What the research found
In the late 1990s, turf researchers at Michigan State University started shredding leaves into test lawns with a mower and watching what happened over the following seasons. The lawns that got the mulched leaves came out healthier than the ones that got none, and the dandelion counts went down, apparently because the leaf layer kept the seeds from reaching soil in spring.
Purdue ran its own trials and reached the same place. Mulching leaves into the turf didn’t raise thatch, disease or weed pressure, and the color and quality of the grass didn’t change. Minnesota’s extension service followed plots for a decade and found no shift in the soil chemistry. The one caveat from Purdue’s turf scientist: the nutrient benefit is small, so a mulched lawn still wants its fall fertilizer.
When mulching stops working
The research used thin layers, shredded as they fell. That’s the condition. A lawn under a big cottonwood that drops everything in one windy week is a different problem, and a 3- or 4-inch mat of wet leaves left for more than a few days smothers the grass under it.
- A little at a time. Mow when the leaves cover about half the lawn, not when you can no longer see grass. In a heavy week that may be twice.
- Dry leaves shred, wet leaves mat. Wait for a dry afternoon. Wet leaves clog the deck and come out in clumps.
- Raise the deck and go slow. Two passes at right angles leave pieces the size of a coin, which disappear into the turf in a week or two.
- Look for grass afterward. If you can’t see blades through the shredded layer, you have too much. Bag the extra.
Bagging in Denver
What doesn’t get mulched can go to the city. Every fall Denver runs LeafDrop, a set of drop-off sites where residents bring bagged leaves and pumpkins to be turned into compost that the city sells back in spring. Paper bags are the preferred container, because they compost with the leaves; the city has offered a free five-pack of them at participating hardware stores with a coupon from denvergov.org/leafdrop. People on the city compost cart can put leaves straight in the cart.
Keep leaves out of the street. Leaves raked into the gutter clog storm drain inlets, jam street sweepers and end up in the South Platte. Pile them on the lawn or in bags, away from the curb.
Beds, gutters and the last pass
Leaves in the flower beds can stay. A 2- or 3-inch layer is free mulch: it holds moisture through Denver’s dry winter, insulates perennials against the freeze-and-thaw cycles that heave roots out of the ground, and shelters whatever overwinters in there. Pull it back from the crowns of plants so they don’t rot, and that’s the whole job.
Gutters are the opposite. A gutter full of leaves holds water, and water in a Denver gutter freezes, lifts the shingles at the edge of the roof and backs up under them. Clear them after the last big drop and before the first real snow, which in this city can be the same week.
Mow the lawn one last time once the trees are bare, a little shorter than summer height, and you’re done until spring. The leaves you mulched are already feeding next year’s grass.