This is the first in Policy Solutions, a Wonkblog continuing series examining policies that work.

AVACANT LOT is a contagious place. Signs of its disorder — graffiti, car parts, trash ditched in the overgrown weeds — have a way of spreading. This is how it happens: First, the one lot drags down neighboring property values, discouraging people who live there from investing in their own homes, deterring banks that could lend them money, and unnerving buyers who might move in. Then the behavior that blight provokes multiplies, too: People who see litter, for instance, are more likely to litter themselves. Finally, blighted lots become good places to stash weapons and sell drugs, and the crime that follows depresses the block even more until what’s the point of picking up the trash when you can just move out, too?

And so these places multiply — “there’s one, and then there’s another, and then there are two more, and then there’s another," says Glenda D. Price, the president of the Detroit Public Schools Foundation and a co-chair of the city’s blight removal task force. "It just seems to creep.”

Cities from Detroit to Philadelphia, though, are increasingly finding novel techniques to fight back. Detroit deployed more than 150 residents, survey software in hand, to document the most extensive census of vacant land that has ever been conducted in a U.S. city. Now the land bank in Detroit is trying to strategically auction abandoned homes worth salvaging, as the city prioritizes neighborhoods with the best chance of recovery.

Chicago has been offering up empty lots for a single dollar to nearby residents and organizations who will care for them (and pay their property taxes). Jacksonville, Fla., passed a new ordinance allowing the city to demolish abandoned homes. Other cities have discovered that a few minor improvements — replacing plywood over windows with plexiglass, requiring owners to at least install new doors — can transform the look of an empty home. And Philadelphia has proven that even a little light landscaping on vacant lots — at the cost of maybe $1,000 a parcel — can significantly curb gun crime, boost property values and even aid residents’ health.

For each of these cities, blight is intimately connected to many of the other problems they would like to solve around health, crime, poverty and economic development. Detroit cannot truly revive its downtown if all the neighborhoods around it remain pockmarked with blight. New Orleans cannot help its low-income residents rebuild wealth if the value of their homes has been wiped out by the vacant land around them.

The scope of the problem, made even more apparent after the foreclosure crisis, is enormous in former industrial hubs where the population has been dwindling for decades. When Detroit took stock of its blight last year, it counted nearly 85,000 structures and abandoned lots in need of intervention. Philadelphia has some 40,000 vacant parcels, Cleveland 20,000. Baltimore has more than 16,000 homes that are still standing but abandoned. Every dot on this map, from Detroit's task force plan, shows a blighted property:

“It is a major problem in every declining city,” says Susan Wachter, a professor at Penn’s Wharton School of Business who has studied blight. “And in a city in decline, these problems don’t solve themselves — they grow over time.”

Philadelphia finds a solution

Philadelphia has been at this work longer than most cities — and has some of the most promising results. The Pennsylvania Horticultural Society has been running programs in the city since the 1970s, originally around community gardening. By the 1990s, the organization realized the city’s vacant land was spreading so fast, it couldn’t create enough gardens to keep up.

“We’d go out in the '90s, and there would be crack vials and needles all around small community gardens that had 80-year-old ladies working in them,” says Bob Grossmann, the senior director of land management at the horticultural society. “How long can you sustain something like that?”

Philadelphia at the time was dealing with blight the way many cities still do today — by responding to the neighbors who complained the loudest, cleaning a lot, encircling it with an eight-foot chain-link fence, then heading across town to the next complaint. That ad-hoc response turns out to be a terrible way to contain a problem that operates like a contagion. And so the horticultural society proposed about 15 years ago that the city work strategically to clean lots and leave them as informal parks instead, regardless of who legally owns them (and even if no one can find the answer).

The horticultural society proposed building a modest fence rather than an imposing one — something just big enough to deter dumping without signaling to neighbors that they are unwelcome, too. At the cost of about a dollar a square foot, the group wanted to remove trash, truck in topsoil, plant grass seed, new trees and a three-foot post-and-rail wooden fence.

The city agreed to try, and since the early 2000s, the horticultural society has had a contract to operate this program, cleaning new lots and managing existing ones. Today, it regularly maintains about 6,500 greened vacant lots, April through October (the program has also created a landscaping industry in the city from scratch). Another 2,100 lots receive care once a month from paid neighbors. Since the start of the program, about 700 lots have gone on to be redeveloped.

These numbers represent modest but real inroads into the city's much larger problem:

All of these treated parcels have made Philadelphia a prime setting for researchers studying blight. Wachter has found that simply cleaning and greening blighted lots can boost the value of neighboring homes by 20 percent. And those benefits keep growing as more lots are greened, eventually producing more property tax revenue for the city.

Plant a new tree in a leafy suburb, Wachter says, and the public gets virtually no return on that investment. Here, though? “There’s a huge return in these places,” she says. The impact, though, isn’t primarily about money. “The point,” she says, “is that improvements in property value are an indicator of the value of neighborhoods to people.”

Charles C. Branas, a professor of epidemiology at Penn, has simultaneously found a significant drop in gun crime around these greened lots (and that’s not just because crime has been pushed elsewhere). Part of the reason, he suggests, is that greened lots are no longer good places to dump weapons. And as communities come to care for and use them, they tend to watch over these new public spaces more themselves. Branas’s health research has also found that exercise went up and stress levels down after the lots were converted.

That last finding isn’t surprising — particularly in light of all the evidence that greenery in hectic cities has a calming effect on us. In one recent study, Branas and researchers led Philadelphia residents on walking tours around vacant lots before and after they’d been greened. The sight of blight actually boosted residents’ heart rates; the sight of the greenery eased it.

Imagine the cumulative biological effects, Branas says, for people who walk by blight every day. Grossmann recalls watching children pass by vacant lots in the city before the horticultural society greened them. Often, they’d walk down the middle of the street — the place that seemed safer.

“Philadelphia had its tipping point a decade ago,” Branas says. He figures that other cities are now approaching that tipping point, too, as the research has become increasingly clear in how blight harms communities and what they might do about it.

Different strategies for different cities

Many of these strategies — greening lots, planting gardens, creating land banks, auctioning homes — are replicable from one city to the next. But the character of blight varies. Philadelphia’s row homes leave narrow slices of blight, compared to the gaping holes left behind when spacious single-family homes decay in Detroit. And Detroit’s blight has been far more pervasive than what New Orleans experienced in concentrated form in the Lower 9th Ward after Katrina. New Orleans also has something Cleveland and Baltimore don’t — a primeval climate that reclaims property faster than most.

That means that what’s worked in Philadelphia will require tweaking and testing anew elsewhere. In the short time Detroit has been trying to fight back, Price says she can already sense an impact.

“Those individuals who have been here in some of these neighborhoods for generations trying to hold on to family property and feeling as though no one cared — they now are so pleased to see that someone does care,” Price says. “That is as contagious as blight.”