Mulching can be done at any time of year. However, winter mulching will help to keep root balls from freezing, as well as keep dirt around the plant from sluffing off due to rain. In turn it can also help to retain moisture in the dry spells that come with summer. It is important to add a quality compost prior to adding fresh bark chips or needles. This will prevent nitrogen being depleted from the soil as the chips degrade. As the mulch bio-degrades, it adds carbons to the soil creating a “fluffy” soil.
In soils, many beneficial bugs will have a hiding place if needed. Plus it adds a clean look to your landscape and impedes weeds from sprouting. There are many kinds of mulch, and colors. Keep it a few inches away from trunk of plant/tree. Mulch should be 3-6” deep to suppress weeds and help with freezing temps.