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A team works to plant trees in the soil.
Louisville Tree Planting The Green Heart Louisville Project planted nearly 8,000 trees in the South Louisville area, and then monitored the effects greening had on the communities health. © Wilkinson Visual

Stories in Kentucky

The Green Heart Louisville Project

This groundbreaking study shows the power of adding trees to a neighborhood as a public health strategy.

In the fall of 2017, The Nature Conservancy, the University of Louisville's Christina Lee Brown Envirome Institute, and other partners launched the Green Heart Louisville Project to examine the link between neighborhood greening and human health. Though previous research found an association between nature and well-being, the Green Heart Louisville Project is the first to measure how a "greening intervention" can improve human health.

Tree Planting Project Blooms (3:47) Does planting trees help improve the health of communities? The Green Heart Louisville Project aims to find out.

A First-of-Its-Kind Project

Aruni Bhatnagar, Ph.D., the project's principal investigator and director of the Envirome Institute, worked for more than two years to develop the project idea and build a team around the study.

In addition to The Nature Conservancy, Dr. Bhatnagar recruited Washington University in St. Louis, Hyphae Design Laboratory, Cornell University, the U.S. Forest Service and other researchers and academics to join the project team and contribute their expertise.

At the time, Bhatnagar said no one knew whether and to what extent neighborhood greenery affected human health and why. The Green Heart Louisville Project could help inform how to design a neighborhood that supports human health.

Quote: David Phemister

The project brings together the expertise of university researchers, nonprofit leaders, community organizations, and volunteers to quantify a link between nature and human health. We intrinsically understand the link, but can we prove it?

David Phemister The Nature Conservancy's Kentucky State Director

The Green Heart Project received affirmation and critical support—just one year after project launch—from the National Institute of Environmental Health Sciences to support the clinical trial portion of the project. With a lead grant from the Owsley Brown II Family Foundation, The Nature Conservancy ultimately raised and provided over $8.7 million for tree planting and maintenance, project management and other key project needs. Additional local donors provided another $3 million.

The growing support for the project was rooted in people wanting to invest at the intersection of nature and human health and to build on a growing body of research highlighting a link between urban greening and health outcomes. 

However, the Green Heart Louisville Project is the first longitudinal clinical trial to test urban greening in the same way a new pharmaceutical intervention would be tested. Specifically, the research team assessed whether a significant increase in trees and shrubs would contribute to better heart health and other health outcomes.

An infographic illustrates the benefits of city trees.
Benefits of Urban Trees Research has linked the presence of urban trees to reduced obesity, better stormwater management, increased property values, reduced stress, fewer particulate pollutants, cooler city streets, reduced disease rates and increased biodiversity. © The Nature Conservancy

The Study: Do City Trees Affect Human Health?

To establish the baseline data for the study, the team documented health data from 745 participants, as well as detailed measurements of tree coverage and levels of air pollution, in targeted South Louisville neighborhoods.

Next, the team planted thousands of trees and shrubs throughout the target neighborhoods to create an urban ecosystem that promotes physical activity while decreasing noise, stress, and air pollution. After that, the participants received annual check-ups to evaluate how the increased greenery has affected their physical and mental health, and their social ties.

“The project brings together the expertise of university researchers, nonprofit leaders, community organizations, and volunteers to quantify a link between nature and human health,” says David Phemister, The Nature Conservancy’s Kentucky state director. “New evidence revealed through the study can inform better policies and investments in nature as critical infrastructure and as a viable public health strategy.”

The Green Heart Louisville Project (3:45) The Nature Conservancy and our partners talk about the importance of the project and how it could change policy in cities around the world. Video credit: Kertis Creative.

Planting Begins

In October of 2019, The Nature Conservancy and Louisville Grows, a local nonprofit that specializes in planting trees on private property, began planting trees and shrubs in the Green Heart Louisville Project study area. The partners went door-to-door, talking with South Louisville residents about the project and asking if they would be willing to allow trees on their properties.

“In the beginning, the project was really unknown,” says Chris Chandler, cities and strategic partnerships director, Global Equitable Conservation team for The Nature Conservancy. “The first pass through a portion of the neighborhood, we had 10 percent of residents say yes. That rate of uptake is pretty standard in Louisville, according to our community partners.”

Once the first planting took place, TNC and Louisville Grows knocked on all the doors again and got 15 percent more residents to say yes. Word was spreading about the project, and residents saw their neighbors receiving trees and wanted to take part.

