The trees are trucked to the Transbay Transit Center in the dead of night.

They’re hoisted by crane 70 feet up onto the quarter-mile-long rooftop City Park that stretches above one of the busiest corridors on the West Coast. A few hours later, thousands of workers from companies like Salesforce and Trulia will be hustling to jobs in the glass towers that line Mission Street.

It’s a little eerie up there at 2 a.m., said Patrick Trollip, who is heading up the tree-planting project for McGuire and Hester, the landscape contractor for the park.

“You are elevated, surrounded by these skyscrapers, yet it is quiet and peaceful and there is no traffic,” Trollip said. “Then all of a sudden these massive trees are lowered onto the roof. I know these trees intimately, but it takes a while to get used to seeing them up there.”

The Transbay Transit Center is set to open late this year and will serve numerous bus lines, including AC Transit, Muni, Golden Gate Transit and Greyhound. It’s still unknown when it might connect to high-speed rail or Caltrain, which will operate the station, how much it will cost to operate, or what shops and restaurants will occupy the 100,000 square feet of retail space.

But what is known is this: Tree by tree, San Francisco’s next significant public open space, a 5.4-acre green rooftop boxed in by towers of glass, is taking shape atop the transit center. And amid all the questions about the future of the transit center, it is the park — with its carefully curated collection of flora — that has the potential to make the project an attraction in the years before the trains arrive.

“This is going to be one of the great parks of San Francisco, and it’s going to be a public space unlike anything else we have,” said Gabriel Metcalf, president of the urban think tank SPUR.

The trees — about 60 have been delivered so far — will eventually number 469.

The oldest trees are 40 years old, and the heaviest weigh 30,000 pounds. For the past 18 months, Trollip and landscape architect Adam Greenspan have scoured 17 nurseries across California and Oregon in search of the perfect specimens.

There are Chinese elms from Rainbow in northern San Diego County, and olive trees from Farmington in San Joaquin County. From Gilroy come island oaks, while Escondido was the source for five or six cork oaks. A Columnar Hornbeam came from a nursery outside Portland, while a rare torpedo-shaped Chilean wine palm was tracked down near San Diego.

Greenspan, a resident of San Francisco’s Duboce Triangle who has designed green spaces around the world, was guided by the trees in his own backyard. Driving down Folsom Street in the Mission late one recent morning, he pointed to the row of Chinese elms growing on both sides of the street — five of them will be planted in the transit center park.

“This is one of my favorite tree streets,” said Greenspan, who is a partner at PWP Landscape Architecture. “I was definitely inspired by Folsom Street.”

The idea for Maytems came from Buena Vista Park, while the inspiration for river birches, with their cinnamon-colored bark, came from Golden Gate Park.

Buying trees is a surprisingly cutthroat business. And it was especially challenging to locate desirable specimens because Apple had been buying trees for its new Cupertino headquarters. When Greenspan and Trollip found a tree they fancied they would “tag it” with a locking yellow tag, to be sure they got it. Eventually all the tagged trees were moved to a nursery in Sunol, where the transbay project team leased 4 acres.

In Sunol, arborists from McGuire and Hester cared for the trees, transferring them to larger boxes when they outgrew the old ones. Some of the trees arrived damaged, and at least one was stressed by a particularly cold night in Sunol.

“We are always checking the trees for wind damage, for damage from hawks and birds of prey that roost on the tips of the branches,” Trollip said.

Once the trees arrive at the rooftop park, they are positioned where they will be planted. Workers are waterproofing the roof and adding a layer of foam that will provide fill underneath the soil.

On a recent morning, Greenspan walked around spray-painting “N” — for north — on tree boxes to indicate which direction the tree should be positioned. He spent 15 minutes studying the five Chinese elms — like those on Folsom Street — that will provide shade in the “picnic meadow.”

“These are important trees,” he said. “And hard to find. Even though you see them around, there are not that many being grown these days.”

Back to Gallery Transbay Transit Center rooftop turning into 5.4-acre... 3 1 of 3 Photo: Michael Short, Special to The Chronicle 2 of 3 Photo: Michael Short, Special to The Chronicle 3 of 3 Photo: Michael Short, Special to The Chronicle





First impressions are important, and Greenspan designed the park to accentuate dramatic entrances. People arriving by escalator will land in a grove of Brisbane boxes. Those coming by elevator from the terminal’s Grand Hall will walk into a bamboo forest. The funicular that will whisk visitors from Mission Street will depart from a group of redwoods and arrive in a second grove on the roof.

The park will have signage explaining the various species and which trees are safe for kids to climb, like the Rhus typhina, or staghorn sumac.

“I think there is going to be a tree for everyone,” Greenspan said. “We have grand and stately trees, and we also have weird trees, quirky ones.”

At 1,400 linear feet, the park will have an area dedicated to South African trees, another to Australian. There will be areas for succulents and cacti, a manzanita and chaparral section, a fern garden and an oak meadow. There will be at least one cafe, a children’s play area, an amphitheater and bridges connecting two abutting towers.

“Because it’s long and skinny instead of it being a big forest or a bowl, transbay is something people will experience episodically. So it’s a lot of small gardens rather than one big garden,” Greenspan said. “The idea we had early was to make it sort of a horticultural botanical gallery of plants that are specially suited to this area.”

While it will be eight or nine months before the park opens, downtown workers are already taking notice of the greenery on the roof. Attorney Michael Sullivan, author of “The Trees of San Francisco,” said he was walking to work at First and Howard streets when he saw what looked like a Norfolk Island pine poking into the sky on top of the transit center.

“I thought, ‘Wow, someone has some imagination around tree planting,’” he said. “It will be so great to have a mini botanical garden right downtown where people actually have an opportunity to see the trees, as opposed to a botanical garden you might visit every year or two.”

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com

Twitter: @sfjkdineen