The best pruning jobs begin with a goal, a determination of what needs to be accomplished--reducing size, controlling shape or rejuvenating growth. As with all pruning jobs, the first step is removing the dead and diseased wood. Then step back and look at the plant's structure before moving ahead with your mission.
The best time to prune azaleas and rhododendrons is immediately after bloom until mid-July. If you wait too long, buds will already have formed for the following year. In general, azaleas and rhododendrons need minimal pruning.
To reduce the height of an azalea while allowing it to retain its natural appearance, follow the branch targeted for removal down to a lower lateral branch, and make a cut just above the point of intersection. The cut should be slightly above where the two branches intersect, so as not to cut into the tissue of the branch that will remain.
Another way to reduce the size of an azalea is to cut it back to just above a whorl of leaves. Also, look for circular scars around the stem, where the leaves once were. Cutting just above these areas should create good bud break. In either case, make the cut just above new buds, whether they are visible or latent.
Evergreen azaleas can be sheared back to form nice mounds if desired. If you prune the azalea into a mound, remember to occasionally open up the plant and let some light penetrate into the dense mass of foliage so that it doesn't get too congested.
If you'd like to make a young azalea or rhododendron more compact and well branched, the easiest thing to do is to pinch off the vegetative buds in spring. First take a look at the plant to familiarize yourself with the bud system. The fat, swollen buds are flower buds. These buds form during the previous season and overwinter; you'll want to leave these alone. They are generally twice the size of the vegetative buds, which are the narrower, pencil-like, smaller buds.
The number of vegetative buds at the tip of a stem determines the number of new stems that will be produced. Sometimes you will find 2 - 4 vegetative buds, but more often there will be just one. If you snap off this single vegetative bud (it will be about 1/2 inch long) with your thumb and index finger early in the season, immediately after flowering, you will induce the plant to produce more buds. Generally, it then produces 2 - 4 buds at the same location, which will turn into 2 - 4 new shoots. In this way you can influence the branching structure of the plant.
Some azaleas and rhododendrons get leggy over time. If the plant is misshapen or too large, you can prune it drastically. Such a rejuvenation pruning should be done early in spring, mid-March to early April in the New York area (usually 2 to 3 weeks before new growth starts). Cut the plant back hard to about 8 to10 inches from the ground. You can cut the entire plant back or leave 1 or 2 smaller stems as a source of energy (these are cut back later once growth resumes). Remember to water the rejuvenated plant well during its first season. New suckers may need to be thinned mid-season.
Some rhododendrons, particularly the Dexter hybrids such as 'Scintillation', do not rejuvenate well. Many other rhododendrons and azaleas do. When rejuvenating an azalea or rhododendron, fertilize the year before to prepare the plant and then fertilize again in spring (end of April) after the hard pruning to stimulate growth.
For smaller azaleas, use bypass pruners to make the cuts. For larger jobs, you may need a folding saw or a pair of loppers. Work with sharp, clean tools. Tools can be sanitized with isopropyl alcohol.
It would be tedious and time-consuming to deadhead azaleas and small flowering rhododendrons. But since seed production does take energy away from the plant, large flowering rhododendrons can benefit from deadheading.
Hold onto the stem and grab hold of the spent flower head. Snap it off, being careful not to damage the new buds forming on the sides of the stem. Your hands will get sticky from this task but you can clean them with an oil-based hand cleaner.
In general, azaleas and rhododendrons are fairly problem-free. Below are some of the most common problems encountered.