This is especially important if the mulched area is large and the plants are young and small. Most inorganic mulches don't get as hot because they transfer some heat to the soil below. Remember, too, that as winter approaches, a sunny day could interfere with the plant's adjustment to cold. Dark mulch can absorb the heat during the day and release it at night, which may interfere with the plant's adjusting to cold winter temperatures. If you're buying mulch from large piles that may not have been turned in a while, make sure you give it a sniff test first.
Good mulch has a clean smell like fresh-cut wood or soil.
Sour mulch can seriously damage and even kill plants within 24 hours after application. Most people believe that mulches like pine straw and pine bark will turn the soil acid below. Actually, that's true only some of the time. Many studies have shown little or no effect on soil pH, even with pine straw on an already acid soil. You can easily suffocate a plant by mulching too deeply.