When azaleas get too tall and leggy, they can be considerably improved by pruning.
The best time to do this is early spring before new growth starts, but it can still be done soon after the flowers fade. Pruning will lessen the display of blooms the following spring, but the long-term result will be much more attractive plants with plenty of flowers.
Select about a third of the stems spaced throughout the plant and cut them back to 8 to 10 inches long. Dormant buds below the cuts will initiate new growth that will eventually fill in the gaps. The following spring do another third, and finish the job the third year.
Azaleas should not be sheared as you would a hedge. It makes the top so dense that light cannot penetrate and the inside foliage dies; even if you cut them back you may not get satisfactory new growth. On deciduous azaleas, the tallest shoots can be cut back a little or a lot. Dead, dying or damaged branches should be cut as you go.
It is generally recognized that azaleas and related species do best in acid soils, but in many cases the soil may be too acid.
Soil samples sent to the University of Maryland soil-testing laboratory show that a high percentage are too acid for the health of the plants. Repeated applications of acid fertilizers on soils that are already sufficiently acid is the main cause, according to Dr. Francis R. Gouin, University of Maryland horticulturist.
Too-acid soil reduces the availability of some plant nutrients (mainly phosphate) while increasing the solubility of other, toxic soil elements.
Over-mulching can also cause azalea decline, Grouin says. Yearly applications of two to three inches of peat or similar materials appear to suffocate roots of these shallow-rooted plants.
To survive, the azaleas must initate new roots from the stems into the new layer of mulch. Damage caused by over-mulching may not become apparent for several years because it is difficult to diagnose without digging up the plant and examining the root system.
"I have seen many cases of two or three different root systems at different levels on stems of plants that are mulched yearly," Gouin says. "The lower root system will be in an advanced state of decay while new roots higher up are growing in the mulch."