Leave the leaves: what fall leaves do for your yard

Leaves aren't trash

Every fall, trees drop a year of stored sunlight onto the ground. In a forest it becomes shelter, food, and soil. In most yards it goes into bags.

The volume is large. EPA figures for 2018, its most recent national data, put yard trimmings at 35.4 million tons, about 12% of U.S. municipal solid waste. About 63% was composted, but around 10.5 million tons went to landfills.

Who lives under the leaves

The Xerces Society notes that most butterflies and moths don't migrate. They spend the winter as eggs, caterpillars, chrysalises, or adults, many of them sheltering in leaf litter. Luna moth cocoons and swallowtail chrysalises even look like dead leaves.

The National Wildlife Federation adds fireflies, which spend most of their lives as larvae under fallen leaves or in the soil, and bumble bee queens sheltering just below the leaf layer. NWF warns that bagging leaves can throw away a large share of a yard's spring butterflies and moths.

Option 1: mow them into the lawn

If you want a lawn and leaves, mow them in. Studies by Purdue University turf specialist Zac Reicher found that mulching leaves into grass didn't hurt its color or quality. He advised shredding a little at a time, when leaves are dry, rather than tackling 6 to 8 inches at once.

A newer USDA-funded study at the University of Wisconsin–Madison and in East Lansing, Michigan, found lawns stayed healthy even with 6 inches of leaves mulched in, while 6 inches left unmulched caused serious damage. The results are preliminary, and the study is ongoing.

One trade-off: the Xerces Society prefers you not shred leaves at all, because mowing destroys the insects living in them. A good compromise is to mow leaves into the lawn and move the rest into beds.

Option 2: use them as mulch

Rake leaves into garden beds and around trees and shrubs. University of Wisconsin–Madison Extension suggests an even layer about 3 to 4 inches deep, and notes that fluffy leaves like oak, sugar maple, silver maple, and birch work best, while Norway maple leaves tend to mat down.

The Xerces Society also suggests leaving plant stems standing until late spring, since many native bees nest in them.

When to rake them up

Not every leaf should stay. A thick, wet mat on a lawn blocks sunlight, smothers grass, and encourages snow mold, as Purdue notes. University of Wisconsin–Madison Extension also advises against leaving leaves in beds with invasive jumping worms or around slug-prone plants like hostas.

Diseased leaves are another exception. University of Minnesota Extension explains that the apple scab fungus overwinters on fallen infected leaves, and recommends raking them up before the first snow. Unless you hot-compost carefully, send diseased leaves to a yard-waste facility.