Mulch's purpose is pretty basic: It acts as a barrier, keeping sunlight and some air away from the soil surface. Sounds simple enough, but mulch's smothering effect brings with it both good news and bad. Consider these positive and negative effects of tucking in your soil beneath a blanket of mulch:
Cypress is a popular bark
mulch because it's
attractive and long-lasting.
Without the summer sun's rays striking it, soil stays cooler and plant roots don't stress from the heat. The bad news is that slugs, earwigs, cutworms, and other eat-and-run types love cool, moist, dark places. To minimize bugs, use only a thin layer of mulch, keeping it several inches away from plant bases.
Water in the soil doesn't thaw on sunny winter days, then refreeze at night. That's good news. The melting-and-freezing cycle makes water shrink and expand, possibly popping shallow-rooted plants right out of the ground -- a phenomenon called heaving. Heaving spells the end for plants.
The ground warms more slowly in the spring. This is good because perennials aren't fooled into breaking dormancy too early. You want the ground to stay cold until it really is spring. The drawback is that perennials may bloom late or soil may not be ready for spring planting. If so, rake back mulch until the soil warms up. Or, if you don't mulch over winter, wait until plants green up before mulching.
Water evaporates more slowly from cool soil protected from the wind. If you mulch, you don't have to water as much, saving time, money, and a precious resource. However, heavy rains can make the ground soggy and puddly for days. If beds become bogs, rake off mulch and let soil dry.
Without sunlight, some seeds can't germinate, and sprouts may not have the oomph to push through the mulch. This prevents weeds, but it thwarts some good seeds, too. Mulch after seedlings are up and have some girth and vigor.
Raindrops don't hit the soil surface, so soil is less likely to wash away or splash onto plants. This keeps plants cleaner and free of some soil-dwelling diseases.