They analyzed crime data from 1999 to 2008 in areas surrounding 4,436 cleaned lots and areas near 13,308 trashed lots, and found a net decrease in gun crimes around the cleaned lots.
They also found an increase in disorderly-conduct reports in cleaned areas. But some say that is a sign of residents taking more pride in communities and becoming less tolerant of bad behavior.
In a separate study, Penn researcher Eugenia Garvin is looking at the health effects of cleaned and greened lots, and anecdotally has learned that people feel safer in cleaned areas.
Take away the junk, and there are fewer places to stash guns and drugs. Mow the grass, and predators lose their hiding places. That cuts stress and makes residents more likely to exercise outside than rush to their front doors and turn the dead bolts.
LandCare is the brainchild of the Pennsylvania Horticultural Society, which a decade ago convinced the city to not only clean but maintain vacant lots, even if the city didn't own them.
"The city didn't own the land but it owned the problem," said Robert Grossman, who runs the program for the Horticultural Society. "We saw our program basically increasing quality of life and stabilizing communities, and found since then it not only stabilized communities, but increased investment and market value."
Another benefit is that the landscapers and community groups mowing grass and pruning trees are creating green jobs.
About 10 percent of the nearly 5,000 cleaned parcels have been developed. An early sign of success came when drug dealers around Fourth Street and Germantown Avenue whined that the clean-and-green program was cutting into business.
The program is not a cure for poverty and its related problems, but it is an important step toward giving residents the support and encouragement they need to take back their neighborhoods.