Casey Trees employees play a game Friday after transforming parking spots near the Metro Center station. The activities were part of Park(ing) Day, an annual effort to create temporary, urban green space. Pop-up parks were also created in front of the Wilson Building in the District and outside Artisphere in Rosslyn. (Bill O'Leary/WASHINGTON POST)

A small forest took over four metered parking spaces on the bustling corner of 12th and G streets Northwest on Friday afternoon. Tall green trees, shrubs, flowers, bags of soil, stumps that were comfy enough to sit on, and even a few picnic tables transformed city pavement into a “Stop, what is that?” moment.

It was all part of Park(ing) Day, the annual effort to create temporary green spaces in urban areas — a pop-up park, if you will.

Workers with Casey Trees, a D.C.-based nonprofit group that helps plant trees, dedicated nearly half its 30-plus member staff to creating a temporary park downtown. They began about 7 a.m., bringing in the picnic tables and shade trees. Plants were brought in from Casey’s tree farm in Berryville, Va.

“We’ve been here since early this morning setting up the plants, the games and the beverages,” said Christopher Horn, a spokesman for the organization. “Our staff has been working in shifts all day, so it only took about an hour for us to get everything for the park set up.”

Park(ing) Day began in 2005, when a San Francisco-based art studio dedicated to environmental projects set up a park for two hours in a metered space. Since then, the event has spread, with temporary parks popping up in 162 cities and 35 countries over the past seven years. Now, the third Saturday in September is designated as the day for creating temporary green space.

More than a dozen cities along the East Coast participated in this year’s effort. Park(ing) Day’s Web site, which allows people to look for events taking place in their area, provides a how-to guide on creating a pop-up park. There’s also a manifesto created by the event’s founders on the ideals of urban planning and answers to potential legal questions about setting up parks while navigating a city’s permit laws.

On Friday, the Washington area played host to three official temporary parks: one outside the Metro Center station; one along Pennsylvania Avenue in front of the Wilson Building, where council members agreed to designate four of their assigned spaces for a park; and one outside Artisphere in Rosslyn. Volunteers handed out fliers and encouraged passersby to create their own parks.

This was the second year workers with Casey Trees participated in the event. Last year they popped up at Dupont Circle, where, Horn said, existing greenery made it a more difficult to get people’s attention.

“At Dupont Circle, it was a little easier for people to already see an urban landscape, but down here there are almost no trees. We’ve definitely had more people stop this year and ask, ‘What’s this?’ ” Horn said.

Park(ing) Day was founded with the goal of creating temporary parks to educate people on city, national and international efforts to re-green. Participating organizations hope that the pop-up parks will encourage debate among residents in urban areas lacking greenery and result in more permanent green space.

The temporary park set up outside Metro Center on Friday provided an opportunity for workers, commuters and others in the area to enjoy a game of bean-bag toss or a free beverage at one of the picnic tables. Or they could take a little bit of the park home with the purchase of one of the potted plants available for sale.

Toby Gohn, who works nearby, said he had read about the plan to bring a park to the parking spaces and knew he would check it out.

“In years past I’ve seen other Park(ing) Day events, and they’ve had cool art exhibits, so I decided to stop by,” Gohn said. His colleague Isha Yilla decided to tag along with him.

“He showed me the link for the Park(ing) Day Web site, and it seemed like something interesting to check out while I went out on my lunch break,” Yilla said.

Maria Wallace of Staunton, Va., said she was in the District to “chase food trucks” when she came upon the park. She decided to make use of the picnic tables to enjoy her street-food findings.

“I walked by and got really excited,” Wallace said. “I love that they’ve set up all these plants and tables here. Now I don’t have to eat my food while sitting on the curb.”