Using Trees and Vegetation to Reduce Heat Islands
Trees and other plants help cool the environment, making vegetation a simple and effective way to reduce urban heat islands.
Trees and vegetation (e.g., bushes, shrubs, and tall grasses) lower surface and air temperatures by providing shade and cooling through evaporation and transpiration, also called evapotranspiration. Transpiration is a process in which trees and vegetation absorb water through their roots and cool surroundings by releasing water vapor into the air through their leaves. Trees and vegetation also provide cooling through evaporation of rainfall collecting on leaves and soil. Research shows that urban forests have temperatures that are on average 2.9°F lower than unforested urban areas.1
Trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings or to shade pavement in parking lots and on streets. Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof.
Benefits and Costs
The use of trees and vegetation in the urban environment brings benefits beyond mitigating urban heat islands including:
- Reduced energy use: Trees and vegetation that directly shade buildings decrease demand for air conditioning.
- Improved air quality and lower greenhouse gas emissions: By reducing energy demand, trees and vegetation decrease the production of associated air pollution and greenhouse gas emissions. They also remove air pollutants and store and sequester carbon dioxide.
- Enhanced stormwater management and water quality: Vegetation reduces runoff and improves water quality by absorbing and filtering rainwater.
- Reduced pavement maintenance: Tree shade can slow deterioration of street pavement, decreasing the amount of maintenance needed.
- Improved quality of life: Trees and vegetation provide aesthetic value, habitat for many species, and can reduce noise.
The primary costs associated with planting and maintaining trees or other vegetation include purchasing materials, initial planting, and ongoing maintenance activities such as pruning, pest and disease control, and irrigation.
A study of urban forestry programs in five U.S. cities showed a range of expenditures: annual costs ranged from almost $15 per tree in the Desert Southwest region to $65 per tree in Berkeley, California. Pruning was often the greatest expenditure, accounting for roughly 25–40% of total annual costs (approximately $4–$20/tree). Administration and inspection costs were the next largest expenditure, ranging from approximately 8–35% of annual expenditures (about $4–$6/tree). Tree planting, surprisingly, accounted for just 2–15% of total annual urban forestry expenditures (roughly $0.50–$4/tree) in these cities.2
Although the benefits of urban forestry can vary considerably by community and tree species, they are almost always higher than the costs. The five-city study discussed above found that, on a per-tree basis, the cities accrued benefits ranging from about $1.50–$3.00 for every dollar invested. These cities spent roughly $15–$65 annually per tree, with net annual benefits ranging from approximately $30–$90 per tree.2
For More Information
More details are available in Chapter Two of EPA’s Reducing Urban Heat Islands: Compendium of Strategies, which covers the following topics:
- How trees and vegetation reduce temperatures
- The benefits and costs associated with trees and vegetation
- Other factors to consider when using trees and vegetation
- Urban forestry initiatives
- Tree and vegetation tools and resources
1. Knight, T., S. Price, D. Bowler, et al. 2021. How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’? An updated systematic review. Environmental Evidence 10, 12.
2. McPherson, E.G., J. R. Simpson, P. J. Peper, S. E. Maco, and Q. Xiao. 2005. Municipal forest benefits and costs in five US cities (PDF) (6 pp, 267K). Journal of Forestry 103(8):411–416.