Source: JARS V47:No.3:p136:y1993
Tips for Beginners: How To Prune Evergreen Azaleas
Many evergreen azaleas never experience the pruning shears, since, for the most part, these plants are naturally well behaved. If they are carefully selected, carefully sited, carefully planted, and properly tended, there is usually little need to prune them in the informal garden. Nevertheless, there are some situations in which pruning can be useful, enough to justify a look at the subject.
Evergreen azaleas have a big advantage over some other garden shrubs when it comes to pruning - lop off the end of most any branch or twig that is not heavily shaded and a healthy plant will soon activate new buds (called adventitious buds) below the cut end. For good results, however, it's best to prune with a plan that takes other factors into account.
Why prune? First of all, to remove dead wood. Branches may be killed by cold over the winter or die in mid-summer after girdling by a hard frost in the spring. They may die back as a result of fungus attack, shading, borers, lack of water or improper fertilizing. Cut out the dead wood back to sound wood, i.e., where a nick with the fingernail reveals a layer of green cambium under the outer bark. If the cut end is more than a quarter of an inch across, I zap it with some aerosol wound dressing to discourage new invasion by fungus or borers. (Incidentally, when fungus is involved, the pruning shears should be sterilized after each cut by dipping in a fairly strong [20%] solution of sodium hypochlorite [Clorox] in water.
One of the most common reasons for pruning evergreen azaleas is to scale back an exuberant bush that is beginning to cover a window or crowd a walkway. Pruning would be unnecessary if a smaller-growing variety had been selected initially and positioned properly when planted. It is possible, however, to keep ambitious plants in bounds by regular judicious pruning. The secret here is to reach below the surface - follow projecting branches back a suitable distance into the bush to functions with other branches and cut them off flush. In this case don't leave stubs, which would probably die from lack of light and could invite fungus invasion. The overall effect is to reduce the size of the plant without destroying its symmetry. The surface of the plant will not be as dense at the outset but should fill in quickly.
Another common reason for pruning is to improve the appearance of tall leggy plants with ugly bare shanks and knobby knees. In these cases major surgery is often needed. Judgment is called for, but in serious cases I usually cut the offending stalks back to within a foot or so of the ground in spring (and spray the cut ends with wound dressing), leaving some low branches intact. Cutting so early will destroy the current year's flowers, of course, but new shoots will start earlier and recovery will be quicker. Strong-growing new shoots should be pinched once or twice up to early August to encourage branching.
Also, some varieties send up suckers from the base of established plants. These usually are shaded out eventually, so it's best to prune them away while they are still small. When the plant is young, however, I like to make sure that it will develop a bushy habit rather than a single trunk like a tree. This means that at least three main branches should be left growing from the base of the plant. That way, if a major calamity such as a borer strikes, all is not lost.
Another reason for pruning is to increase the density of twigs and flower buds on plants of naturally rather open growth. This is accomplished by pinching or shearing the new growth during the growing season to induce branching. Timing is rather critical if optimum results are to be achieved. Not all shoots on a plant are ready for pinching at the same time, and not all varieties ripen at the same time. If the new shoots are pinched too early, while they are still soft, the likely result will be a single new branchlet replacing the nipped growing point, and nothing is accomplished. If pinching is done at the right time, however, several buds will break below the point pruned. I usually wait till late June or early July, depending on variety, when the new shoots tend to snap when bent double. This is also the time to take cuttings for propagation, doubling one's reward for waiting.
Selective pinching of the terminal buds to increase branching can continue up until about the first week of August (later in the South). Pinching or shearing must stop while there is still enough of the growing season left to allow the plants to set flower buds for the next spring's show. Late pruning will take away the flower buds for the following year.
In formal garden, like some Japanese gardens, evergreen azaleas are often sheared to smooth rounded shapes. This is fine and gives results that are pleasing to many people, but, as we indicated earlier, shearing must stop in time for the plants to set blower buds.
In summary, the first rule of pruning is to select the right varieties in the first place and plant them where they'll do what you want without a lot of pruning. There are hundreds of varieties to choose from, dwarf, medium and tall, in many colors, blooming early and late. The second rule is to stop pruning while there is still time for the plant to set flower buds for the next year: around the first of August in the Mid-Atlantic region, and late August in the South. Third, don't leave stubs when pruning larger branches; cut them off flush with another branch.