Hidden Talents Lawn and Landscape recommends that leaves be mulched as much as possible. With that said, we are in Michigan and there is an abundance of leaves and mulching isn't always possible. Although mulching leaves are a healthy and natural way of replenishing nutrients back into the soil and providing a healthier root system, it can also become harmful as well.
Cool season grasses require enough sunlight, nutrients, and water, and temperatures that are neither too cold nor too hot. Cool-season grasses such as Kentucky bluegrass revitalize themselves in fall. This is when they must "make hay," strengthening their root systems.
But a thick layer of fallen leaves can get in the way of the growth of these grasses. Why? Because, for one thing, the leaves can deprive the grass of one of the key elements just mentioned: sunlight. If not raked up at times, a thick or matted layer of fallen leaves casts too much shade over the grass below.