“Once the community saw that we were making good on what we were talking about, that these weren’t empty promises, we had people say, ‘Hey, my neighbor got that beautiful tree last fall, can I get one?’” says Chandler. “Now they’re starting to see tangible outcomes.”

The Power of Trees for Public Health

Support for the Green Heart Louisville Project was rooted in people wanting to invest at the intersection of nature and human health and to build on a growing body of research highlighting a link between urban greening and health outcomes. Photos © Mike Wilkinson.

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A front loader dumps soil into a waiting dump truck.
Two men stand together watching soil being spread from the back of a dump truck onto a planting area tucked between a highway and tall sound barrier wall.
Two people watch as soil is distributed onto a planting area adjacent to a highway as cars pass by.
Cars speed by a flatbed semi truck parked on the shoulder of a highway, laden with mature trees ready for planting.
Two men stand in knee deep holes, preparing them to receive newly planted trees.
Two workers remove ropes from the burlap sacks protecting the root balls of mature trees awaiting planting.
A man stands next to a large hole, gesturing to guide the placement of a mature tree as it is lowered into place by a small dozer.
An aerial shot of Watterson Expressway as plants are brought in on a flatbed truck for installation along the highway in Louisville, Kentucky.
A small tree with its exposed root ball is moved and ready for planting in the hands of a volunteer.
Teams work with shovel in hand and heavy machinery to help plant trees along the Waterson Expressway in Louisville.

COVID-19 slowed down planting activity in the spring of 2020, but trees continued to go in the ground. Ultimately, The Nature Conservancy, Louisville Grows, and a number of private contractors planted over 8,000 trees, including hundreds of trees along the Waterson Expressway, a large highway that bisects the Green Heart neighborhoods. These plantings are designed to create a living wall to filter air from the roadway before it reaches the community. Within the community, thousands of medium-sized trees up to 15 feet tall were planted on private property.

“When we plant trees on private properties, a part of the agreement with the landowners is that they will take care of the trees,” Chandler says. “We educate them, we give them information and pamphlets, and we talk about the cycle of watering.”

Louisville Grows regularly checked on the trees, making sure they were watered and offering supplemental watering if needed. Thanks to a capacity-building grant from TNC, the partner had a 1,000-gallon watering tank to provide additional water as they drove through the neighborhoods.

“The most important thing is that these trees not only survive but thrive, and to thrive we need all hands on deck,” says Chandler. “That’s how the landowner makes an investment in the project, in the care of the trees. The Nature Conservancy and our partners at the Envirome Institute are here to support the community to keep these trees alive.”

Young trees in various stages of growth fill and shade a once open field of grass.
A once open field of grass is covered in medium sized trees at a green park in Louisville, Kentucky.
Wyandotte Park Planting Trees were installed at Wyandotte Park to bring shade and greenery to the community. Partners in the Green Heart Louisville Project tested the effects of this work. © Mike Wilkinson

Celebrating Hope for a Greener, Healthier Future

In August 2024, after years of caring for the trees and monitoring results, the partners announced the study’s groundbreaking findings.

Researchers at the University of Louisville's Envirome Institute found that people living in the neighborhoods where the project conducted its “greening intervention” showed lower levels of a blood marker of inflammation strongly associated with cardiovascular health, as well as diabetes and some cancers. (Source: UofL news.) This finding is significant, because it means that adding trees to a neighborhood may reduce community members’ risk of heart disease. The Green Heart Louisville Project is the first study to show that an intentional increase in trees and shrubs in a neighborhood can indeed improve human health.

“Most of us intuitively understand that nature is good for our health. But scientific research testing, verifying, and evaluating this connection is rare,” said Katharine Hayhoe, chief scientist of The Nature Conservancy. “These recent findings from the Green Heart Project build the scientific case for the powerful connections between the health of our planet and the health of all of us.”

Thanks to the Green Heart Louisville Project, cities beyond Louisville and across the world have groundbreaking research to guide their efforts to create healthier communities. Investing in trees is an investment in human health–while also making our cities a cooler, more beautiful, and more enjoyable place to live.

A volunteer gently places a tree into the hole prepared for it in the soil.
One Tree at a Time Trees were planted all over Louisville communities as part of the Green Heart Louisville Project. © Wilkinson Visual