On a Saturday morning this spring, about 80 people, and more than a few dogs, gathered on San Bruno Avenue to be photographed under a colorful new mural that announced: “Welcome to the Portola: San Francisco’s Garden District.”

For many of the assembled residents, the freshly painted mural — featuring green vines and a fat red rose — wasn’t just a pretty picture. It represented a milestone in the neighborhood’s multiyear effort to re-establish the Portola, once home to 19 blocks of greenhouses and farms, as San Francisco’s center of urban agriculture.

For nearly a decade a small but growing group called the Greenhouse Project has been working toward that goal. Neighbors have been successful in getting pocket parks and pedestrian trails built. They came up with a neighborhood “green plan” that includes planting 160 new street trees later this summer. They have raised $400,000 through private donations, government grants and fundraisers, and the Board of Supervisors has designated the area the city’s Garden District.

But now a top goal of the greening initiative — to build a new urban agriculture center where the ruins of the Portola’s last nursery lie — is facing its biggest challenge yet. Last month, Group I, a San Francisco development group active in the Mid-Market neighborhood, bought the 2.2-acre property at 770 Woolsey St., where it plans to construct as many as 60 single-family homes.

In a statement, Group I President Joy Ou said her company “is excited at the prospect of providing critically needed housing at the 770 Woolsey site. This unique opportunity to create detached single-family homes for working families is unparalleled in San Francisco at this time.”

While Group I has yet to submit a proposal to the city, it is likely that its plan for housing would be incompatible with Greenhouse Project’s vision. A 110-page feasibility study by the architecture firm Sitelab calls for five new greenhouses, a commercial farm, a community garden, a farmers’ market, events space and a community center-commercial kitchen. The complex would take up the entire site. The San Francisco Recreation and Park Department has agreed to review the study and provide feedback, with the goal of acquiring the site if it should become available, according to a letter from department Director Phil Ginsburg.

“The progress of the Greenhouse Project and the neighborhood greening has grown by leaps and bounds over the past few years,” said David Gabriner, a Greenhouse Project co-founder who lives near the site. “We continue to develop a great deal of momentum toward the ultimate goal. It has become a real highly legitimate, freestanding project.”

Gabriner said his group has had little direct communication with Group I. “There has not been a lot of outreach to understand where the neighborhood is with their vision,” he said.

Ou said community outreach has just started, but initial conversations indicate that Portola residents are “as concerned as the rest of the city about the lack of affordable housing” and would be more likely to support single-family homes than a denser project.

The Portola — pronounced PORtola and not to be confused with Portola Drive — is a residential enclave with Visitacion Valley to the south, Highway 101 to the east, Highway 280 to the north, and McLaren Park and the Excelsior to the west. Its main commercial street, San Bruno Avenue, is a lively stretch of ethnic restaurants, services and shops.

In the early 1900s, the neighborhood was home to farms and greenhouses that produced enough flowers to supply much of the Bay Area. The horticultural opportunities, along with affordable-single family homes, drew a mix of Italian, Maltese, Irish and German immigrants. Today the largest ethnic group in the neighborhood is Chinese.

Supervisor Hillary Ronen, who lives in the Portola, said she will “remain unbiased” and won’t take a position on the Group I proposal until it comes before the Board of Supervisors, which would have to approve a change of use from agriculture to residential zoning.

But Ronen did say she “loves the vision of the Greenhouse Project” and that it could attract folks to one of San Francisco’s lesser-known neighborhoods.

“The culture of the Portola District is unique and worth preserving, and the greenhouses are the last-standing historic asset of an important era in the neighborhood,” she said. “It would be beautiful to see them restored.”

The Greenhouse Project also has an ally in San Francisco Heritage, the city’s preservation nonprofit, which recently started a fundraising effort aimed at landmarking significant properties in the city’s southern neighborhoods, including the Portola. S.F. Heritage Executive Director Mike Buhler says designating the 770 Woolsey property a landmark will be a priority.

“The bottom line is not only is this a unique cultural landscape worthy of preservation and eligible for landmark designation, it’s the heart of the garden district that the board established less than two years ago,” he said.

While the greenhouses are dilapidated, he said, some of the buildings could be restored and others re-created. He pointed to other San Francisco sites that were essentially rebuilt, including the Sunnyside Conservatory on Monterey Boulevard and the Conservatory of Flowers in Golden Gate Park. The storage tanks, small office building and smokestack on the property could be saved, he said.

Ou says the plan for housing honors the vision of the Garibaldi family, the previous property owner and the descendants of the original owner of the rose farm on the site. The family “always envisioned homes on the site,” Ou said.

“Our priority is to create a project that matches the existing character of the Portola neighborhood and community,” she said.

But while a quest to save the greenhouses may be difficult, given that the proponents don’t own the property, it has already helped spark a wider neighborhood renaissance.

The Greenhouse Project spearheaded the construction of a pocket park at the end of Burrows Street, right off of San Bruno Avenue. Overlooking the little park are a Four Barrel Coffee and a community space for concerts and talks. The mayor’s office recently funded the restoration of the blade sign at the Avenue Theater, built in 1927, and a new restaurant is close to signing a deal at the long-vacant Johnson BBQ on the ground floor, Next door, Ferment Drink Repeat, a brewery and brewing supply store, has become a community meeting place for neighbors involved in the Greenhouse Project advocates and other locals.

Resident Elisa Laird-Metke, who is active in the greening efforts, said she and her neighbors embrace change in the neighborhood, and that the greenhouses are a “valuable symbol of the Portola’s past and future.”

“We are not being NIMBYs and saying no new housing,” she said. “But we want to protect this piece of land that has so much history and significance.”

But in a city grappling with a housing crisis, it’s likely that at least some Portola residents will accept Group I’s proposal. Resident Marc Arreola said he likes the sound of both proposals, but given that Group I owns the property, it should be allowed to go ahead and build.

“It’s just about time someone did something about that place —it’s been an eyesore since I was a kid,” Arreola said. “Hopefully this will be something that houses teachers and nurses and those kinds of people.”

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

Twitter: @sfjkdineen