Spring is here, and your lawn will soon be out of control. Time to start up the old mower — but besides the refreshing color of the grass, is any part of your slow trip across the yard going to be "green"?

Not quite, say environmental experts. The exhaust from traditional gasoline-powered mowers is polluting and puts lots of carbon dioxide into the air, contributing to global warming.

And because of their endurance, power and speed, about 90 percent of all lawn mowers in the U.S. have gasoline engines.

So what about electric mowers, either the plug-in or rechargeable kind? Or the old-fashioned human-powered push mowers?

Well, each of those has its own problems — but let's tackle gasoline mowers first.

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Gasoline mowers

There's a simple reason most people have gasoline mowers — they work well and they never need to be plugged in.

"Mowing large lawns is impractical with anything other than a gas-powered unit," says Peter Sawchuk, program leader for home improvement at Consumer Reports magazine. "It's really your only choice — you'd have to break [the lawn] into sections with an electric."

That's why such lawn-care giants as John Deere and Toro make only gasoline mowers for environmental use.

But lest you think you're pushing — or riding — a gas-guzzling hog, there's reason not to. The EPA just last September tightened regulations on gasoline-mower engine emissions.

It's demanding that a 35 percent reduction in exhaust emissions, and a 45 percent reduction in total evaporative emissions (i.e., fumes from the inefficient two-stroke engines), begin in 2009 and be fully implemented by 2015.

"EPA's new small-engine standards will allow Americans to cut air pollution as well as grass," said EPA Administrator Stephen L. Johnson in a statement.

Sawchuk says it's time to ditch your old gas mower, which probably doesn't conform to the new EPA standards.

"If your gasoline-powered lawn mower is older than 5 years old," he says, "replacing it with a new mower will give you significantly reduced emissions."

The big lawn-care companies would argue that they're already on top of it.

"We take a lot of time to ensure that we don't have harmful emissions," says Greg Doherty, group director of worldwide product and technology marketing for John Deere. "There are standards for that, and we meet or exceed all of those."

But to some environmentalists, they're all bad.

"The absolute worst thing you can do is use a gas mower on a typical American lawn," says Lloyd Alter, design and architecture writer for Discovery Communications' PlanetGreen and TreeHugger blogs. "It still has carbon dioxide and other gases that are harmful. You can't call anything with gas safe for the environment."

Todd Larsen, director of corporate responsibility at the nonprofit environmentalist group Green America, appreciates the efforts by the gas-mower makers and the EPA, but he says they're not enough.

"We don't recommend people use [gas mowers]," he says. "If you want to go green, use an electric or push mower ... Anything that runs on fossil fuel is going to emit carbon dioxide."

So what can you do if you've got a large lawn and can't give up the gas mower? Buy a no-spill gas can, Consumer Reports' Sawchuk says.

"Spilled raw gasoline in the environment is a much more significant contributor to the ozone degradation and the buildup of smog," he says.

Sawchuk also advises using a gas mower with an overhead-valve engine, sharpening your mower blades at least three times a year and buying a fuel stabilizer to keep the gas in the tank fresh for up to two years.

Electric mowers

Flipping an extension cord back and forth across your lawn is inconvenient, but an electric mower has a cleaner environmental footprint than its gasoline-fueled cousin.

"Even if the electricity that powers them comes mostly from coal, they would still be less polluting," says Green America's Larsen.

Among corded mowers, Consumer Reports' Sawchuk recommends Black & Decker's models.

"They're totally green — no emissions," says Joe Newland, Black & Decker group product manager, who added that electric mowers take about as much power as a 100-watt light bulb.

When it comes to battery-powered mowers, they're best for those with small lawns — or lots of time.

Most models need nine to 24 hours to fully recharge, so "once you've exhausted the battery, you're done for the day," says Sawchuk.

John Deere's Doherty points out that trailing an extension cord in the grass you're mowing makes electric mowers "not the safest in the world."

Push mowers

Back-to-basics reel, or manual push-powered, mowers, still made by the American Lawn Mower Company and others, are the greenest and cut the closest, according to Sawchuk.

Plus, they cost only about $100, when even low-end gas or electric mowers start at $150, and top-end riding mowers can cost $3,000.

"The best option for the environment would be a push mower," says Green America's Larsen, adding that they're silent and provide an opportunity for exercise. He recommends push mowers by Sunlawn.

Reel mowers are commonly used on golf courses and at stadiums. The downside, however, is that because of the low cut, the grass often dries out.

"At less than two inches in hot situations, the grass gets brown and you're going to lose it for the summer," Sawchuk explains.

PlanetGreen's Alter insists that suffering through brown patches is the way to go.

"It's the virtuous way to mow your lawn," he says. "It does not use fuel, and it does not create pollution."

It may not be the most practical option, however.

John Deere's Doherty says that push-powered mowers end up being expensive and time-consuming because you need to sharpen the blades regularly. Motor-powered blades travel much faster and can whack down grass even when relatively blunt.

Solar mowers

One intriguing option is the robotic, partly solar-powered Husqvarna Auto Mower Solar Hybrid, part of the Swedish maker's line of lawn-mowing robots, all of which return to charging stations when done. (Roomba maker iRobot has patented a lawn-mowing robot, but hasn't yet brought it to market.)

There's a catch, though — the Husqvarna costs close to $3,000. And it may not be manly enough to handle the American lawn.

"A lot [of these] are designed for Europe, and they just don't cut high enough," Sawchuk says.

Alter doesn't think the solar charger would do much for the unit, pointing out that it can do only about 900 square feet per charge.

"There's not enough power from a solar cell to mow a whole lawn," he says. "If you have a real sunny day and left it out for a week, how much time in the sun versus how quickly it runs out?"

Lose the lawn?

So is there an even greener option than a push or solar mower?

"It's much more sustainable not to have a lawn," says Green America's Larsen, adding that lawns are not part of the natural environment. "They were created by people."

Try replacing the lawn with plants native to your area, he advises.

"There are wonderful plantings that people use that consume less water."