As leaves across America make their annual autumn pilgrimage from the treetops to the ground, lawn and wildlife experts say it's better to leave them around than to bag them.

First, because it keeps leaves out of landfills. Every year, about 8 million tons of leaves end up there.

And second, because leaves help the grass.

Leaves are full of nutrients, including nitrogen, phosphorus and potassium.

"Those nutrients are being returned to the soil," Susan Barton, a professor and extension specialist in landscape horticulture at the University of Delaware, told NPR. "But probably even more important than that, it's the organic matter. It's the fact that you've got this tissue that then eventually decomposes and improves the soil health."

They also provide a habitat for insects, spiders, slugs — and depending on where you live — possibly turtles, toads and small mammals, according to the University of Delaware's College of Agriculture & Natural Resources.

In order to optimize your fallen leaves, some maintenance is recommended. It's best to run over a thin layer of leaves with a lawn mover or cut them up via other means so that they will break down more quickly. Thick layers of leaves are actually bad for the grass as well.

"If you just leave the leaves on the grass, it will exclude light. And then the grass won't be able to photosynthesize. Eventually, it would die under a thick layer of leaves," Barton said.

Rake excess leaves into a landscape bed and it will turn into mulch. Shredded leaves can also be piled into a garden.

"Ideally, you want to let them decompose a little bit and they'll form a very nice mulch. Instead of going out and buying hardwood bark mulch, which is expensive, you can have a better mulch that's free," she said.

At the same time, city dwellers should be mindful that wind and rain can push leaves into streets and clog up drainage systems — creating a flooding hazard.

Some cities actually collect leaves for composting at a central facility, where it turns to mulch that residents can collect for free. On the other hand, leaves in landfills that don't have enough oxygen to decompose will end up releasing a significant amount of methane.

How people deal with leaves is just one part of a longer-term issue of environmental sustainability.

"We want to think about those leaves as being a resource," and not a problem, Barton said. "And when you think about sustainable landscaping, well, one of the things we say about sustainable landscaping is let natural processes happen. And that's a natural process."