As of this month, the 5.4-acre rooftop park atop the Salesforce Transit Center is once again accessible for commuters, SoMa residents, lunch-goers, plant lovers, and anyone else who just wants to take a stroll.

Perched 70 feet above the Transit Center's Grand Hall, the park is home to 13 different garden zones, containing more than 600 trees and 16,000 plants in total. Some are native to the Bay Area, while others hail from around the globe.

One of those exotic trees was the first to catch the eye of Mike Sullivan, former Friends of the Urban Forest board president and author of the 2013 book "The Trees of San Francisco."

While the Transit Center was still under construction, "I was walking down Howard Street one day and I looked up and saw a monkey puzzle," Sullivan said. Endangered in its native Chile, where it's the national tree, the monkey puzzle is a rare sight in North America.

"That was the first time I had a sense there was something special going on up here," Sullivan said.

Now, he's created a walking tour to introduce locals to what he says is "the best collection of plants in San Francisco outside of the Botanical Garden."

A monkey puzzle tree (left) with rock purslane flowers, native to Chile.



On a sunny summer day, we joined Sullivan, a Parnassus Heights resident, to take the tour, which starts from the ground-floor escalator and loops counterclockwise around the park.

Sullivan said the idea for the tour was born when Adam Greenspan, the landscape architect behind the park, invited him on a private visit just before its opening day.

Laypeople might not realize, he said, that the gardens are organized around regional themes, with special sections dedicated to the plants of Chile, South Africa and the Mediterranean.

Over in the South African garden, you can find silver trees (endangered in the wild) and agapanthus plants, which are currently in full bloom.

"You see these plants all over San Francisco," Sullivan noted, laughing. "If you are from California, you think they're boring — but I'm from upstate New York, so I think they're cool."

Silver trees and blooming agapanthus plants.

Many of the plants and trees are native to California, like the sycamores that surround the tables and chairs in the courtyard plaza.

While Sullivan's never seen them in San Francisco before, he says they are very closely related to the London plane, which can be found all the way up and down Market Street.

The park also features several strawberry trees, whose botanical name, Arbutus x Marina, is closely connected to the city's history. Sullivan says this hybrid was first presented at the 1915 Panama-Pacific International Exposition in Golden Gate Park, which got the trees from a closing sale of a nursery in the Marina.

"This is now the most commonly planted tree in San Francisco," Sullivan said.

Sullivan points at the fruit of a strawberry tree.

Even as a tree expert, Sullivan encountered some new species during his tour with landscape designer Greenspan. He was previously unfamiliar with the Queensland bottle tree, which is native to Australia and "would only do well in the eastern, warmer side of the city," he says.

Sullivan explains that the Transit Center's location is unique, allowing different types of plants to thrive based on the position of the sun and wind. In some areas, SoMa's high-rise buildings block the wind; in others, they create a wind tunnel. Shady spots abound, as do sunny ones, amplified by the reflection of sunlight on the glass of the surrounding buildings.

A sign invites visitors to the park.

During the Transit Center's unexpected nine-month closure for repairs, officials touted one positive impact: the garden was allowed to grow without any human interference. Ashley Langworthy, western region director for Biederman Redevelopment Ventures, even described the closure as "a blessing" for the park's plants and trees.

Sullivan disagrees: "I don't think the closure had much of an impact," he explained, noting that most visitors aren't trampling on the plants anyway.

In fact, he explained, the conditions of the garden will make it difficult for the trees to achieve the kind of growth they would in the wild.

"There are only three to four feet of dirt under this whole garden," he said. "So eventually, the trees will slow their growth down, almost like a bonsai."

Mike Sullivan's self-guided walking tour of Salesforce Park is available for free on his website. If you're interested in exploring more of the city's green spaces, he also offers a guide to his "Top 10" landmarked trees in San Francisco.