Southern magnolia (Magnolia grandiflora), by Jessica Voss.
Summer in the Triangle is heating up, and 2025 is projected to be one of the hottest years on record, following a series of increasingly sweltering seasons. In addition to all of the typical ways to beat the heat—drinking lots of water, staying in the shade, limiting vigorous activity to early morning and evening—there’s one other surefire method of staying cool that is often overlooked: spending time near plants and trees.
Trees and shrubs are nature’s air conditioning, capable of lowering the temperature around them by 3°F (1.6 °C) on average. This might not seem like much on the surface, but on a landscape level, it can make the difference between bearable and unbearable even on the hottest days. This isn’t solely because trees provide shade. Much of this cooling effect is the result of evapotranspiration, where trees absorb water from the ground and release it into the air.
Here’s how it works. Trees are like giant straws, sucking anywhere from 10-150 gallons of water a day through their roots up to their leaves. They use this water, combined with carbon dioxide pulled from the air and energy from the sun, to make food for themselves, releasing oxygen as a byproduct. However, in order to do this, special pores (stomata) on the surface of their leaves must remain open to allow carbon dioxide in and oxygen out, which exposes the water in the leaf to the air. The result? Evaporation. These tiny water particles in the atmosphere are just like spraygrounds, special playgrounds equipped with water jets and fountains so kids can cool off on hot days. On a microscopic level, even if you don’t consciously notice the water vapor in the air, you’ll certainly notice the results!
Thus, from a climate perspective, trees are a two-for-one deal, capable of both removing carbon dioxide from the atmosphere and cooling the local environment. Combine that with their aesthetic appeal and the benefits to mental health that come with simply being around plants, and you’ve got a simple, practical solution that is a win on almost every level: plant more trees.
Of course, it’s not quite that simple. When it comes to transpiration, some plants have a bigger impact than others—woody plants transpire more than herbaceous plants, deep-rooted trees generally absorb more water (and therefore transpire more) than shallow-rooted ones, and evergreens transpire more than deciduous trees because they keep their leaves year-round instead of shedding them. Some plants that thrive in arid or harsh environments like cacti and bromeliads use a special form of photosynthesis known as crassulacean acid metabolism, which allows them to photosynthesize at night and keep their stomata closed during the day to avoid water loss. There are many excellent reasons to plant these drought-resistant and heat-tolerant species, but localized cooling isn’t one of them.
It’s important to note that summer is a great time to enjoy trees, but a terrible time to plant new ones, as young plants may be stressed by extreme conditions (save planting for later in the fall). It can also take several years to observe a measurable cooling effect. There are many important questions to consider when selecting which kind of tree to plant in any given space, including its size at maturity, tolerance for site conditions, pruning needs and benefits for pollinators and other wildlife, along with aesthetics and personal preference.
We tend to think of tree planting and care as an individual act, but trees are so important and beneficial that they should also be considered a public resource accessible to everyone. However, a quick glance at the City of Durham’s interactive Street Tree Inventory will quickly demonstrate we have a long way to go to realize this goal, with the presence or absence of trees in any given area tied to a longstanding history of racial and economic discrimination. In an effort to close the gaps in tree coverage, the city has established an Urban Forestry department with a special focus on underserved areas, including specifically for requesting street trees at locations that need them.
With over 17,000 trees on its main campuses as of 2020—which doesn’t include all of the trees in Duke Gardens, Duke Forest and other Duke institutions—Duke University is no stranger to trees and tree care, having received the “Tree Campus Higher Education” designation by the National Arbor Day Foundation for fifteen years. Maintaining and preserving existing trees, as well expanding and planting new ones both on-site and off, is a key component not only of Duke Garden’s mission, but also of the larger Duke Office of Climate and Sustainability of which we are a part, as well as the Duke Climate Commitment for a more just, equitable, sustainable, beautiful, tree-filled—and above all, cooler—world.