When New York City first set a goal to plant a million new trees in a decade, it was more ambitious than any city tree planting project that had ever been attempted before. Now they’ve finished two years ahead of schedule.

Update: Today’s millionth tree ceremony has been canceled, so New York has planted just 999,999 trees (still very impressive!). The millionth tree will go into the ground soon.

The MillionTreesNYC campaign–dreamed up by former Mayor Bloomberg and Bette Midler, who runs the nonprofit New York Restoration Project, while they were on a walk in a park–became part of PlaNYC, a massive strategy to make the city more sustainable.

The long list of benefits of city trees, from sucking pollution out of the air to filtering stormwater, keeps getting longer. “There’s been an avalanche of new research,” says Deborah Marton, executive director of the New York Restoration Project (NYRP). Trees can make city dwellers happier and smarter, and reduce diseases like obesity and diabetes. More trees on a city block can even make us feel younger and richer.

Basically, it’s hard to have too many trees. “We partnered with the city to plant a million trees because the truth is that when it comes to ecology–even urban ecology–more is more,” says Marton. “In order to have healthy urban forests, you have to work citywide.”

New York isn’t the only city to try a massive tree-planting campaign. But cities like Boston, Denver, and L.A. haven’t moved as quickly or gotten as far. Part of the project’s success may have come from the fact that the Parks Department partnered with the nonprofit to plant more trees wherever the city couldn’t.

“Because we’re private, we’re able to tend to the urban forests in the way that the average New Yorker experiences it–which is without regard to ownership,” Marton says. “We can work on property owned by anyone.” While the city planted the majority of the trees (750,000) in parks and along streets, NYRP filled in the gaps on other public land–places like housing projects, libraries, airports, churches and synagogues, and hospitals. They also gave away trees to anyone with a yard who wanted one.

The partnership also helped give the city some extra cash for the project. Because they weren’t limited to taxpayer dollars, NYRP was able to raise another $30 million for tree planting through a combination of grants, donations, and corporate gifts. “There was a kind of flexibility and nimbleness to what we do,” she says. “We couldn’t work at the same scale, because we’re not the City, but we can work in places where the City would have difficulty.”