Saturday was "Grow This Block" day on Germantown's West Rockland Street, and just like last year, residents were in their front gardens spreading mushroom compost and planting flowers. It was exactly the kind of event you would expect from a couple of savvy, Gen Y marketing whizzes. Preparations were chronicled on a blog, rocklandstreet.com. Journalists were alerted in advance, donations obtained. And, naturally, the day's highlights were broadcast to the world via regular Twitter blasts.

The brainchild of two residents, Ainé and Emaleigh Doley, "Grow This Block" is about more than just prettying up their little street with impatiens and coleus. The sisters, who spent their early childhoods on West Rockland and returned to live there after college, conceived the event as part of a larger campaign to keep their street from slipping over the precipice into blight.

It's too soon to gauge the long-term impact of their ambitious undertaking, but the methods the Doleys employ are worth watching — and imitating all over Philadelphia. There's even a name for the approach: tactical urbanism.

The appeal of modest projects such as "Grow This Block" is that they produce immediate and visible change, while building community. A new approach to planning, tactical urbanism is gaining favor in cities around the country, especially among a younger generation of urbanites such as the Doleys, who have grown frustrated waiting for financially strapped government agencies to fix their neighborhoods. The movement's rallying cry is "short-term action, long-term change." Let's hope so, for Rockland Street's sake.

Tactical urbanism projects can take any number of forms, but they all share a low-budget, seat-of-the-pants style. One Baltimore neighborhood painted a crosswalk at a troublesome intersection after the city failed to respond to repeated requests to install one. A group of Miami residents was even more artful with paint. They sprayed the weeds in an abandoned lot with bright colors to get city officials to pay attention to the overgrown mess. (The tactic worked.)

In some cases, the interventions are officially sanctioned and financed, like the small, shaded seating deck, or parklet, that the University City District built last year across from Clark Park. Other tactical urbanism projects resemble street theater. In Dallas, two residents staged a one-day event where they outfitted a derelict commercial strip with pop-up stores and cafes. It was so successful that city officials changed the zoning to make it easier to attract retail. The pair went on to found betterblock.org and serve as consultants on similar pop-up projects.

Urban planner Mike Lydon says he was among the first to identify the growing movement and give it a name. A founder of the Brooklyn-based Street Plans Collaborative, he attributes the rising interest in citizen planning to a confluence of three trends: the Great Recession; the migration of younger, DIY-oriented residents back into cities; and the rise of the Web. After Lydon's Street Plans group posted a free manual of effective tactics online, his office was deluged with requests for advice; in April, he spoke to a packed house at the Storefront for Urban Innovation in Philadelphia's Brewerytown

Still, Lydon concedes that tactical urbanism is hardly a cure-all for everything that ails cities. "This kind of approach doesn't replace the need for conventional planning," he admits. A big city like Philadelphia also needs big-vision, big-ticket projects, such as those outlined in the master plans for the Schuylkill and Delaware waterfronts. Ordinary citizens such as the Doley sisters can't begin to address the deep structural issues that plague their blocks.

It also seems that the most successful tactical-planning interventions, like Manhattan's Broadway or Center City's bike lanes, are realized in neighborhoods that are already on the way up. The Doleys are unusual in that they are applying the same tricks to a neighborhood that is struggling to reverse a downward spiral.

The sisters were spurred to action after the 2008 foreclosure crisis began to wreak havoc in lower Germantown, which blends into the weaker parts of North Philadelphia. Within a few months, Emaleigh Doley calculates, nearly 15 percent of the families on Rockland Street had lost their homes. The foreclosed properties were bought up at bargain prices by companies that turned them into Section 8 rentals, a subsidized-housing program run by the Philadelphia Housing Authority. What had largely been a stable block of working-class homeowners — albeit dotted with several vacant lots and abandoned houses — was suddenly inundated with renters. It was clear from her observations that many were unemployed, single-parent families struggling with a trifecta of poverty, drugs, and violence.

The change "was like someone flipped the light switch, " Emaleigh recalls "All of a sudden there was more trash, more people, more noise, more cop cars, more ambulances. You couldn't go to sleep with the windows open because it was so noisy. The block started to look worse."

Natural-born organizers, the Doley sisters appointed themselves co-block captains and began talking to neighbors about the mounting trash. From their first cleanup day, they have chronicled the block's progress on their website. All their interventions are intended to involve their neighbors and give them a sense of shared struggle. When the stray-cat population got out of hand, the Doleys arranged a low-cost neutering program. When people began to take an interest in gardening, the sisters got Home Depot to donate lumber so they could build a community garden on a vacant lot. Now, Ainé says, "People are starting to come forward with their own ideas."

"If I wanted the city to clean the vacant lot, I knew I'd get put on a list," Emaleigh says. "So I bought a lawn mower on Craigslist for $50." Unfortunately, the machine gave up after a few uses and wouldn't run at Saturday's event.

The Doleys' projects have gotten a lot of applause in City Hall, yet they are still pretty much on their own in their fight to save their block.

Last year, they got a boost when Mayor Nutter visited the street after reading an Inquirer article about three dangerous vacant properties that had been abandoned for 20 years. Nutter had them torn down within days. The Doleys considered the demolitions a great victory until they realized the city had just left a rubble-strewn lot that didn't even have enough soil for a lawn. Now they have to contend with residents parking their cars there.

The Doleys' new cause is trying to get control of the space, which has three owners — the city, PHA, and a private owner who has disappeared — so they can turn it into a garden. They'd like to partner with the DePaul Catholic School, across the street, but just learned the archdiocese is closing the adjacent church, St. Francis of Assisi. They fear another white elephant.

Occasionally, weariness creeps into their voices. "Germantown is so old, so many houses are just falling down," says Ainé, sighing, while Emaleigh adds that she wonders what it would be like to live in a less problematic, more walkable neighborhood. Half a century ago, in the urban-renewal era, the city might have dealt with a problem area such as Lower Germantown by wholesale clearance, observes Matt Wysong, the city planner for Germantown, who pitched in Saturday. Fortunately, those days are over. He's working on a plan for the nearby Wayne Junction station that could spur development, but who knows when it will get City Council approval.

Cities just don't plan like they used to. That leaves tactical urbanism as one of the few tools for making change. It's not clear if the Doleys' tactics can succeed, but right now they're Rockland Street's best hope.

Contact Inga Saffron at 215-854-2213 or isaffron@phillynews.com, or follow on Twitter @ingasaffron.