Science of Fall Leaves: Better to Rake or Leave?

The best bet may be a mulch mower or compost: leaf collection a big waste.

Falling leaves are clogging streets, backyards and parks across the country. The question remains: is it better to rake them up leave them alone?

Researchers who study plant science may have several answers to this perennial question. They take into account the amount of work done by manual raking or gas-powered leaf blowers, the trucks that come to suck up those leaves from your local street, and the impacts on both your lawn and the global carbon cycle that affects the entire planet.

"Our basic position is that a tremendous amount of energy is used to rake and remove leaves from the landscape," said Joe Rimelspach, program specialist in turf grass pathology at Ohio State University. "The best thing is to use a mulching lawnmower. You recycle the nutrients. You aren't having a loss of nutrients from the trees. Also you are recycling the organic matter and aren't removing it from the ecosystem of the landscape."

Rimelspach says that curbside leaf pickup programs aren't so great for the environment.

"In many of our urban settings, we are using a lot of carbon to maintain these programs," he said. "Leaf blowers cause pollution. A lot of cities contract landscape waste to a compost stream which is excellent. But then there's the cost of the fuel to haul everything around, and more gas to haul around the compost."

In Toledo, Ohio, for example, the city's leaf program is underway this month with a combination of 100 street sweepers, front-end bucket loaders, packers, semis, and dump trucks, all of which produce diesel fumes as well as carbon dioxide.

Sweeping leaves into the street is also expensive. New York City recently canceled its leaf pickup program because of budget cuts, and Philadelphia is just now resuming its program after cancelling it five years ago.

In some ways, trucking leaves to a landfill can interrupt nature's carbon cycle. Every year, the amount of carbon dioxide in the atmosphere falls in the spring and summer when plants absorb CO2 as part of photosynthesis. The pattern reverses when leaves fall during the cold months, decompose and release their carbon back into the air.

Of course, dead leaves also clog storm sewers, make sidewalks and streets slippery for cars and walkers, and always seem to blow around after you rake them into a pile. Leaving them alone on your lawn may seem like a good idea, but they also block the light needed for the grass underneath.

James Crum, professor of plant and microbial science at Michigan State University, agrees that a mulching lawnmower is the way to go.

"There's no reason to fill landfills with leaves," Crum said. "They should go back on the lawn."

In fact, a recent study at Michigan State found that lawns do better with mulched leaves on top, up to six inches. "Healthier grass and fewer weeds," Crum said.

The alternative, according to Ohio State's Rimelspach, is a backyard compost pile. Microbes break down dead leaves, clippings and other plant material in just a few months. But beware the sulfuric smell that signals that the compost pile is using up too much oxygen.

"High sulfur smell is decaying is sign of anaerobic conditions. A proper compost pile should smell like a fine cigar," Rimelspach said. "Not a rotten egg."

Leave them or rake them?

The transition from Autumn into Winter is many people's favorite time of the year. The air freshens, the heat abates, and we anticipate, with a thrill, winter's chill. Take a look at nature's cool season around the world in these photos. First up: Aspen trees with fall color, San Juan National Forest, Colo.

A farmhouse in Vermont peacefully passes through pastel fall colors.

The autumn harvest is ready to pick at this time of year.

Fall colors blanket the vineyards in Montefalco, Umbria, Italy.

Alaska's winters are legendary; here, the landscape moves into the cold season.

Colorful autumn leaves glow at sunset in the Carpathian Mountains.

A pumpkin patch sits ready for autumn pickers.

Fall colors add beauty to Mt. Hood, Mt. Hood National Forest, Ore.

Japan's Mt. Fuji is framed by a reddening maple tree.

A lake glows with autumnal change in Japan.

A romantic street in Madrid's downtown braces for the oncoming winter.

A Colorado river is backed by colorful mountains during foliage season.