PEACEFUL TAKEOVER In Gowanus, a group from Feedback Farms works on planters to grow vegetables.

THE city of New York owns thousands of slivers of unused land, and about a year ago, a group of Brooklyn gardeners had an idea: identify all the vacant lots in the borough, then help neighborhood residents take them over. They built an online map, then a mobile app, with information about the plots, including the names and phone numbers of the agencies that owned them. They called themselves 596 Acres, after the total area of unused public land in Brooklyn, according to city data.

On a recent Saturday, Paula Z. Segal, 34, a founder of the group, loaded up a bicycle trailer with handwritten wooden signs and set off for points on her interactive map, starting with 406 Nostrand Avenue in Bedford-Stuyvesant, a narrow ribbon that belongs to the Department of Housing Preservation and Development. Passersby stopped to watch Ms. Segal and a fellow volunteer, Eric Brelsford, hang signs on the chain-link fence.

“This lot is public land,” read one of the signs. “It’s very likely that they would let you and your neighbors do something nice here — maybe a farm or an outdoor movie theater.” When a woman pushing a shopping cart said she might call the agency’s phone number, Ms. Segal gently steered her in another direction.

“Calling the number is an O.K. place to start,” she said. “It’s better to talk to your neighbors and see what they want to do here.”

Ms. Segal has a history of contesting the lines between public and private space. In 2005, she and two friends docked an abandoned Navy rescue boat in the Gowanus Canal, converting the 63-foot ship into an open space for art, politics and hanging out. Amid complaints from neighbors, they were eventually evicted — in part, Ms. Segal said, because of an article in The New York Times. Earlier this year, as a law clerk at the firm Rankin & Taylor, she helped lead a successful campaign to reopen Zuccotti Park, which city officials had fenced off after evicting the Occupy Wall Street protesters. Afterward, she told The Village Voice, “I work to take fences down,” adding, “That’s mostly what I do.”

Born Paula Zaslavsky, she adopted the surname Segal in 2008, from her maternal grandfather, the only member of his family to survive the Holocaust and the last to carry the Segal name, she said.

The idea for 596 Acres grew out of a long and still ongoing effort to convert a city-owned site in Clinton Hill into a park. In the course of organizing, Ms. Segal became interested in a city database of publicly owned vacant properties, which she felt could be useful for community groups. The raw data showed close to 2,000 lots in Brooklyn. Working with researchers at the Center for the Study of Brooklyn at Brooklyn College, Ms. Segal and her colleagues first eliminated lots that already had community gardens or plans for use, and parcels that were inaccessible from the street. They came up with a map of 1,044 properties, covering about 250 to 300 acres, mostly in low-income areas with little green space, said Mr. Brelsford, 29, a freelance computer programmer. With a $324 grant from the online fund-raising platform In Our Back Yard, which works like Kickstarter for green projects, they printed 1,000 maps and began handing them out or posting them on vacant lots. They put the map online last August. E-mail messages started to come in, hundreds of them, from people who said they wanted to help or be helped.

Tami Johnson had been trying for two years to create a community garden near her home in Gowanus when she came across 596 Acres.

“My whole neighborhood is totally full of holes,” said Ms. Johnson, 37, who tests video games and plays drums in several bands. “Developers came in, the economy crashed, projects stopped, and they left holes all over the neighborhood. And there are all these other lots that are trash pits.”

Ms. Johnson had tried to convert two lots, but had been unsuccessful. Through the 596 Acres Web site, she found a lot on Bergen Street, and the name of another gardener, Tom Hallaran, who was also interested in converting part of it to grow produce. Mr. Hallaran, in turn, had been attracted to the lot — long a neighborhood eyesore — by a sign posted by Ms. Segal on the fence around it. Last summer, Ms. Johnson began lobbying city agencies for the land, this time with better results. By March, volunteers had cleared off 30 to 40 trash bags of debris.

On a cold Tuesday afternoon earlier this month, Mr. Hallaran and three other volunteers tended to tomato seedlings at what they are now calling Feedback Farms, which shares the Bergen Street lot with a traditional community garden and the Textile Arts Center, which grows plants for botanical textile dyes.

Feedback Farms is an experiment in movable urban gardening. Because the soil was considered contaminated, all the gardening is done in containers. And to get the land, Ms. Johnson stressed that the garden was temporary, and the city or a private owner could reclaim it at any time. So Mr. Hallaran, working with other volunteers, designed a vegetable garden that could be moved on short notice, using forklift pallets. In Our Back Yard provided a grant of $4,484 for the garden.

Mr. Hallaran’s online biography cites his work “in bioinformatics at the Washington University Genome sequencing Center, at a collective bakery, a biofuels production plant, and in online advertising,” as well as his interest in gardening. The gardeners are experimenting with planters that are irrigated from below, and equipped with electronic sensors that monitor their moisture levels and relay the information to a server nearby.

Testing two soils and two types of planters, they expect to grow between 1,200 and 1,900 pounds of produce this season — mainly tomatoes, Shishito peppers, lettuce, kale and bok choy — which they plan to sell to pay for the garden, said Clare Sullivan, 32, an environment coordinator at the Earth Institute at Columbia University (she is also married to Mr. Hallaran). The goal is to replicate the most effective combinations on other lots around the city, she said. “We want a test case where we show we can use the land and leave,” she added.

Letitia James, a Brooklyn city councilwoman who has worked with 596 Acres, said that city agencies were wary of community gardens becoming permanent institutions, difficult to displace, as happened in the East Village. And often, agencies face competing requests from groups that want affordable housing on empty lots.

“They’re afraid of the Occupy Wall Street mentality, that the gardens are going to inherit legal rights, squatters’ rights,” Ms. James said. “596 Acres has made a huge difference, because it organizes people. But getting the city to embrace the concept has been challenging. They move at a glacial speed.”

Beatriz De La Torre, an assistant commissioner of planning for the Department of Housing Preservation and Development, said that the agency recognized that gardens were a benefit to a community, but that its mandate was to build affordable housing. The agency owns 673 unused lots in Brooklyn, of which some are inaccessible or committed for use, and has made 141 more available to gardeners, either temporarily or permanently.

“We take everything on a case-by-case basis,” she said.

For 596 Acres, the progress has been slow. So far three gardens have been started, covering just 0.233 acres. On a recent volunteer day, no volunteers showed up to help Ms. Segal and her colleagues hang signs.

But earlier this month the group won a $4,000 award as the best green app from the city’s Big Apps contest, and a $1,000 grant from a group called the Awesome Foundation for the Arts and Sciences to expand its map citywide.

In the meantime, would-be farmers are eager to get at some land, however far from bucolic. At a scarred lot in the shadow of the Long Island Rail Road in Bushwick, Ms. Segal hung a sign on the fence, hoping to attract volunteers for a group called Brooklyn Permaculture, which has applied for use of the site. The group has ambitious plans for what it calls a food forest, with trees and chicken coops to feed 50 families, said Frank Addeo, 31, who works at a health food store. He said Ms. Segal had helped him contact other groups to learn about how they have navigated city agencies.

He remained guarded. “I’m hoping H.P.D. will follow through, but I’ve been disappointed in the past,” he said. “At this point, we haven’t received a clear answer.”