July is Heat Awareness Month. This month, we’re talking about how trees help mitigate heat and what you can do when the temperature goes up.
Heat Awareness Month and How Trees Reduce the Heat Island Effect
Every summer when temperatures rise, we naturally shift our habits to keep cooler. Around the Twin Cities, it might look like searching for a shaded parking spot, crossing to walk on the shady side of the street, or stopping in the shade of a tree to sip from a water bottle or to reapply sunscreen.
During Heat Awareness Month this July, looking at these seasonal habit shifts helps us understand one of our most effective tools for combating extreme heat: trees.
Nowhere is the value of trees more evident than in urban areas, where the urban heat island effect can intensify already dangerous summer temperatures.
Cities and the Urban Heat Island Effect
Keeping cool during warmer months can be especially challenging in cities, where a combination of factors create the urban heat island effect.
The urban heat island effect is a phenomenon where more developed areas become significantly hotter than surrounding rural landscapes because roads, parking lots, sidewalks, and buildings absorb and store heat throughout the day, then slowly release it at night.
According to the U.S. Environmental Protection Agency (EPA), daytime temperatures in urban neighborhoods are often 1–7°F warmer, with nighttime temperatures remaining 2–5°F higher, than those in outlying areas.
Finding and Growing Solutions
In the Twin Cities, recent urban heat mapping efforts found that highly developed areas such as downtown Minneapolis and downtown Saint Paul remain significantly warmer than neighborhoods with more tree canopy and green space. Local studies also found that street trees, parks, and nearby water bodies help lower neighborhood temperatures and provide relief during periods of extreme heat.
How Trees Help Mitigate Heat: Doing More Than Just Cast a Shadow
There are a number of unique ways trees help us fight the Heat Island Effect and keep us cooler in the heat of summer.
Have you ever wondered why standing under a tree feels better than standing under an umbrella? While umbrellas, tents, and temporary shade structures can provide relief from direct sunlight, they do so by creating a shadow, though they often trap heat and can even create uncomfortable “hot pockets.”
Trees, on the other hand, do much more than cast a shadow. They actively cool the air through a process called evapotranspiration.
What is Evapotranspiration?
Evapotranspiration occurs when trees release water vapor through their leaves and draw heat from the air. This is a natural cooling process that can significantly reduce air temperatures. This process effectively makes trees nature’s air conditioning system.
Unlike an umbrella or a shade structure that blocks sun, trees not only block sun and create shade, the process of evapotranspiration also cools the surrounding environment and creates a more comfortable type of shade.
Trees and Airflow
Alongside evapotranspiration, tree canopies also facilitate natural airflow while preventing the humidity-trapping effect commonly experienced under man-made shade. The shadows cast by a tree can block sun while also allowing air circulation between branches and leaves, creating an overall more balanced, breathable microclimate.
Healthy Trees Better Cool Communities
Trees can only provide maximum cooling benefits when they are healthy and well-hydrated. During hot, dry periods, watering young and newly planted trees is especially important because drought stress can reduce growth, limit evapotranspiration, and make trees more vulnerable to pests and disease. By caring for trees and providing adequate water during the summer months, homeowners and communities help ensure trees remain healthy, resilient, and capable of delivering the shade and cooling benefits that make neighborhoods more comfortable during extreme heat.
Watering Trees: How to Help Trees Help Us
- Regular watering is the single most important factor in a tree’s successful establishment. It is critical to keep newly planted trees watered for at least the first two growing seasons, although watering for the first three to five years (especially in drought) can be highly beneficial. Trees need at least 10-15 gallons of water every week, unless it rains at least 1” that week. If you are unsure whether the tree needs water, test the soil using your finger—if it’s dry to the touch when you press into the soil, it’s time to water your tree.
- Water efficiently. Water trees in the evening to reduce water loss through evaporation and keep a mulch ring around the tree to maintain soil moisture. When watering mature trees, pour water around the tree’s drip line (the edge of the above-ground canopy). This is where the roots will be actively seeking water.
Plant More Trees: A Heat Remedy with Lasting Benefits
All through July, during Heat Awareness Month, and through the warmer months of summer and fall, it is worth recognizing that trees do much more than beautify our neighborhoods. They also serve as natural cooling systems, create shade, improve air quality, reduce stormwater runoff, and help protect communities from the growing risks of extreme heat.
Whether you are giving a tree a little extra water during a heat wave, planting a tree in your yard this fall, supporting community tree-planting efforts, giving to Tree Trust’s Green Futures fund, or simply enjoying the shade of a mature canopy on a hot summer day, take a moment to remember that every healthy, well-watered, cared-for tree contributes to cooler, healthier cities.
