direction. Cut close to a side branch, as stubs tend to die back to the side branch and leave dead wood which may become infected later. If you remove a lot of the branches, it is a good idea to mulch around the plant to make up for the reduced shade on the roots. The new shoots, which will appear when the weather warms up, may come from the stubs and from the base of the plant.

Use clean cutters, and sterilize them as the work progresses, particularly after a cut in infected wood. Denatured alcohol is an effective sterilizing agent, as is rubbing alcohol or household bleach.

Next year take out two or three more branches using the same process, spreading the pruning over a three year cycle. This approach will result in the plant sending out new growth near the base, and lets you manage the shaping of the plant to achieve a nicely shaped bush. It also minimizes the shock to the plant.