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.