You’ve probably heard that lawns, too, have to "breathe," and that they can be smothered if a thick layer of unshredded leaves is left on top of them over the winter. That is true, but it is only part of the reason why we rake lawns.
Most lawns in the Northern U.S. are composed of one or more cool-season grasses. "Cool-season" lawn grasses are so called because their prime growing time is during those periods of the year when moderately cool weather predominates. Fall is one of those times. Blessed with sufficient sunlight, nutrients and water, and enjoying 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," if you’ll pardon the pun.
But a thick layer of fallen leaves can impede the growth of these grasses. Why? Because they can deprive the grass of one of the key elements I mentioned: sunlight. If not raked up, a thick and/or matted layer of fallen leaves casts excessive shade over the grass below. You don't have to rake up every last leaf; a shortcut is to mow, so as to shred left-over leaves.
