LOS ANGELES — Spurred by visions of their cities frying in a warmer world, mayors around the nation have grasped a green solution: trees! Like Johnny Appleseed, they have vowed to sow their seeds in great profusion, promising millions of new trees in the coming years. Arbor Day, that old fusty holiday, is getting a makeover.

Cities once planted trees because they were beautiful. Now trees are being retasked as "green infrastructure" managed by "urban foresters" to work as powerful energy-saving, carbon-sucking, wastewater-treating tools to save the planet. But as the mayors spin their green dreams, their releaf teams have had to confront a brutal reality: Planting a tree is a lot harder than it looks.

Urban tree farming can be a time-consuming, expensive and exasperating experience — like children, trees require years of maintenance. Businesses complain about the cost, neighbors about the sap. Their roots are murder on sidewalks; their limbs tangle with power lines.

"The city sidewalk can be one of the most hostile environments for a young tree," a cramped cell of garbage soil surrounded by smothering asphalt, says Gregory McPherson, a scientist with the federal Center for Urban Forest Research. "A virtual conflict zone," as one arborist put it, beset by disease, pollution, drought, insects — not to mention drunken drivers and staple guns and trip-and-fall lawsuits. "It's a tough life," sighs Marcia Bansley, executive director of Trees Atlanta. It's hard out there for a poplar.

Trees are the new potholes. On his first day in office, Los Angeles Mayor Antonio Villaraigosa helped pat moist mulch around a golden medallion sapling, the first in an audacious promise to transform this dense, dirty, dry city by planting 1 million new trees. That was almost three years ago. Lessons learned? "We have learned that a million is a really big number," says Nancy Sutley, a deputy mayor who oversees the mass reforestation project, which has experienced some serious growing pains.

Boston Mayor Thomas Menino last year promised to add 100,000 trees by 2020, a goal that sounds almost humble compared with those of his counterparts. Seattle Mayor Greg Nickels envisions a new tree for every man, woman and child in the city — 649,000 maples, sweet gums and cherries over the next 30 years. Denver Mayor John Hickenlooper, announcing his "Tree by Tree" project, is going for a million by 2025.

A million just has that aspirational ring.

Indeed, Salt Lake County Mayor Peter Corroon is calling his bid "One Million Trees for One Million People." The state of Nebraska is shooting for a million in a decade. New Mexico recently unveiled its "Plant a Million More" campaign. The Sacramento region is betting it can add 5 million. Going global, the United Nations launched the Billion Tree Campaign. (Less numerically ambitious programs are under way in cities such as Atlanta, Baltimore, Chicago, Indianapolis and Washington.)

Not to be outdone, on Earth Day last year Mayor Michael Bloomberg promised New Yorkers a million trees in 10 years. The cost of planting a single street tree in Manhattan? About $1,000. Estimated cost of the urban reforestation project is $600 million, annual maintenance not included. An early hurdle faced by New York? The city can't hire trained arborists fast enough.

Driving around Los Angeles in his Prius is Andy Lipkis, the founder of TreePeople, one of the nation's most experienced organizations of "citizen foresters," who is helping Mayor Villaraigosa reach his million mark. Lipkis points to shady boulevards lined with ficus trees and then to entire neighborhoods devoid of any shrubbery, and he confirms what satellite imagery tells us: Poor people don't have plants. The thinnest tree cover is, no surprise, over the city's most impoverished neighborhoods. Where ritzy Bel Air has 53 percent canopy coverage, gritty South Central has only 7 percent.

Nationwide, three dozen cities have lost a quarter of their tree canopy since 1972, according to the group American Forests, which discovered America is missing 600 million trees as our major metropolitan areas fade from green to gray. But here's the problem: The increased density of American cities means there is less room for new trees to replace the missing. The same is true in the suburbs: All those new minimansions built to the edge of the property line don't have big yards.

But everybody likes trees, right? Apparently, no. According to the community tree planters toiling on the streets, businesses don't like trees (when foliage blocks signage). Bureaucrats don't like trees (because they're a hassle). And despite what they say now, politicians have not been tree huggers. The first item cut in any tight budget year is usually tree maintenance. "When you say, 'What's the cost of a tree?' it is more than buying a tree and putting it in the ground. It's also taking care of it," says Ray Tretheway, executive director of the Sacramento Tree Foundation, who explains it takes two or three years for a tree to establish a home.

Planting street trees often requires both a city permit and the permission of neighbors, who give many reasons for not wanting a tree at the curb. They don't like dogs, who do like trees. Tree activists have heard people complain about sap, birds, squirrels, spiders, leaves and shade. Oh, and they don't want their views blocked.

Burry explains that neighborhoods with the fewest trees are the toughest places to plant. "These are often the harshest environments, communities on the bad side of environmental justice issues — lots of renters, working class, two-job individuals. For us it is much more costly." Burry says it takes intense community outreach — many meetings, much door knocking, to get neighbors to agree to plant and care for new trees. Then: "Sometimes there has been no organic life on the street in 20 years. The soil is extremely dry, nutrient-poor, compacted. We have to pour gallons and gallons of water and just let it sit, and still sometimes you dig and it's like concrete. This loud clang." Other challenges? "They don't have water hoses," Burry says.