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.

'That was the first time I had a sense there was something special going on up here,' Sullivan said. Endangered in its native Chile, where it's the national tree, the monkey puzzle is a rare sight in North America. 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. 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.' Some are native to the Bay Area, while others hail from around the globe.

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. 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.

Salesforce roof garden