To accomplish Mayor Bloomberg’s goal of planting 1 million new trees throughout the five boroughs by 2030, the city is counting on New York residents to do about half the work and foot a large chunk of the bill.

While the city has earmarked $250 million over the next 10 years to plant more than 15,000 trees yearly on city streets, residents and homeowners must plant and buy 500,000 saplings, officials said, or the city will fail to meet the goal of increasing the number of trees in the city by 20%.

“There’s no way to reach that goal without a private push,” the deputy director of forestry and horticulture in the Parks Department, Bram Gunther, said.

The tree-planting program is one of 127 goals Mr. Bloomberg outlined in April that could reduce the city’s carbon emissions by 30% by 2030. Greener streets could help reduce asthma incidents and improve real estate values in neighborhoods currently dominated by concrete, city officials said.

Even environmentalists who have voiced support for the tree-planting initiative said they were surprised to hear that individual New Yorkers planting on their own property were part of the formula devised to reach the goal.

“I don’t have a shovel, and I don’t know where I’d plant a tree,” the executive director of the nonprofit group Citizens for NYC, Peter Kostmayer, said. “How do you get people in Manhattan to do that?” Mr. Kostmayer said he would like to help the city achieve the goal even if it seemed daunting.

The city is putting together a coalition of about 100 nonprofit organizations, headed up by the New York Restoration Project, to help raise the funds for the planting and to persuade New Yorkers to begin greening their own city.

Perhaps more difficult than planting 1 million trees over the next two decades is keeping those trees alive. Vehicles damage tree trunks when they bump them while parallel parking, and the salt used to melt street ice can seep into roots and kill trees, a former parks commissioner, Henry Stern, said. “The city also doesn’t have the resources to water every tree,” Mr. Stern said.

“It’s a huge goal, and we can’t do this alone,” the PlaNYC project manager for parks, Susan Donoghue, said. The city is looking to community groups to teach New Yorkers how to keep delicate saplings alive, she said.

“Trees give you the biggest bang for the buck,” the executive director of the New York Restoration Project, Drew Becher, said. “They’re relatively cheap and make neighborhoods feel different. That’s why people love the West Village so much: There are trees.” Planting a tree on a Manhattan street costs about $17,000, a Parks Department official said. Homeowners buying big saplings for their yards could be spending upwards of $1,000 apiece.

As part of the tree planting initiative it plans to unveil as early as next month when planting season begins, officials are considering marking new trees with the slogan, “Hey, I’m one in a million.”

“Every one of these PlaNYC initiatives is going to be partnership,” Mr. Becher said. “It’s going to take all of New York.”