Just enough leaves on a lawn to stop weeds but not good grass
People are way too afraid of leaves on lawns. Perennial grass is stronger than you might think. A pile of leaves will leave a hole in a lawn. Two inches or more will allow grass to grow through only here and there. But a few layers will not stop perennial grasses from jumping right up in the spring. They have enough food in their roots to push through light layers of leaves. Leaves keep the soil warmer than no leaves and make it more fertile as the worms eat them through the winter, so grasses grow stronger in the spring.
Leaves not only feed soil and keep it warmer to grow grass better, but they stop small seeds like crabgrass; annual rye; Bermuda grass, nut sedge; dandelions; wild lettuce; and groundsel, the first of the wind-blown weeds to bloom in the spring, by keeping sun off the soil. They also stop mosses, which like these weeds, need sun on bare soil to germinate. Large annual grass seeds, like cheat and foxtails, can push through a few layers of leaves and need to be pulled. You can tell them from perennial grass sprouts by their color, which is a lighter green with wider blades than perennial grass, or by their seed stalks later in the spring. Crab grass, which sprouts in late spring, are very fat even when very short, and are best pulled small.
The weakest perennial grass is probably perennial rye sod, which most lawns on new construction are built with. It is not a true sod grass; it is a clumping grass held together by plastic net. Its roots are short for cutting off its clay soil base and transplanting, so it only lasts about 5 years before being invaded by stronger grasses. Clumping grasses don’t spread by rhizomes, underground stems, but depend on reseeding to refill bare spots that form when clumps die, so it pays to reseed it in the fall before allowing leaves to cover, or rake it into the leaves in spring. Its seeds are large enough to push through a few layers of leaves when they are ready to grow. Perennial rye seed grows deeper roots than rye sod, and can create a stronger lawn as the sod grass dies out if it is reseeded yearly.
Perennial grasses prefer to grow longer north of California, as the sun is lower and they need to be taller to catch the sunlight. Two inches tall is good in summer, but 3-4 inches is better in winter. Not cutting grass too short in summer or winter also helps keep sun off soil to prevent weeds and moss.
A mulching mower can break up the leaves and help them to lie heavy enough between the grasses to keep small weed seeds from sprouting, but whole leaves will not stop the good grass from growing from roots or seed if they are not too thick.