Program Profile: Pennsylvania Horticultural Society’s Vacant Lot Greening Program

This was an intervention to reduce crime and delinquency in urban areas by remediating vacant land. The program is rated Promising. Compared with the control area, treatment areas experienced statistically significant reductions in gun assaults, burglary, nuisances, shootings (overall and per kilometer), and all crimes overall. However, the intervention showed mixed results with regard to drug offense rates and no statistically significant effect on robbery/theft rates.

A Promising rating implies that implementing the program may result in the intended outcome(s).

This program's rating is based on evidence that includes at least one high-quality randomized controlled trial.

Program Goals/Target Sites

Blighted, vacant land is a term for property that is considered unsafe or unhabitable. This could be due to a variety of reasons, such as illegal dumping or dilapidated, uncared for structures. Excessive trash, abandoned cars, and unmanaged vegetation growth often are indicators the property is blighted and abandoned. These spaces make up about 15 percent of land in U.S. cities and are considered significant threats to the health and safety of nearby residents (Branas et al. 2018).

A program, implemented in Philadelphia (Pa.), focused on remediating blighted vacant land through an inexpensive cleaning and greening intervention. The goal was to reduce crime and delinquency in neighborhoods with areas that were blighted and abandoned.

Program Activities

The Pennsylvania Horticultural Society partnered with residents living in the Kensington area of Philadelphia, Pa., to develop the “land and care” project, which evolved into the Philadelphia LandCare program. The aim of the intervention was to improve the damaged space inexpensively without taking large urban transformation initiatives, such as luxury amenities or housing. While such initiatives can buoy depopulating neighborhoods and change local economic conditions, they often are expensive, displace existing residents, and create further entrenched neighborhood segregation (Branas et al. 2018).

First, trash and debris were removed from the vacant lot. Next, the land was graded and new grass was planted using a hydroseeding method to cover large areas of land quickly. A small number of trees were planted in each lot to create a park-like setting. In addition, low wooden fences were installed around the lot’s perimeter, to deter illegal dumping and create a visible sign that the lot was cared for. However, the wooden fences were low and included multiple openings to encourage entry and use by residents. This process was completed across a 2-month period by a team of local landscape contractors, and the lots were regularly maintained afterward.

Program Theory

The logic of the remediation of blighted vacant land to deter crime and delinquency is based on the theory of collective efficacy and routine activities theory. The theory of collective efficacy posits that communities with members who have strong social ties to that community are more likely to act in the favor of common good. This suggests that remediated vacant lots would encourage a stronger sense of social cohesion and thus normalize nonviolent behavior in these spaces.

Routine activities theory speculates that crime is most likely to occur when three elements co-occur: 1) a person motivated to commit a crime, 2) a suitable target, and 3) an absence of an effective guardian. Remediating vacant lots encourages residents to socialize and gather in previously unsuitable areas. According to routine activities theory, an increase in the presence of residents would provide effective guardianship of the area and therefore reduce the amount of crime (Moyer et al. 2019).

Study 1

Branas and colleagues (2018) conducted a cluster randomized controlled trial in Philadelphia, Pa., to examine the impact that remediation of vacant land made on crime and delinquency. In January 2011, a total of 34,149 vacant lots were determined eligible because they had existing violations signaling blight—such as illegal dumping, abandoned cars, and unmanaged vegetation growth—and were confirmed abandoned through either contact or nonresponse from the owner on record. Of these lots, 541 were randomly selected to be included in the study and were separated into 110 clusters. Clusters were assigned to one of three groups: 1) a greening intervention (n = 201 lots), 2) a less-intensive mowing intervention (n = 174 lots), or 3) no intervention (n = 166 lots). The greening intervention consisted of the full intervention (planting new grass and trees, erecting fences, and regularly maintaining the lot). The mowing intervention involved only removing trash, mowing the existing grass, and regularly maintaining the lot. Following the analysis, the vacant lots in the control areas were scheduled to receive remediation in the future. The CrimeSolutions review focused on the differences between the treatment areas that received the greening intervention and the control areas that received no intervention.

Contamination-adjusted intent to treat analyses were conducted to examine the impact of the intervention. The outcomes of interest were all crimes, gun assaults, robbery/theft, burglary, illicit drug crimes, and nuisances. All outcomes were police-reported crime, and data were provided by the Philadelphia Police Department 18 months after the intervention period. Subgroup analyses were conducted to specifically examine the impact the intervention made on neighborhoods below the poverty line.

Study 2

Moyer and colleagues (2019) conducted a difference-in-differences analysis using a mixed-effects regression model to examine the impact of remediation of vacant land using the sample described in Study 1 (Branas et al. 2018) The sample size, eligibility, randomization process, and interventions were the same. The CrimeSolutions review focused on the differences between the treatment areas that received the greening intervention (n = 201 lots) and the control areas that received no intervention (n = 166 lots).

Shootings were the main outcome of interest, and data were provided by the Philadelphia Police Department 21 months following the intervention period. Both point-based and area-based geographic metrics of shootings were calculated. The latitude–longitude coordinates of each shooting were used to calculate a kernel density estimate of the monthly rate of shootings per square kilometer. This method assigns more weight to shootings that occurred closer to the vacant lots. Area-based metrics of monthly shootings were measured by calculating the number of shootings that occurred within 300 meters of each cluster. This method allowed researchers to observe displacement 300 to 600 meters outside the treatment areas. Two regression models were used, one for the points-based metric and one for the area-based metric. No subgroup analyses were conducted.

Study 3

MacDonald and colleagues (2021) conducted a difference-in-difference analysis using a fixed-effect model to examine the impact of remediation of vacant land, using the sample described in Study 1 (Branas et al. 2018). The sample size, eligibility, randomization process, and interventions were the same. In this analysis, both treatment groups (the greening intervention and the less-intensive mowing intervention) were combined to create one treatment group that consisted of 380 lots. The control group remained the same (n = 166 lots).

The outcomes of interest included the rate of total crime, robbery, aggravated assault, drug offenses, and public order offenses 24 months following the intervention. Measures of aggravated assault and robbery consisted of both armed and unarmed incidents, and public order offenses included disorderly conduct, vandalism, mischief, and prostitution. The Philadelphia Police Department provided crime incident data. The latitude–longitude coordinates for each crime were used to calculate a kernel density estimate of the monthly rate of crimes per square feet. No subgroup analyses were conducted.

Subgroup Analysis

Branas and colleagues (2018) conducted subgroup analyses to examine the impact of the greening intervention on neighborhoods below the poverty line. Treatment areas that consisted of neighborhoods below the poverty line experienced a statistically significant reduction in all crimes, gun assaults, burglary, and nuisances, compared with control areas that consisted of neighborhoods below the poverty line, at the 18-month follow-up. However, there was no statistically significant impact on robbery/theft or illicit drug crimes.

Diffusion and Displacement

Moyer and colleagues (2019) conducted analyses to examine the potential displacement of crime in a buffer area 300 to 600 meters outside of the treatment area. There was no statistically significant difference in shootings in the buffer areas at the 21-month follow-up, suggesting there was no displacement of crime.

These sources were used in the development of the program profile:

These sources were used in the development of the program profile: