"Trees form the basic integrity of the landscape here, and it's the largest living component, and this is the first life coming back to the memorial," said Tom Cox, the CEO of Environmental Design.
There are new signs of life at ground zero.
One by one, crews began planting 16 Swamp White Oaks at the former World Trade Center site.
The trees will dot a cobblestone plaza surrounding two huge pools built on the footprints of the destroyed towers.
"It is just a beautiful reminder that this is going to be a sacred place where people are going to come together. So we are all thrilled with all of the work that has been done," said Joe Daniels, the President of the National 9/11 Memorial.
Once they are in place, the White Oaks are expected to soar 60 to 80 feet high.
400 trees are to be planted at the site and should all be in place by next year's 10th anniversary commemoration of the attacks.
"You look at every corner of the site and you can see that One World Trade Center building is 36 stories high, Four World Trade Center is going up, and the transportation hub is being built. 1,000 workers are here around the clock," said Bill Baroni of the Port Authority of New York and New Jersey.
A lot of those workers say it is an honor to be part of such a meaningful project.
"The trees are showing there is growth, and we are growing beyond what happened, and we are slowly recovering," said Chuck Turcott, a ground zero technician.
Cultivated for four years at a nursery in Millstone, N.J., the 16 trees were loaded onto eight tractor-trailers at midnight Friday for the 35-mile trip to Manhattan. Several were planted overnight and into Saturday morning on the western side of the memorial plaza.
The memorial plaza will essentially become a rooftop garden, built atop the deep chasm left by the destroyed towers. It will cover the museum commemorating the 2001 attacks, commuter train platforms and a parking garage that are being built as far as 70 feet below ground.
The trees were irrigated and fertilized for four years at 15-acre nursery in New Jersey. Daniels said the swamp white oaks were selected for their beauty and their ability to withstand Manhattan's cold, snowy winters and steamy summers.
The trees come from New York, Pennsylvania and Washington, D.C. - the places where the attacks hit.
Once they are planted, an arborist will work full-time to prevent the construction site's daily dust and clutter from damaging the oaks.
An elaborate subterranean irrigation system, with individual tubes running to each tree, will water and fertilizer the grove.
The trees' condition, soil moisture and temperature can be monitored remotely through sensors embedded into their root balls.
"Our expectations are we will have 100 percent survival of the trees," Cox said.
National Sept. 11 Memorial & Museum: http://www.national911memorial.org/
(The Associated Press contributed to this report)